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Alexander Markov, Elena Naimark
EVOLUTION
Classic ideas in the light of new discoveries

Preface. Why is life wonderful?

The amazing complexity of living beings, their fantastic diversity, their almost perfect adaptability to the environment, to each other, to their “place in the economy of nature” 1
Case [shaggy in front]>This is how ecological niches used to be romantically called.

– the facts are remarkable and require explanation. In the past they amazed the imagination no less than now. However, in the pre-scientific era, the explanations were, frankly speaking, simpler: almost any aesthetically balanced invention was suitable for this role.

As science developed, the attitude of literate people towards traditional mythological “explanations” became cooler. “It is in vain that many people think that everything, as we see, was created from the beginning by the Creator... Such reasoning is very harmful to the growth of all sciences, and therefore to natural knowledge of the globe. Although it is easy for these clever people to be philosophers, having learned three words by heart: God created this way, and giving this in response instead of all reasons,” wrote M. V. Lomonosov.

But how can we explain the amazing harmony of living nature without invoking hypotheses about the supernatural? Despite the attempts of many extraordinary minds - from Empedocles to Lamarck - to offer a rational explanation, until 1859 the generally accepted answer to this question remained a resounding “no way.” The complexity and adaptability of living organisms were considered almost the most visual and irrefutable evidence of the divine creation of the world. The “Book of Nature” was called the second Scripture, and its study was called “natural theology.” We read, for example, from the same Lomonosov: “The Creator gave the human race two books. In one he showed his majesty, in the other his will. The first is this visible world, created by him, so that man, looking at the enormity, beauty and harmony of its buildings, would recognize the divine omnipotence of the concept given to himself. The second book is Holy Scripture. It shows the Creator’s favor for our salvation.”

It seemed that the more new facts we discovered, the more clearly we would comprehend the highest plan.

Everything went wrong after the publication of Darwin's book On the Origin of Species by Means of Natural Selection (1859). Before Darwin, humanity knew only one reliable way to create complex, purposefully designed objects: intelligent design. Early evolutionary hypotheses, such as Lamarck's in his Philosophy of Zoology (1809), offered only untestable and incomplete alternatives. For example, Lamarck's idea of ​​inheritance of the results of exercise and disuse of organs offered a rational explanation (albeit incorrect, as we now know) for the increase or decrease of existing structures, but did not explain the origin of new organs. Therefore, in addition to the exercise, Lamarck had to postulate a special driving force that forces organisms to develop along the path of complexity - the mysterious “striving for perfection.” It was little better than God's providence.

Darwin proposed another, much simpler, more elegant and obvious way for the spontaneous improvement of living beings: natural selection of random hereditary changes. Darwin's hypothesis did not postulate any unknown forces or new laws of nature and, in general, seemed to lie on the surface 2
No wonder Thomas Huxley (1825–1895), the famous English biologist, after reading “The Origin of Species,” exclaimed: “How incredibly stupid it was not to think of that!”

If objects are able to reproduce, if they pass on their individual traits to their descendants, if these traits sometimes change randomly and, finally, if at least some of these changes increase the efficiency of reproduction, then such objects simply must - and will! - by themselves, without any reasonable intervention, become more and more perfect over generations. In this case, perfection means fitness, aka the efficiency of reproduction.

The scientific world was shocked. True, Laplace (in response to Napoleon’s question, where is God in his model of the solar system) half a century ago declared that he “does not need this hypothesis.” But Laplace was talking about physics. It seemed to almost everyone that there was nowhere in biology without “this hypothesis.”

Of course, expressing a beautiful guess is not enough; to justify it logically is also not enough; the guess must still be correct (and verifiable). Darwin's theory may not have been confirmed in the course of further development of science. But he had a special gift for putting forward correct hypotheses based on incomplete data. Without knowing genetics, without knowing the nature of heredity, long before the discovery of DNA, Darwin was able to correctly formulate the main law of life.

For “natural theology,” Darwin’s book was the beginning of the end. This is precisely what orthodox and fundamentalists will never forgive Darwin (the notorious “man descended from apes” is, in general, a trifle, a special case). The vector of development of biology, and indeed the entire scientific understanding of the world, has changed to the opposite. New discoveries have proven Darwin over creation over and over again. Natural selection, the blind force of nature, triumphed over “intelligent design.” Darwin overturned the universe as it had been previously imagined, replacing a beautiful fairy tale with an equally beautiful, but, alas, more difficult to understand scientific theory.

Darwin's model of selection of small hereditary changes seems to be simple - but its simplicity is only apparent. It is no coincidence that this model, which now underlies biology, entered science so late - in the second half of the 19th century. In other sciences - mathematics, physics, astronomy - theoretical breakthroughs comparable to it in their significance and level began one or two centuries earlier. Even today, there are frequent cases when not the most stupid people “stumble” on the Darwinian model, cannot understand how it works, how it explains the observed facts (and some even think that these facts do not need explanations).

Perhaps one of the reasons for the misunderstanding is lack of attention to detail. Based on general principles, almost any phenomenon can be interpreted one way or another: philosophical reasoning, as we know, is a good tool for justifying directly opposite conclusions. Evolution is counterintuitive. We are accustomed to the fact that everything will work out as it should only with a clearly defined goal and competent leadership. In this sense, it is more comfortable for us to exist when someone has outlined a development plan for five years in advance, and not in the conditions of the cruel elements of the “free market”. We know very well that it is easier to win if the attack is carried out by an army deployed in planned positions, and not by many disparate individuals pursuing their own personal goals. One way or another, intuition usually tells us that without conscious planning and control, nothing good will come of it, only chaos. Evolutionary biology will forever remain incomprehensible to those whose minds cannot free themselves from the captivity of these intuitive sensations.

The task we set ourselves when starting to work on the book was to try, based on new scientific data, to understand how Darwinian selection works. Why And How From the chaos of random hereditary changes, something new, useful, beautiful, harmonious and complex is born. Here it is important to abandon general reasoning and conversations at the level of “common sense” - they will convince few people now. We must take a closer look at the facts, details, examples, from which the mechanism of the great historical engine - natural selection - will appear in all its complexity and logic.

How new genes, new traits, new adaptation3
Adaptation- a trait that has adaptive significance. For example, protective (camouflage) coloration is an adaptation that helps an animal quietly sneak up on prey or hide from a predator.

New species, new types? What is the general biological meaning of these words: new, useful, beautiful, harmonious, complex? After all, all these terms in biology have special shades of meaning. What is considered a real “innovation” - is it the acquisition of a new mutation, a new appearance, a new gene, a new function or a new place of residence? Try to answer such questions on the fly... What is “beauty” from the point of view of a bee or a colored lake fish? It’s probably not the same as for the jury of the Miss World competition. To understand the structure of living nature, in order to understand the meaning of all its components and interrelations, it is necessary first of all to understand their evolutionary context. We want to see evolution up close. We want to unscrew the evolutionary mechanism into all its cogs and gears, study them, understand how they connect, and then screw them back together and make sure that it is still ticking. But this work will give us an idea of ​​the whole device - if it ticks, then we understand its mechanics.

In The Birth of Complexity, our first book in evolutionary biology, the emphasis was on “challenging dogma.” Indeed, many conclusions that seemed absolute 50 years ago now have to be revised. It became clear that life is more complicated than it seemed just recently. In the natural sciences in general, and in biology in particular, there is a problem with absolute truths. There are exceptions to any rule. On the other hand, The Birth of Complexity may create a kind of “dogma-refuting bias” in the minds of readers. One might think that too much of what the classics of evolutionary biology wrote about has not stood the test of time.

So, correcting this imbalance is another task of the book you are holding in your hands. After all, in fact, classical ideas are not so much refuted by new discoveries as they are concretized, refined and developed. Thus, paleontologists can correct the systematic position of trilobites as much as they like, bringing them closer to crustaceans, then to arachnids, or separating them into a separate subtype - it does not at all follow from this that our knowledge about trilobites is unreliable or that science is marking time, lost in conjectures, – on the contrary, these processes reflect an increasingly complete and correct scientists' understanding of this extinct group of animals, and the most fundamental, classical truths remain unshakable and are only strengthened (for example, the belief that trilobites are representatives of arthropods, which means that the last common ancestor of the trilobite and the fly lived later than the last common ancestor of the trilobite and the sparrow ). Classic ideas are often classic because they have been reliably confirmed from many angles. They allow us to develop and modify ideas about the world without any damage to them. This, of course, is the best version of “classical ideas”: sometimes truly outdated dogmas are successfully disguised as them. Both are boring clichés, but what can you do - these are the ones you encounter every now and then in scientific life. One way or another, those classic ideas that will be discussed in the book are classics in the good sense of the word. We will try to support this statement with the latest scientific data.

In recent years, biologists have received a whole body of new data that has allowed them to better understand how the gears of the evolutionary mechanism turn. Miracles are happening right before our eyes. The skeletons of theories are overgrown with the flesh of real facts. Many beautiful hypotheses and models, which until now could not be verified experimentally, have finally been subjected to such testing. Regularities that until now existed only in the imagination of theorists, we can now examine through a microscope. We can use them! “Applied evolutionary biology” is no longer fiction, but reality. The book is dedicated to such discoveries.

We have to look at specific examples of how heredity, variability, selection, competition, isolation, drift and other components of the great natural machine work, tirelessly creating new types of living beings.

We hope that the reader already has minimal knowledge of biology - and if he does not, he will be able to look up the missing information himself in available sources, for example, in a textbook or on Wikipedia. 4
Or in our previous book “The Birth of Complexity”: there is a glossary of terms.

>. In the end, you can't each a popular science book to repeat the same information from the school curriculum. It’s a pity for the time, paper and those readers for whom this will not be the first biological book they have picked up. Therefore, we will not retell in detail for the hundredth time what DNA replication and a cell membrane are, but will get straight to the point.

A few terms you can't do without

The hereditary information contained in DNA is heterogeneous and written in several different “languages.” Best learned language protein-coding regions of DNA5
We will highlight special terms in bold the first time they are mentioned. These terms will be explained directly in the text or in footnotes.

The sequence of nucleotides in such a region represents instructions for the synthesis of a protein molecule, written using genetic code– a system of correspondences common to all living things between certain triplets of DNA nucleotides (triplets, or codons) and the amino acids that make up the protein. For example, the triplet of nucleotides AAA codes for the amino acid lysine, and CGG for arginine.

To synthesize a protein based on such instructions, information must first be rewritten from DNA to RNA - a molecule that differs from DNA in some details: for example, instead of the nucleotide T (thymidine), RNA uses U (uridine). Rewriting information from DNA to RNA (RNA synthesis on a DNA template) is called transcription. A gene can be transcribed frequently, and then the cell will produce many molecules of this protein, or rarely, and then there will be little protein. It's called expression level gene. The expression level is controlled by special regulatory proteins.

The resulting RNA molecule is then used to synthesize protein. The molecular “machine” for protein synthesis based on instructions written in RNA is called ribosome, and the process of protein synthesis itself is broadcast.

Chapter 1. Heredity: where is the world heading?

DNA is the main “gear” of heredity

There have been several critical moments in the history of biology over the past century and a half when Darwin's model was seriously tested - and would certainly have been refuted if it had been wrong. One of them came in the early 1950s, when several talented biologists and chemists 6
James Watson, Francis Crick, Rosalind Franklin, Maurice Wilkins.

They began to seriously decipher the structure of DNA, the mysterious “substance of heredity.” The case, as we know, culminated with the discovery of the famous double helix in 1953. According to legend, Francis Crick exclaimed in joy: “We have discovered the main secret of life!” What did he mean? Didn't you get excited?

Four most important events in the history of biology

1. 1859 The theory of evolution by natural selection.

2. 1900–1910s. Classical genetics, chromosomal theory of heredity. At first, it seemed to many that genetics contradicted Darwinism: after all, Darwin prioritized small, smooth changes, and early geneticists worked with “gross and visible” discrete changes - mutations with a strong effect.

3. 1930s. The apparent contradiction was successfully resolved. A genetic, or synthetic, theory of evolution (GTE or STE) has emerged - a triumphant unification of Darwinism with genetics.

4. 1950–1960s - discovery of the material nature of heredity and variability: DNA structure, replication, transcription, translation, genetic code.

To understand why there was so much fuss about the discovery of the DNA double helix, we need just a little historical context. There was already genetics. It was already known that hereditary information consists of discrete units - genes, which are located in chromosomes and are located there linearly, one after the other. Chromosomes contain proteins and DNA. At first it seemed quite possible that hereditary information was recorded in proteins. After all, proteins consist of 20 different amino acids, and DNA consists of only four nucleotides. DNA seemed too “uniform” a molecule. There is logic in this: it would seem easier to write a long text with a 20-letter alphabet than with a four-letter one 7
An example of “general reasoning,” which in biology sometimes works and sometimes doesn’t, so should be used with caution.

But then it turned out that hereditary information is still stored in DNA. This was shown in experiments with labeled viruses. It turned out that in order for a bacteriophage virus to multiply in a bacterial cell, it is necessary and sufficient for viral DNA to enter the cell. Viral proteins do not need to penetrate bacteria. All the information necessary for their production is in the DNA. This means that DNA is the “substance of heredity.”

Two great questions remained.

In what way and in what language is hereditary information written in the DNA molecule?

How does a cell copy this information before each division?

The answer to the first question has already suggested itself. It was known that DNA is a long molecule, a polymer consisting of four types of nucleotides. They, as you, of course, remember, are designated by the letters A, G, T, C. Hereditary information is somehow encoded in a sequence of nucleotides - written down in this four-letter alphabet.

The second question seemed more mysterious. Here it is necessary to clarify that Life (not any life, but one whose development is adequately described by the genetic theory of evolution) may not be based on every polymer in which something is encoded. This must be a molecule, firstly, capable of reproduction, and secondly, possessing hereditary variability. Proteins, by the way, do not have these properties (with one interesting exception, which we will talk about soon).

Let's start with reproduction. The molecule underlying life must contain instructions for making copies of itself. Chemically speaking, it must be a molecule capable of catalyzing the synthesis of its copies. Without this, living beings simply cannot reproduce. Hereditary information must be copied so that parents can pass it on to their offspring.

The ability of a “heredity molecule” to reproduce is a sufficient condition for Life based on such a molecule to exist. For example, if it were Life artificially created by someone, not subject to evolutionary changes, then it would be enough for the “molecule of heredity” to simply be able to reproduce.

This, however, is not enough for Life capable of evolution based on the Darwinian mechanism. If the GTE adequately describes reality, then the “molecule of heredity” must have one more property – hereditary variability. This means that not only the “general characteristics” of the parent molecule must be transmitted to descendant molecules, but also its individual, particular characteristics, which from time to time undergo small random changes. These changes must also be inherited.

Systems capable of reproducing and possessing hereditary variability are called replicators.

Reproduction without heredity

An example of reproduction without hereditary variability is the Butlerov autocatalytic reaction. During this reaction, formaldehyde (CH 2 O) is converted into a complex mixture of different sugars, and the catalyst for the reaction is the very sugars that are formed in it. This is why the reaction is autocatalytic: it is catalyzed by its own products.

This response can be described in terms of reproduction, variability and heredity. Sugar molecules catalyze the synthesis of other sugar molecules: they can be said to reproduce using formaldehyde as “food”. They also have variability, because the end result is a mixture different sugars But here hereditary this variability is not because the composition of the resulting mixture is virtually independent of which sugars catalyzed the reaction. Now, if, say, ribose selectively catalyzed the synthesis of ribose, but sometimes glucose molecules were synthesized “by mistake” and began to selectively catalyze the synthesis of other glucose molecules, then we could say that there is hereditary variability in the system.

Richard Dawkins gives another example in his books: combustion. We can light a match, use its fire to light a candle, and use the fire of a candle to light something else. The lights seem to reproduce, but the individual characteristics of a particular light - for example, its color - do not depend on the properties of the parent light. They depend only on the “environment”, for example, on the composition of the combustible material. Like sugar in Butlerov’s reaction, fire transmits by inheritance only its “general characteristics”, but not individual characteristics. In such a system there is reproduction and variability, but there is no heredity in the narrow sense. Such systems could form the basis of some living beings, but only artificial ones, created ready-made, as if robots assembled other robots from scrap materials. Such creatures would not be able to independently develop and become more complex, evolving “according to Darwin.”

Long before deciphering the structure of DNA, geneticists already knew for sure that mutations(randomly occurring changes in genetic material) are also copied and inherited. This means that the system for copying hereditary information does not depend on what information is copied with its help. This is a universal system: not a stamp that produces identical texts over and over again, but something like a photocopier that reproduces any text regardless of its content. If a change accidentally occurs in the text, then copies taken from the changed text will also contain this change.

Thus, the DNA molecule must have an amazing property - it must be capable of self-copying, and all the nucleotides in the copy must be in the same sequence as in the original. If a mutation occurs, then copies of the mutant molecule must also contain this mutation. Only such a molecule is a molecule- replicator- may lie at the basis of earthly life. This followed from Darwin's theory. This followed from genetic data. And this was brilliantly confirmed by the discovery of Watson and Crick.

The main thing they discovered was the principle specific nucleotide pairing, or complementarity. In the DNA double helix, adenine (A) is always connected to thymine (T), and guanine (G) is always connected to cytosine (C). Complementary nucleotides match each other in size (pairs A - T and G - C are the same size, and therefore the DNA helix is ​​smooth and strong), as well as in the location of positively and negatively charged sites. Due to the latter circumstance, two hydrogen bonds are formed between A and T, and in pairs G – C there are three such bonds.

At the end of their landmark paper describing the structure of DNA, the authors remarked: “It has not escaped our attention that the specific mating postulated by us directly points to a possible mechanism for copying genetic material” ( Watson, Crick, 1953). This phrase is considered one of the two most modest statements in the history of biology. 8
We talked about the first in the book “Human Evolution”. This is a phrase dropped by Darwin in the final chapter of The Origin of Species, that his theory would shed light on the origin of man.

Of course, it did not escape their attention that they discovered the main secret of life!

So, the discovery was that the DNA molecule was designed in such a way that it was very easy to copy. To do this, it is enough to unravel the double helix into two strands, and then add a second strand to each of them in accordance with the principle of complementarity. This means that the DNA molecule encodes itself. Each strand of the double helix encodes the second strand, precisely defining its structure. DNA also encodes many other things - the entire structure of the organism, ultimately, but first of all it encodes itself. She controls the production of her copies herself. Copy mechanism ( replication) DNA is embedded directly in its structure. Naturally, any mutation error, any nucleotide substitution with this copying method will be inherited by the daughter DNA molecules.

The molecule of heredity turned out to be exactly what it should have been according to the predictions of the GTE. Scientists love it when everything fits together, when theoretical constructions are confirmed by facts. So Crick had a worthy reason for making loud statements about the “secret of life.”

Another important conclusion is that the structure of the DNA molecule directly implies the inevitability of Darwinian evolution. Living beings possessing such a molecule of heredity simply cannot help but evolve “according to Darwin.” No copying system can be absolutely accurate. From time to time, failures and errors, i.e. mutations, will definitely occur. They will be inherited. Since DNA determines the lion's share of the hereditary properties of the body (let's leave a little for all kinds of epigenetics and maternal effects 9
We talked about these phenomena in the book “The Birth of Complexity.”

), some mutations will certainly affect the efficiency of reproduction - both the DNA molecules themselves and the organisms whose structure is controlled by them. Thus, DNA ensures the fulfillment of a set of conditions necessary and sufficient for Darwinian evolution: 1) reproduction; 2) hereditary variability; 3) the influence of the second on the effectiveness of the first 10
This set of conditions can be formulated in another way: “hereditary variability and differential reproduction” or “heredity, variability and selection.”

Selection is a game by the rules

Using the random search method (random walks) it is completely unrealistic to find one specific point in the vast “protein universe” that corresponds to the optimal performance of a given function by a protein. Fortunately, such optimums, as a rule, are not points, but vast areas. And most importantly, the search for the optimum in the course of evolution is not carried out by the method of random walks. It uses the method of directed selection. Directionality occurs due to the fact that random deviations in the “wrong” direction, which worsen the performance of the protein, are rejected, canceled and forgotten, while random deviations in the “right” direction are remembered and stored.

If the original sequence is already at the foot of some hill on the fitness landscape - where there is already at least a slight slope (this means that the protein, at least to a minimal extent, but already performs some function), selection drives the sequence to the top of the mountain with amazing efficiency. This can be demonstrated using a computer program first described by Richard Dawkins in The Blind Watchmaker. Let the initial sequence be an arbitrary set of letters, for example, this:

The program will reproduce this sequence in a thousand copies, introducing random mutations into it at a given frequency. For example, let each letter in each copy mutate with a probability of 0.05, that is, be replaced by another random letter. As a result, every twentieth (on average!) letter of each child will differ from the parent. Let's set the optimum to which we need to strive - some meaningful phrase. Let's first try to reach the optimum without the help of selection, using the method of random walks. To do this, out of 1000 descendants, choose one at random and again multiply into 1000 copies with mutations. And again, and again.

This is what we got. On the right are the generation number and the degree of difference from the target sequence, i.e. the distance from the optimum:

Useless! For 100 generations - not the slightest progress, no approach to the optimum. The phrase remained as meaningless as it was. A random walk in the sequence space that we have now simulated gives virtually no chance of finding a specific point in the protein universe.

No, of course, if we had an infinite supply of time, someday we would still stumble upon the optimum. When, that is the question. We used 26 letters of the Latin alphabet and a space, for a total of 27 characters. The length of the phrase is 60 characters. The optimum is one combination out of 27 60 (≈7.6 × 10 85) possible. There are more options than there are atoms in the universe. It will take us approximately this many generations to accidentally stumble upon the right point.

The well-known arguments about a hurricane flying over a landfill, which will never collect a Boeing 747 from the garbage, and about a monkey, who, mindlessly knocking on the keyboard, will never write “War and Peace,” are quite applicable to this method of searching. In this way, he really won’t write. Fortunately for us, evolution does not proceed this way at all. Or, to be completely precise, Not only in this manner.

After all, it was not in vain that we modeled random walks - they also have an analogue among evolutionary processes. That's just how it goes neutral evolution. This is how sequences (amino acid or nucleotide) change, on which the fitness of the organism does not depend and which are therefore not under the influence of selection.

By the way, even though the wanderings are random, certain patterns can be noticed in this case. Note that the evolving sequence gradually became less and less similar to the original one. The phrase from generation No. 1 differs from the original one in only three characters, in generation No. 2 we already see seven differences, in generation No. 4–13, in the fifth generation - 16 differences. By the 50th generation, no resemblance to the original sequence remained. But during the first 20–25 generations, the similarity remained, constantly decreasing. Therefore, based on the degree of this similarity, we could approximately determine the generation number to which this phrase belongs. We could, by comparing this phrase with the original one and knowing the mutation rate, roughly estimate how many generations separate these two phrases! This principle is the basis of the remarkable molecular clock method, which we will introduce in the next section. For now, let's get back to the selection.

Let's change our program a little. Let us now assume that not any of the 1000 sequences are selected at random for reproduction, but the best one - the one most similar to the “optimal” one. Let's leave everything else as it was. We run the program and get the following:

Well, that's a completely different matter! Under the influence of selection, our sequence confidently moved towards the optimum - climbed to the peak of the fitness landscape - and reached it in the 89th generation, for which we congratulate it.

The main lesson from these exercises is that evolution under selection is very different from trying to type “War and Peace” by randomly hitting keys. Selection is a powerful organizing force that gives direction to evolution, forming a natural, orderly result from the chaos of random mutations.

But excuse me, have we not retreated from reality by arbitrarily setting the optimal sequence - the phrase to which we had to strive? Doesn't this sound like "divine intervention"?

Not at all. For any useful function performed by a protein under given conditions (temperature, environmental composition, etc.), there is indeed one or more optimal amino acid sequences that will perform this function best. Perhaps a real protein with such a sequence does not yet exist in nature - evolution did not have time to create it - but the ideal sequence still exists, just as the chemical element carbon potentially existed with all its properties even before the first carbon atoms began to be synthesized in the depths of stars that burst into flames in the young Universe. Selection will move the evolving sequence towards this ideal, regardless of whether such proteins already exist in the world or have yet to appear.

Where we really went wrong was that we implicitly introduced into the model several assumptions about the shape of the fitness landscape that greatly simplified reality. We have assumed that the landscape is one big mountain with smooth slopes and a single peak, and any random sequence with which evolution begins is already on the slope of this mountain, so that moving “up” (toward the ideal) increases its fitness. Being on a slope means that the original sequence, at least a little, even very poorly, still copes with this function.

The model considered is valid only for a situation where selection already has something to “catch on to”, when the evolving sequence is already good for something.

How do evolving sequences manage to move from one mountain to another and from slope to slope? In other words, how can a protein that has already adapted to perform some function - having begun to climb the slope of one of the mountains - acquire another function, i.e., move to another mountain? How will he overcome the lowland that separates them?

This is an important question, and we will return to it later. For now, let's limit ourselves to a few comments.

Firstly, indeed, the higher the squirrel climbed on one slope, the less chance it has of moving to another. Deep depressions between mountain ranges corresponding to the main groups of protein functions - the so-called superfamilies proteins are usually impassable. It is extremely rare to overcome the abyss with one desperate leap - macromutation. In the vast majority of cases, such jumps in mountainous areas end with obvious results. But there are still successful landings. And then, to everyone’s surprise, some enzyme, which has been converting one carbohydrate into another for billions of years, suddenly turns into crystallin - the protein of the eye lens, and the digestive enzyme trypsin - into an antifreeze protein that protects the blood of Antarctic fish from freezing.

But these are exceptions. Typically, the evolutionary movement of a large and complex modern protein is limited to one mountain range - one group of related functions. At high altitudes, the fitness landscape of proteins is highly disjointed and fragmented, so that it is almost impossible to get from one massif to another. But at low altitudes, at the foot of majestic mountain ranges, there is a hilly area, over which it was much easier for simple and primitive ancient squirrels to move. Among the artificially synthesized short protein molecules with an arbitrary sequence of amino acids, one can find molecules that perform, albeit with low efficiency, various functions performed by natural proteins in the cell. This means that, moving through these lowlands, you can stumble upon the foothills of some mountain range, even by random wandering!

Most likely, the main functions of proteins corresponding to protein superfamilies were “found” at the very beginning of the evolutionary formation of the genetic code and protein synthesis in ancient RNA organisms. Then these functions were endlessly improved and divided into many variations. The evolving sequences climbed higher and higher along the slopes of “their” mountain ranges, scattering along forks and spurs, and the higher they rose, the less chance they had of moving from the once chosen mountain system to some other.

Apparently, only for the simplest and shortest (but still useful) protein molecules there is a real probability of arising “out of nothing,” “out of the blue” - from a random combination of nucleotides that accidentally encoded a certain sequence of amino acids. This method of forming new proteins has exhausted itself in the RNA world. Since then, new proteins appear only from old ones - not from scratch, but by modifying what is already there. This is one of the main rules of evolution. Almost everything new is a remake of the old.

Neutral mutations and genetic drift - movement without rules

The fitness landscape is a vivid and useful image, but like any model, it is imperfect. Many aspects of the evolutionary process are difficult or impossible to reflect with its help. The real landscape of fitness is changeable (as are real mountain systems). If one protein in a cell has changed, this will, at least slightly, change the properties of the cell, its behavior and internal environment - and therefore the “requirements” imposed by selection on other proteins. Their fitness landscapes will be slightly different. A change in one species in a community will inevitably affect the selection factors acting on other species, etc.

In addition, it is difficult to imagine a landscape that would accurately reflect the real ratio of harmful, beneficial and neutral mutations. As a rule, most mutations are neutral - at least in higher organisms, eukaryotes, whose genomes have many regions that weakly affect fitness. Harmful mutations are in second place in number, and beneficial mutations are in last place. It is not easy to imagine a slope so shaped that from each point there are more different paths parallel to the horizon than there are paths leading up or down. But the real slopes of real fitness landscapes are exactly like that.

Neutral mutations are, by definition, those mutations that do not affect fitness, or, what is the same, are not subject to selection. It's time for us to take a closer look at this class of mutations, the most common in nature. Let's forget about the fitness landscape for a moment and turn to another model that describes the processes that occur with genetic variants (alleles) in a population.

Let's say we have a small population of mumziks of 40 individuals. For simplicity, let us assume that mumziki haploid, i.e. they have a single set of chromosomes - one single copy of the genome, and not two, like you and me, diploid organisms. Having reached the age of one year, each myumzik gives birth to several cubs, and then immediately dies of old age. All the cubs are the same, but only 40 of them can survive - more mumziks simply cannot fit in the old iron barrel at the bottom of the pond where the studied population lives. The mortality of cubs is random - exactly 40 randomly selected lucky ones will become adults and produce offspring in a year.

Since mortality is random, the fitness of mumsiks is determined only by their fertility, i.e., the number of cubs. Fertility depends on the genotype. Let's say the mumzik has one gene that affects fertility. Let's denote it by the letter A. By the way, don't laugh: this model is quite suitable for studying some laws of population genetics.

Once upon a time, all myumziks had only one variant (allele) of the A gene. Let's denote it A 1. But then a mutation occurred in one individual, and as a result, a second variant of the gene appeared - allele A 2. Let us assume (again for simplicity) that in our population, when we began to study it, in half of the myumzians gene A was represented by the first variant, in the other half - by the second. Thus, the frequency of the A 2 allele is 0.5 (q 2 = 0.5) and the frequency of the A 1 allele is the same (q 1 = 0.5).

We need to answer the question: how will the frequency of the A 2 allele change over time if the mutation that led to its occurrence was neutral?

Since the mutation was neutral, it means that the fertility of the owners of both alleles is the same. Let's say they all give birth to exactly ten babies. Of course, the offspring inherits the parental allele of gene A.

It might seem that since the alleles have the same fitness, then there will remain an equal number of them. This is the wrong answer. The figure shows the result of four runs of our model. We see that the A2 frequency fluctuated chaotically in all four cases (in scientific terms, such fluctuations are precisely called random walks). The wandering continues until the allele frequency reaches either the upper “point of no return” (q 2 = 1, the allele frequency has reached 100%, the allele fixed in the gene pool), or to the lower one (q 2 = 0, frequency dropped to zero, allele eliminated from the gene pool).

This always happens. If the allele is neutral, its frequency will “randomly wander” between zero and one until it hits either an upper or lower limit. Sooner or later, the neutral allele will either be fixed (reach frequency 1) or eliminated - disappear from the gene pool. There is no third. Thus, although the wanderings are random, their outcome is predictable. We know for sure that the matter will end either in fixation or elimination. The larger the population, the longer you will have to wait on average for the outcome, but it is still inevitable.

Here is the time to say that random, fitness-independent changes in allele frequencies are called genetic drift. All neutral mutations are under the power of drift (and in small populations, partly also harmful and beneficial ones, but more on that later).

Is it possible to calculate the probability that a neutral allele will eventually become fixed rather than eliminated? Yes, it's simple. In our example, alleles A 1 and A 2 were initially in equal position because both had a frequency of 0.5. Obviously, in such a situation, their chances of fixation should be the same and equal to 0.5. In half of the cases A 1 is fixed, A 2 is eliminated, in half - vice versa.

Well, what if there are not two competing neutral alleles, but, say, four and their initial frequencies are also the same (0.25)? In this case, the matter will end with the fixation of one of the alleles and the elimination of the other three, and for each allele the probability of fixation is 0.25. Thus, it is obvious that the probability of fixation of a neutral mutation in the future is simply equal to its frequency at the moment: P fix = q.

If you, dear readers, are not yet tired of this primitive mathematics, then let me introduce you to two more simple, interesting and useful formulas.

How many neutral mutations will be recorded in the population in each generation? (This means mutations not in the same, but in different genes.) If we can calculate this, we will get the most wonderful tool - a molecular clock. Then we will be able to determine by the number of neutral genetic differences when the last common ancestor of the organisms being compared lived.

The derivation of this formula is a true masterpiece of “biological mathematics”. Judge for yourself. Let us first determine how many new mutations appear in the population in each generation. For simplicity, we will assume that the vast majority of mutations are neutral (this is not far from the truth). The answer is obvious: U × N, where U is the rate of mutagenesis (the average number of new mutations in each newborn individual), N is the population size. U can be determined simply by comparing the genomes of children and parents.

Now we need to understand which part of these U × N newly appeared mutations will eventually be fixed. This will be the desired value - the rate of fixation of neutral mutations in the gene pool of the population in one generation. Let's denote it by the letter V.

We already know that the probability of fixing a mutation is equal to its frequency: P fix = q. What is the frequency of a newly appeared mutation? It's quite simple. Since the mutation has just appeared, only one individual has it so far. Therefore, its frequency is 1/N. That's all, actually. We multiply the number of new mutations by the probability of fixation of each of them (i.e., by the proportion of mutations that will eventually be fixed) and we get the answer: V = U × N × 1/N. Amazing! The population magically shrinks and the N value disappears from the equation. We come to the conclusion that the rate of fixation of neutral mutations does not depend on the number and is simply equal to the rate of mutagenesis: V = U. I don’t know about you, dear readers, but we, biologists, are delighted with such beauty. We are one step away from the molecular clock. After two species descended from a common ancestor split, neutral mutations independently accumulate in their gene pools. Over time t (measured in generations), the first species will accumulate V × t = U × t neutral mutations, and the second species will accumulate the same amount. By joint efforts they will accumulate 2U × t neutral differences from each other. Knowing the rate of mutagenesis U and counting the number of differences between the genomes of the compared species (we denote it by the letter D), we determine the lifespan of their last common ancestor: t = D/2U. This is the famous molecular clock.

If the rate of mutagenesis and the rate of change of generations were the same in all living beings, everything would be quite simple. But, of course, they are not the same, so corrections have to be made to the calculations. And there is also the problem of saturation: at some point - usually after tens, and more often hundreds of millions of years - the genes of species that once separated become so “overwhelmed” with neutral differences that the value of D stops growing, although neutral mutations continue to be recorded. Below we will see an example of a study that showed how the level of similarity between “randomly walking” sequences approached the minimum possible and could no longer decrease.

Fortunately, different regions of the genome accumulate neutral changes at very different rates. Sections of DNA that change quickly are used to date recent events, while sections that change slowly are good for dating events from ancient times.

Drift and selection: who wins?

Genetic drift reigns over neutral mutations (alleles), selection reigns over beneficial and harmful ones. Selection that increases the frequency of beneficial mutations is called positive. Selection that rejects harmful mutations is negative, or cleansing.

However, everything is so good and simple only in large populations. In small ones the situation is more complicated, because selection and drift - the two main driving forces of evolution - begin to compete with each other for control over weakly harmful and weakly beneficial mutations.

Let's see how this happens. Let's use the same model with mums for this. Let us only change the nature of the mutation that led to the appearance of the A 2 allele. Until now, we believed that the mutation was neutral. Since it was neutral, those with the A 1 and A 2 alleles had the same fertility. Let's now assume that the mutation was beneficial, that it increased the fecundity of the mumsiks by 5%. This can be modeled as follows: let the myumziki with genotype A 1 give birth to 20 cubs, and the myumziki with genotype A 2 - 21. Let the initial frequency of the A 2 allele be still equal to 0.5. Only now we will consider populations with different numbers (N).

The figure (pp. 54–55) shows how the frequency of A 2 will change if its carriers have a five percent adaptive advantage. We see that in a large population (N = 5000) the frequency of A 2 is steadily increasing, approaching unity. This is how selection works under ideal conditions, that is, in large populations where the influence of drift on beneficial and harmful alleles is negligible. The shape of the curve is quite regular, and this suggests that it can be described by some kind of mathematical formula. This is true, but we will not derive the formula so as not to bore the readers (and mathematics lovers can do this on their own).

Before us, by the way, is the most important evolutionary process - allelic substitution, i.e., displacement by a more fit allele of a less fit one. The process is not going very quickly. In a large population, say one of a million individuals, it takes about 560 generations for a new beneficial mutation to become established that provides a 5 percent adaptive advantage. But 5% is a serious advantage. Such mutations are rare. It will take 2800 generations to wait until a mutation with a 1% advantage is fixed! However, in a large population, selection senses even the smallest differences in fitness. This ensures effective (albeit slow) fixation of weakly beneficial mutations and elimination of weakly harmful ones.

In the middle graph we see what fate awaits the same beneficial mutation that increases fitness by 5% in a smaller population (N = 200). The frequency of the beneficial allele in this case also increased and eventually reached 1 (the mutation was fixed), but the path was difficult and tortuous. Perhaps our mutation might not have been lucky; its chances of being fixed were not one hundred percent.

Finally, in this graph we see what happens to the exact same beneficial allele in a very tiny population (N = 30). The figure shows the results of two runs of the model. In one case the mutation was fixed, in the other it was eliminated. Isn't it true that the picture looks like the result of drift rather than selection?

The way it is. This is the main lesson we can learn from our experiments. The smaller the population, the weaker the power of selection in it and the more powerful the drift. In small populations, weakly beneficial and weakly harmful mutations begin to behave virtually like neutral ones. Their frequencies "randomly wander" until they hit an upper or lower threshold. In small populations where drift reigns, small differences in fitness become invisible to selection. Therefore, a weakly harmful mutation can easily be fixed, and a weakly beneficial one can be eliminated.

Is it good or bad? In most cases, of course, it’s bad. The loss of weakly beneficial mutations prevents a small population from adapting to changing conditions. The uncontrolled accumulation of mildly harmful mutations can even bring it to the brink of extinction. This, by the way, is what biologists see as one of the reasons why large animals die out on average more often than small ones. Large animals such as rhinoceroses or elephants may not have populations as large as mice or insects. This reduces the adaptability of large animals.

But this medal also has a flip side. Small populations have a greater chance of escaping the “local optimum trap,” that is, sliding down from a low peak in the fitness landscape and climbing to another, higher one. After all, selection drives organisms upward and only upward. If the population is large and the slightest difference in fitness is "noticeable" to selection, downhill descent becomes impossible. Once a large population climbs a lonely hill, it never leaves it. As for drift, it guides organisms across the fitness landscape in a chaotic manner, oblivious to ups and downs. If the population is small and drift is strong, organisms sometimes have a chance to move not only up, but also a little down (and to the side). Having descended into the hollow, organisms can “discover” that there is another, more promising ascent from here. Unless, of course, they die out before they stumble upon it.

Chapter 7. Transitional forms

The problem of transitional forms worried Darwin, alarmed his supporters and invariably delighted his opponents. Looking ahead, let's say that now for expert biologists this problem is seen from a completely different angle, although it still alarms poorly informed adherents of evolution and, as before, excites its opponents. In this chapter we will look at why the attitude of biologists towards transitional forms has changed, and we will leave the emotions of non-specialists out of the equation.

Transitional forms are those that line up in a series of gradual transitions from ancestors to descendants. If it is known what appearance a distant ancestor had, then any sets of intermediate characteristics between him and his descendants will be transitional. Based on the phylogenetic tree, it is possible to predict which intermediate forms may have existed (and therefore can be found) and which may not. According to the scientific method, predictions that come true confirm the theory. For example, knowing the structure of dinosaurs and birds, one can predict some morphological features of the transitional forms between them. We predict the possibility of finding the remains of animals similar to reptiles, but with feathers, or the remains of animals similar to birds, but with teeth and a long tail. However, we predict that transitional forms between birds and mammals, such as mammal fossils with feathers or bird-like fossils with mammal-like middle ear bones, will not be found. In other words, the evolutionary tree will show us where and what to look for, and what cannot be and what is not worth looking for.

In addition, ancestral forms with mixed characteristics of daughter groups are considered transitional. If the daughter groups descend directly from some common ancestor, then some (not necessarily all) primitive characteristics will be collected in its appearance. Its other characteristics may, on the contrary, be as advanced as those of one or another descendant. This form can also be called transitional - it combines the characteristics of several lines in their primitive state. However, such a fossil will most likely, on closer inspection, turn out to be not the direct ancestor of the daughter groups, but one of the early branching and little modified descendants of this ancestor (because the likelihood of finding one's direct ancestor in a very incomplete fossil record is low). This form can serve as a good portrait approximation of the common ancestor.

Species with an intermediate state of a complex character are also called transitional (if we consider as the final state what is observed in modern animals; from the point of view of our descendants, the current form will be intermediate in comparison with their updated, changed world).

Darwin outlined the essence of the problem of transitional forms as follows. If evolution proceeds gradually through the selection of ever more perfect forms, then it would seem that we should everywhere see rows of endless smooth transitions between forms. In reality, we more often see discrete species that either cannot interbreed with each other at all, or do so with difficulty and reluctantly. In the extreme case, if living “improved” species, having won competition, replaced their less adapted ancestors, these latter should be found in the fossil record. Even though modern species turned out to be luckier and more resilient, and even though they have now settled everywhere, but once upon a time their ancestors were winners and lived as free kings on their territory. This means that their remains should be buried in large numbers, turn into fossils over time (petrify) and become the property of paleontologists. But Darwin had few examples of fossil transitional forms in the mid-nineteenth century.

Another part of the problem is the gradual formation of a complex trait. It would seem that the eye sees only because all its parts are perfectly fitted to each other (in fact, not everything is perfect, but these are minor things). The lungs breathe in and out because the entire chest cavity is designed like a perfect vacuum pump. The design of the ear is light, elegant and mathematically technical, and therefore the ear serves us, allowing us to capture the smallest shades of vocal emotions, navigate in space and build sound harmonies. Bird feather - light and durable flight surface; Without a set of such perfect, orderly overlapping surfaces, a bird will not fly...

Indeed, acquiring a clearly seeing eye seems to be an extremely difficult matter. After all, it had to be assembled gradually, having at the very beginning only a set of cells that capture light, and in the end get an accurate optical device. And at every stage of evolution, this proto-eye had to serve the animal usefully, so that evolution would not abandon the matter halfway.

Imagining this path in your mind, you can’t help but wonder why animals would need all these organs in a half-assembled form. Why do we need a half-eye that registers only fuzzy shadows, a feather unsuitable for flight, an ear that can’t hear well, respiratory bubbles that can’t breathe properly? Is it worth evolution's effort and time to make these adaptations if the end result is unknown? And if indeed the entire path from a photosensitive pigment spot to a perfect eye has been traveled, then there should be transitional forms with semi-sighted under-eyes, flightless feathers, poorly functioning lungs and similar imperfect organs. Did such forms exist?

In “The Origin of Species,” where this question is identified, a clear theoretical answer is given to it - yes, they should have existed and did exist! - and examples are given. Now we know many, many more excellent examples - transitional forms with seemingly “unfinished” morphological structures, which nevertheless diligently served their owners.

A little about the eyes

In fact, it is not so difficult to make eyes if you have light-sensitive proteins (and single-celled animals already have them) and photoreceptor cells that produce these proteins (which already appeared in the most ancient animals). Evolution has coped with this task more than once, each time inventing a special organ and constructing it gradually from available materials (tissues and cells) according to the needs of different animals, but never looking ahead and “caring” exclusively about small momentary improvements.

The eyes of the bivalve mollusk Swift's scallop, box jellyfish, diving fish, trilobite - all these are completely different organs of vision that help animals capture light and distinguish objects in conditions characteristic of the animal. These are examples of the parallel appearance of a complex organ - the eye.

Big-eyed creature on the color insert- Swift comb ( Swiftopecten swifti), a bivalve mollusk 10–12 cm in size. Tentacles grow along the edge of its mantle - organs of touch - and numerous small eyes - organs of vision. The scallop's eyes, of course, are completely different from ours and are not connected to the brain (it simply does not exist, there are nerve ganglia), but still they are not so simple. They are vesicles of transparent epithelium, the back side of which is lined with a layer of light-sensitive cells, followed by a pigment layer and the so-called mirror. The bubble contains a light-refracting lens - the lens. Due to the reflection of light from the mirror, the eyes shimmer in a wonderful green color. With the help of their eyes, scallops can see only at a short distance. Only when the scallops' worst enemy - the starfish - approaches it almost closely, does it, flapping its doors, make a jump and take flight. These little eyes are always on alert and warn their owner of danger.

Jellyfish (representatives of the coelenterate type, quite simply structured animals) can also develop eyes. Thus, box jellyfish - a group with a more or less square dome - acquired vision during evolution. The eyes of box jellyfish are very unusual. They sit on special outgrowths - ropalia; box jellyfish have only four of them, according to the number of sides of the dome. Each rhopalia has six eyes - four simple and two more complex. They are designed according to the scheme of any complex eye: a light-sensitive layer, a pigment layer, a cornea and a lens. One of the six eyes of the rhopalia also has a diaphragm. A change in illumination leads to its contraction or expansion: this is how the eye responds to a light signal. These eyes are designed to see only large objects. The jellyfish simply does not notice any unnecessary little things. In box jellyfish, the signal from the eyes goes directly through sensory neurons to the motor neurons of the dome and tentacles. Therefore, having barely seen a large object, the jellyfish immediately reacts: it accelerates and turns. The box jellyfish does not need an analyzing intermediary - the brain or at least a nerve ganglion: it does not waste time thinking - it sees and acts ( Skogh et al., 2006).

The eyes of trilobites - extinct arthropods that inhabited the seas in large numbers 530-252 million years ago - were completely unique. The trilobite's eye is faceted, like an insect's: it is assembled from many individual lenses. The lenses, like those of other big-eyed arthropods, are transparent. But unlike all animals without exception, they are not made of proteins, but are built from the ultra-transparent mineral calcite. Such an eye is like glass glasses permanently attached to the eye. So the trilobite literally had a stone gaze. Each lens was strictly oriented along the main axis of light refraction. Lenticular lenses focused light onto the retina (recently traces of pigment cells under calcite lenses were found in trilobites), from which nerves emerged. The lenses, however, were highly convex, almost spherical, so image clarity suffered from spherical aberration. But trilobites coped with this problem. The light was focused at a point using a thin figured insert of high-magnesium calcite, which has a different refractive index. Such lenses, with the same exact shaped shape as in the eyes of trilobites, are used in modern telescopes. It can be argued that natural selection solved the problem of constructing modern optical devices much earlier than Descartes and Huygens, who eliminated spherical aberration in telescopes. The evolution of eyes in trilobites is well documented: from simple multiple biconvex lenses to few and almost spherical lenses with a compacted corrective insert. Both eyes helped the animal determine the distance to an object in the water and examine it from afar. The difference, apparently, was the degree of detail of the image and farsightedness.

Thus, the problem of transitional forms breaks down into three questions. The first of them is why in the modern world there are not so many gradual transitions between species, why most species are clearly separated from one another. We have already discussed this question in Chapter 6. The second question can be formulated as follows: why are there few extinct transitional forms in the fossil record that were once replaced by more adaptable competitors? And finally, the last: how can a complex organ gradually form, which seems useful only in its completed form? We will consider the last two questions below.

There are many transitional forms

Until now, sometimes one hears the surprising statement that, supposedly, transitional forms do not exist. This opinion is a simple legacy of the century before last or a deliberate concealment (primarily from oneself) of factual knowledge. In fact, a huge number of transitional forms are known today - after all, a century and a half has passed since the time of Darwin. And all this time, antiquity hunters did not cease to find more and more new forms, including many transitional ones. We no longer have to doubt their existence and, like Darwin, convince opponents that they did exist, explaining their rarity by the incompleteness of evidence of extinct life. A hundred and fifty years ago, about 200 species of fossils were known, today approximately 250 thousand of them have been described. Not only have thousands of new species been discovered, but a whole field of knowledge has emerged and gained strength - taphonomy. Taphonomy is the discipline of the patterns of formation of fossil sites. It explains where, how and why fossil layers were formed. The founder of this science was the outstanding paleontologist and writer I. A. Efremov (1908–1972). The laws of taphonomy help to predict in which places, in which layers and rocks it is necessary to look for new fauna, and where it is not worth wasting any effort.

In some cases, fossil transitional forms have not yet been discovered. For example, there are no traces of the evolution of the ancestors of chimpanzees (taphonomy explains this by the lack of conditions for the formation of fossils in tropical rainforests), there are no reliable traces of the existence of ciliated worms, and this class unites more than 2,500 modern species (they have neither a skeleton nor dense membranes, they, probably too soft and delicate to survive in fossil form). Such gaps in the fossil record are inevitable, but they are certainly no reason to assume that chimpanzees and eyelash worms never evolved and only recently appeared ready-made.

Here are some remarkable examples of transitional forms that have recently been found, despite centuries of skepticism regarding the very possibility of the existence of such creatures.

Halfway to flounder

“With a small crumpled face on the edge...” - this is how one could, following Babel, describe the flounder. Indeed, looking at her, you can’t help but think - she’s really been mangled! Why should she do this, poor thing? On the other hand, it is difficult to imagine a creature more suitable for life on the seabed. It is completely flat, one side has become lighter and pretends to be a belly, the other

darkened and turned his back. The dorsal and anal fins extended along the edges, turning into a uniform flexible frill. And the eyes ended up on one side of the head: one eye, the one that should have remained on the light, pseudo-abdominal side, moved to the other side, on the pseudo-back.

The larvae of all flounder fish are much more similar to normal fish: their eyes, as expected, are located on the sides on different sides of the head. During individual development, one of the eye sockets gradually shifts, first to the upper edge of the head - to the top of the head, and then to the opposite side.

Darwin and his followers assumed that changes in the structure of the skull observed during the growth of the fry reproduced the evolution of the group. Darwin admitted that the “Lamarckian” evolutionary mechanism could be at work in this case: the ancestors of flounders, who got into the habit of lying on their sides, had to constantly squint their eyes, and the lifetime changes developed in the fish as a result of such “training” were passed on to their offspring. At the beginning of the twentieth century, however, it became clear that the results of training cannot be inherited. You can only rely on natural selection. In other words, the ancestors of flounders, whose “lower” eye was slightly shifted towards the top of the head from birth, should have left more offspring than those whose skulls were strictly symmetrical, and whose “lower” eye looked straight into the sand. This assumption seemed implausible to many.

If evolution proceeded through the selection of small hereditary changes, then the eye should have moved from one side to the other in tiny steps. And what advantage could a slight displacement of the eye give to the ancestors of flounders if it still remained on the underside of the head and, therefore, could not see anything except sand? This structure seemed non-adaptive.

The most primitive representatives of flounders were still considered psettods (genus Psettodes with only two types). Their “displaced” eye is located on the upper rib of the head. But still, this eye no longer buries itself in the sand when the fish lies at the bottom. And in that fish, which, as gradual evolution assumed, should have existed before the psettode, the “abdominal” eye would look meaninglessly into the mud.

Therefore, flounders were a headache for evolutionists. Not only did the hypothetical transitional forms seem non-adaptive, but there was no evidence of their existence in the past. There was only a stubborn certainty that they must exist if the theory was correct. And now, in the Eocene deposits (55.8-40.4 million years ago) of Italy and France, these long-awaited primitive flounder-like creatures, long-awaited and suffered through disputes, were still found. They were discovered and described by Matt Friedman from the University of Chicago. The first of the two primitive flounders was previously unknown and, when described, received the name Heteronectes chaneti. Paleontologists already knew the second (this is the genus Amphistium with two types: paradoxum And altum), but Friedman was the first to accurately reconstruct the structure of her skull using X-ray tomography ( Friedman, 2008).

These two fish have the same skull structure that skeptics thought impossible: the skull is sharply asymmetrical, one eye socket has already moved upward, but both eyes are still on opposite sides of the head. And one of them - yes, yes - looks into the sand. This is clearly not an error or the result of post-mortem changes in fossil bones. Among the facts confirming this, we can mention the asymmetrical structure of the frontal bones in Heteronectes. The right frontal bone is large, rectangular in shape, while the left has turned into a narrow curved plate, limiting the upper edge of the orbit that has shifted upward. Genus Amphistium known from many specimens, and the structure of the skull is approximately the same in all. In all cases, the skull is asymmetrical, one of the eye sockets is displaced upward, but remains on its side of the skull.

All found copies Heteronectes And Amphistium at least an order of magnitude larger than the fry of modern flounders at the stage when their eyes are on one side of the head. Even in psettodes, the eye appears on the upper edge of the head already at a body length of 13 mm, whereas the length of the type specimen Heteronectes- 142 mm. Consequently, there can be no question that the found transitional forms represent fry in which the migration of the orbit has not yet completed. This is also evidenced by other signs, including complete ossification of the skull and the absence of age-related changes in the position of the orbit in Amphistium in the process of growth from 103 to 200 mm. Undoubtedly, these fish are adults and their eye sockets are in their final position. By the way, now we can state that Darwin was right and the gradual transformations of the skull in the individual development of modern flounders actually reproduce the course of their evolution.

Not only the structure of the skull, but also other features Heteronectes And Amphistium show that they are the most primitive flounder known. Their skeleton contains archaic features characteristic of the ancestors of flounders - ancient representatives of the group of perciformes. Some of these features are not preserved in any of the modern flatfishes, others are preserved only in psettods (for example, spines in the dorsal and anal fins).

It is curious that among the specimens Amphistium There are both “left” and “right” individuals. The same is observed in psettodas, but in more advanced flounders such variations are extremely rare: each species is usually represented either by only “left” or only “right” fish. This is an example of the strict regulation of ontogenesis. At the beginning of evolutionary development, ontogenesis received more freedom in random variations, but later ontogenesis stabilized and there was no room left for randomness (we will talk more about this general pattern of the evolution of ontogenesis in Chapter 8).

The fossil fish described by Friedman refute the claim that transitional forms between flounder and normal fish with a symmetrical skull are impossible and unviable. However, I would also like to understand how the ancient flounders used their lower eye, which, in theory, should have looked straight into the sand. Friedman notes that a clue can be found in the behavior of some modern flatfishes, which from time to time raise their bodies above the surface of the bottom, resting on the rays of the dorsal and anal fins. Heteronectes And Amphistium they could also “do push-ups” in this way, since their dorsal and anal fins were very powerful. Raising their heads above the sand, these fish, with their displaced lower eye, examined the surface of the bottom in front of them in search of prey.

Heteronectes And Amphistium were not the direct ancestors of modern flatfishes. They are relatives of these ancestors, or more precisely, little-changed descendants of those fish that gave rise to all flounder-like fish, modern and extinct. However, the same can be said about most fossil transitional forms. The likelihood of finding someone’s direct ancestor in the fossil record is very low, and even if they do find one, then try to prove that it is definitely the ancestor himself, and not his second cousin. Therefore, “by default” among biologists it is customary not to consider this possibility at all. As a result, any transitional shape is automatically interpreted as a “side branch” rather than a “trunk section.” But if the branch does not move far from the trunk, as in this case, calling it a “transitional form” is quite correct.

The first flatfishes appeared in the Paleocene (65.655.8 million years ago) and quickly divided into many evolutionary lineages. In the Eocene, when they lived Heteronectes And Amphistium, more advanced flatfishes with eyes on one side of the head already existed. Thus, the amazing fish described by Friedman were already “living fossils” in the Eocene, that is, little changed descendants of the common ancestor of the group. If an evolutionist used a time machine and went to the Eocene, he would be pleased to observe a whole series of transitional forms: here are proto-flounders with eyes on different sides of the head, here are psettodes with an eye on the top of the head, here are advanced flounders with eyes on one side.

What kind of guy is this?

At the beginning of the 20th century, paleontologists could still assume that life on Earth began in the Cambrian period (542-488 million years ago). This was due to the fact that the first fossil fauna suddenly, as if out of nowhere, were discovered in the lower Cambrian layers. Cambrian animals presented to the biologist completely ready-made morphological types. If these were arthropods, then they would have graceful jointed legs, a segmented shell and faceted eyes - ready-made, real arthropods. If they are mollusks, then they are ready-made mollusks, with characteristic shells; if they are brachiopods, then they are also ready-made, with shells, with imprints of dexterous hands and muscles, with attachment legs.

Both Darwin and his followers understood that such advanced creatures had to be preceded by a long preliminary stage of development. But where is he? Beneath the Cambrian layers lay Proterozoic layers, which theoretically should have preserved the remains of more ancient and primitive life. But there, in the Proterozoic layers, no one found anything interesting. Therefore, the mystery of the Cambrian explosion - the so-called sudden appearance of various animals in the lower Cambrian layers - for a long time seemed insoluble.

During the twentieth century, a solution emerged. "Darwin's Lost World", the fauna of Precambrian sediments, began to gradually be discovered. The most famous of them was called Vendian in Russia; in other parts of the world it is more often called Ediacaran. It turned out that in the late Precambrian a variety of soft-bodied multicellular organisms already existed. Burials of multicellular Precambrian age were found in China, Australia, Canada, Europe and Russia (on the White Sea). So the Cambrian had to be content with a more modest role: from “the beginning of beginnings” it turned into “one of the stages.”

Yet the Cambrian period was a revolutionary time for the formation of the most important groups of animals. The revolution was the acquisition of hard skeletons at once by many groups of organisms, both multicellular and unicellular, and therefore they became abundantly preserved in the fossil record. In Cambrian time, the appearance of all known types of the animal kingdom (arthropods, molluscs, brachiopods, chordates, echinoderms, etc.) and many more creatures that cannot be attributed to any of them is recorded. These latter have a mixed morphology. They have characteristics of different types and classes: mollusks and annelids, arthropods and cephalopods, chordates, hemichordates and echinoderms, crustaceans and chelicerates. And there are also those whose structure does not give any clues about a possible relationship. These are called “problematics” without specifying the taxonomic type.

In Precambrian times, there were even more species and animals with mixed morphology, and about some of them scientists even argue whether they are animals or lichens. And the point here is not that the remains are rare or poorly preserved, or that scientists are not making enough efforts to understand them. No, this is an objective difficulty that evolution poses for biologists. Indeed, in the initial periods of the evolution of multicellular organisms, before the characteristics of the main phylogenetic branches of the animal kingdom stabilized, there were common ancestors of these branches. And they, naturally, had a mixed set of characteristics from different phylogenetic lines. Because of this, they cannot be confidently correlated with any of them and placed in the Procrustean bed of modern taxonomy. Different researchers interpret these forms in different ways, either singling them out into separate types, or classifying them as “stem” (ancestral) groups among known types, or bringing them closer to the common ancestors of several known types. It is these creatures, without a specific taxonomic registration, that provide the best idea of ​​transitional forms. Those that mark the path from ancestral species with a wide range of variability to taxa or groups of species that took as the basis of their structural plan only part of the available possibilities.

Nature, at first seemingly laughing at morphologists and taxonomists and making them suffer from uncertainty, itself offered a clue. The hint appeared in the form of a series Lagerstätt- fossil sites with unique preservation. In Lagerstätt, fossils preserve the smallest structural details, and not only the hard parts, but also the soft ones. These are imprints of the integument, muscles, digestive organs, circulatory and excretory systems - everything that is so valuable to the anatomist and allows you to reconstruct an extinct organism and determine its place in the system of the animal world. Lagerstätt, surprisingly, are especially characteristic of the Cambrian and Ordovician layers, when the formation of modern types and classes took place and when all these amazing intermediate forms lived. Most of the established early transitional forms are known from Lagerstätt.

Chalkieria

Chalkieria ( Halkieria) - an animal 5-6 cm long, found in deposits of the Early and Middle Cambrian (500-540 million years ago), combines the characteristics of three types - annelids, mollusks and brachiopods. It is not surprising that Chalkieria has been assigned alternately to different taxonomic types. The history of its discovery and reconstruction is very instructive.

The name Chalkieria was originally given to a hypothetical fossil from which only flat spines were found. It was believed that the animal itself was squeezed into the narrow gap of the thorn and had to somehow be kept on the substrate in the flow of water flowing around it. This reconstruction was not very convincing, so in the end it was decided that the thorn was not the whole skeleton of the animal, but only a part. Halkieria spines were usually found together with flat scales, and both had similar patterns. Having calculated the ratio of scales and spines, the animal began to be depicted as a worm, completely covered with scales and spines. It was still better than an incomprehensible monster living in streams of water.

But in one of the lagerstätts in Greenland (the place is called Sirius Passet) they discovered the imprint of a whole animal, which - oh horror! - there were halkieria thorns. And here it is, a real Chalkieria - a multi-segmented worm, slightly similar to a gastropod - a slug, covered with rows of scales of several types, and in addition scutes resembling brachiopod shells are attached to the front and back. Of course, no brave zoologist could imagine such a chimera. But now we know approximately what the common ancestor of mollusks, brachiopods and annelids might have looked like.

Here is a simplified tree of types of the animal kingdom. It shows the position of the most important fossil forms, shedding light on the origin and relationships of modern types.

The Cambrian and Vendian (Ediacaran) part of the tree is replete with dead-end branches - extinct forms close to the ancestors of one or more types of modern animals. There are branches that extend not from the powerful trunks of today's types, but from the dotted, basal parts of the tree. These are most common in the Vendian and early Cambrian.

Most Middle Cambrian transitional forms are known from the Lagerstättätt, which is called the Burgess Shale ( Burgess Shale). Discovered in southwest Canada by Charles Walcott in 1909, it is the oldest, best known and most researched Lagerstätt in the world.

Here is one of the characteristic transitional forms found at this location. This creature, like Chalkieria, combines the characteristics of annelids, brachiopods and mollusks. It was described by Simon Conway Morris, a paleontologist from Great Britain, an attentive, meticulous and courageous morphologist, and Jean-Bernard Caron, a specialist from Canada, who created a virtual museum on the Burgess Shale ( Conway Morris, Caron, 2007).

A bizarre creature 6-10 mm long is called Orthrozanclus reburrus. The soft body of the animal was covered from above and from the sides with hard, but not mineralized sclerites - spines and spines of various shapes and lengths, located in several rows. In addition, the front end of the body was covered with a small shell. The animal led a bottom lifestyle and crawled on its ventral side like a slug. The spines and bristles served a protective and possibly sensory function. The purpose of the shell is unclear. Perhaps it served as a support for the muscles of the oral apparatus.

Problematic Orthrozanclus reburrus bears characteristics of two other problematic Cambrian groups - Chalkieriids (see above) and Vivaxiids, which are interpreted by different researchers either as primitive mollusks, or as annelids, or as brachiopods. According to modern ideas, supported by DNA comparisons, these three types are related and are combined into a supertype Lophotrochozoa. Their most important common feature is the presence of a special floating larva, trochophore. The same supertype includes bryozoans, echiurids, nemerteans and some other groups.

Spikes and bristles Orthrozanclus almost the same as in vivaxiids, and the shell - like in chalkieriids. Despite some significant differences (for example, in Halkieria, unlike Orthrozanclus, in addition to the anterior shell, there is also a posterior one, and the sclerites are mineralized), the authors believe that their find proves the close relationship and common origin of the chalkieriids, vivaxiids and Orthrozanclus. They propose to combine all these forms into a single group “halvaxiids”.

Position of halvaxiids on the evolutionary tree of the superphylum Lophotrochozoa cannot yet be determined unambiguously. Two possible options are being considered: either this group is close to the ancestors of mollusks, or it is closer to the common ancestors of annelids and brachiopods. However, by and large, the difference between these hypotheses is small, since the common ancestors of mollusks, brachiopods and annelids were themselves very close to each other. So it is not surprising that extinct animals with characteristics of all three types were found: on the contrary, this is a striking example of the prediction of evolutionary theory coming true.

"Micro" or "macro"?

We have come close to the question of why biologists have become uninterested in proving that transitional forms really exist. And the point is not only that they exist in the predicted multitude and order, but also that interest in them has now shifted to a different plane. Today we are primarily interested in How major changes occurred and adaptations to completely new conditions arose - for example, during the development of land and fresh waters by the first invertebrates, and then by vertebrates, during the development of the air environment by insects and birds, and the return of mammals to the ocean. Is it possible to trace the formation of large groups using fossils? Do fossils provide insight into the routes of evolutionary creativity?

If we are talking about ordinary speciation, it is not difficult to imagine the tiny steps with which evolution went on century after century and adaptations were improved. In the case of reaching land or inventing flight, such steps are more difficult to imagine. In order to survive on land, animals had to completely reorganize themselves: force the swim bladder to breathe, force the heart to drive blood from the bubble directly to the head, force the body to rest on its legs, and force these legs to step, the kidneys to conserve water, the ears and eyes to hear and see in the air environment. And all this happened gradually, and each intermediate link was adapted to its environment, without planning and a long-term view of the future... yes, it is not easy to imagine such a route step by step at first glance.

But as biology developed (including the experimental study of evolution and observations of cases of rapid adaptive radiation in nature), it became increasingly clear that macroevolution is a series of the same microevents, only longer (see Chapter 2). This is the total result of many successive microevolutionary transformations. To definitively prove this conclusion, it is important to study fossil transitional forms that show the possibility of gradual rearrangements, from which step by step a revolutionary change in the organism is formed. It is precisely such transitional forms that are now the object of close attention of paleontologists.

The more carefully the evolutionary series is studied, the clearer the gradual course of change becomes. After all, if the researcher has only the initial and final links of the chain, then the gap between them seems huge, insurmountable. And then, in the face of the enormity of the task, they come up with a special term “macro-” and try to invent a special macro-mechanism to bridge the chasm. But if you search better and take a closer look, then from the initial to the final version a path of successive “micro” steps begins to be visible. And here the main help, of course, was the new wonderful finds of paleontologists. Many of the most important finds that filled illusory chasms are finds in Lagerstätt. And not only Cambrian, but also Mesozoic, formed not only by marine sediments, but also by freshwater ones.

Of course, when dealing with the macroscale, one cannot rely on direct observations. We can, as shown in Chapters 5 and 6, observe the emergence of new species, but not new families and orders - those levels of classification that, as a rule, mark entry into a new adaptive zone. You will ask why - and you will do the right thing. This is a natural question. And the answer to it is very simple: we can’t a-priory. The point is not at all that carriers of a new character, comparable to those by which large taxa (families, classes, etc.) are established, cannot appear before our eyes. This is exactly what you're welcome to do. But we will never recognize such a form as a representative of a new large taxon. Biologists will not classify a creature that has just appeared before our eyes into a special taxon, no matter how much it differs from its ancestors. One of the reasons is that the prospects for the new form and its newfound characteristic are unknown. It is not known whether a series of species with different morphology and ecology will be able to appear on the basis of this innovation. Or maybe the innovation has no prospects, its carriers will soon become extinct, and then they should be classified not as the ancestors of a large taxon, but as freaks. Whether we like it or not, biologists assign large taxonomic ranks to a group only in retrospect, when the group has already “gained strength”, accumulated a reserve of variability, divided into genera and species, demonstrated its evolutionary-ecological capabilities that are different from other groups - in a word, proven with its entire history, which deserves a high rank. Even if as a result macromutations If an unusual form with a changed body plan arises, it will take millions of years to find out whether this form can be considered the ancestor of a new large taxon or whether it was just a freak doomed to extinction - an aberrant representative of the parent taxon, from which nothing worthy of attention came.

A good illustration could be the emergence of a new order of bryozoans Fenestellida. These bryozoans differ from others in the form of colonies and the high specialization of zooids. The first genus of these bryozoans appeared in the Silurian. And over the next 25 million years, several species of bryozoans with a characteristic fenestellid morphology existed simultaneously with their ancestors, modestly contenting themselves with the waters of the seas washing Laurentia (future North America). Only at the beginning of the Carboniferous period did these several species give rise to a whole bouquet of promising forms that spread throughout the world. On the basis of the morphology “proposed” by the first Silurian genus, they gave rise to many genera and families that made up the order Fenestellida. If paleontologists began to work with Silurian species, then, without knowing further history, they would have attached isolated species with an unusual fenestellid character to the ancestral order, separating them into a separate genus, but nothing more. But as soon as all the subsequent variety falls into their hands, a whole new squad immediately appears. So who arose in the Silurian and existed without changes for two whole periods - a new genus or order? This is a question that requires a formal solution and a long-term view. Any new, deviating form can become the progenitor of a large taxon, or it can become ingloriously extinct.

One way or another, the beginning of a large taxon is a species that always deviates from its ancestral morphology, which received some kind of advantage that opened up new opportunities for its descendants. This is the first macroevolutionary step.

Forward into airspace

Let's consider how fish turned into four-legged animals that became land conquerors: how fish went into terrestrial space. This event dates back to the end of the Devonian period, 385–360 million years ago. The first tetrapods were not the first creatures to leave the aquatic environment: plants had already settled on land at the end of the Ordovician; apparently, even in the Cambrian, a rich fauna of invertebrates appeared in the soils, and even earlier, bacteria and fungi became inhabitants of land. So the fish, having sacrificed harmony with their usual aquatic environment and turned into slow, clumsy quadrupeds, came to a populated and hospitable world. If in the water they were in wait for predators, and for food they had to compete with many hungry eaters, then on land there were no large predators and were attracted by the abundance of available food. Therefore, the risky and difficult landfall was a profitable venture. It was a gigantic unoccupied ecological niche. And evolution hastened to fill it as soon as the opportunity presented itself.

The opportunity, apparently, was provided by the conditions of the Devonian period. The paleoecological interpretation of localities with Devonian transitional forms depicts small, low-flowing fresh water bodies of the subtropical or tropical zone. A reduced oxygen content in the atmosphere and water is expected. Perhaps, due to strong periodic shallowing, Late Devonian lobe-finned fish had to adapt to life in conditions where the water no longer serves as a support for the body, because it is too shallow, where they have to breathe atmospheric air without the help of gills.

These reservoirs were chosen by various fish - and there were many of them, including large and small predators. It is no coincidence that in the Devonian seas, 17% of fish genera acquired shells - peaceful inhabitants acquired means of protection. This was the heyday of fish - in addition to jawless fish, which appeared no later than the Ordovician, cartilaginous and bony fish, which separated in the Silurian, swam in the Devonian seas. And bony fish in the Devonian were already represented by two groups - ray-finned and lobe-finned. These groups differ, among other things, in the structure of their fins: in lobe-finned fish, the fin sits on a fleshy base, the skeleton of which is made up of elongated bones, the fin blade is supported by a series of symmetrical bony plates. In ray-finned fish, the fin blades are asymmetrical, and the bones of the fin base are shortened.

It was one of the groups of lobe-finned animals with powerful pectoral and ventral fins resembling paws - rhipidistia - that gave rise to terrestrial vertebrates. The lobe-finned species include modern lobe-finned species (coelacanths) and lungfishes (horntooth, protoptera, lepidoptera). Genetic analysis has shown that among modern fish, the closest relatives of tetrapods are lungfish.

The evolutionary changes that occurred with Rhipidistia, the ancestors of the first tetrapods (amphibians), were gradual. More than a dozen representatives are known from Late Devonian deposits, combining the characteristics of amphibians and fish. They line up in a relatively orderly series of fossil forms: eusthenopteron ( Eusthenopteron), pandericht ( Panderichthys), Tiktaalik ( Tiktaalik), elpistostega ( Elpistostega), Livonian ( Livoniana), elginerpeton ( Elginerpeton), ventastega ( Ventastega), metaxygnathus ( Metaxygnathus), Acanthostega ( Acanthostega), Ichthyostega ( Ichthyostega), tulerpeton ( Tulerpeton) and greenerpeton ( Greererpeton). In this series, fishy traits gradually decrease and tetrapod traits accumulate, but different organ systems do not make this transition simultaneously; some come to the tetrapod state faster, some more slowly. Following the famous Swedish paleozoologist Erik Jarvik, these creatures are often called “four-legged fish.”

The main thing that needed to be changed in order to gain the right to be called “true quadrupeds” were the limbs. However, quadrupeds and walking as such also arose long before the quadrupeds themselves. Many lobe-finned fish were able to deftly walk along the bottom on four fins, lifting their bodies above the ground. So pre-adaptations for walking were already present at the “fish” stage. But in order to walk effectively on land, where the body weighs much more, it is still advisable to have a specialized limb. It was necessary to form a movable joint of the paw and detach the belt of the forelimbs from the skull. After all, in fish, the girdle of pectoral fins is rigidly attached to the posterior temporal bone of the skull, and this limits the movements of both the head and fins.

But if you take a closer look, it turns out that the limbs (fins) of Devonian lobe-finned fishes were not so different in their structural plan from the paws of primitive tetrapods. Although not all fossil transitional forms have preserved limbs, we know in sufficient detail what changed and how. Already in eusthenopteron there was a bone in the front fin corresponding to the humerus of tetrapods, and two bones corresponding to the ulna and radius. The structure of the “distal elements” corresponding to the future tetrapod manus in Eustenopteron and Panderichthus still remained relatively disordered.

Next in this series is Tiktaalik, discovered in 2006 by American paleontologists Edward Deshler, Neil Shubin and Farish Jenkins on Ellesmere Island (Arctic Canada). "Tiktaalik" means "large freshwater fish that lives in shallow water" in the Eskimo language. Indeed, Tiktaalik is a flat fish covered with large scales with a crocodile head, on which eyes sit on top, two nostrils and a large toothy mouth in front. This fish, like other representatives of the series of transitional forms that interest us, has some features similar to lobe-finned fish, while other features bring it closer to tetrapods. Fishy characteristics are scales, fin rays, almost the same as those of lobe fins, a complex lower jaw and palatine bones. Tetrapod features - a shortened skull, a head separated from the girdle of the forelimbs and therefore relatively mobile, the presence of elbow and shoulder joints ( Daeschler et al., 2006).

Compared to Pandericht, Tiktaalik’s skeleton of the forelimbs acquired a slightly more formed appearance, so it becomes more or less clear where the bones of the metacarpus and fingers came from in Tiktaalik’s descendants. Tiktaalik and pandericht could already bend their front fin or paw at the joint, which their descendants would call the elbow.

Tiktaalik also tried one small but important innovation - it almost got rid of the gill cover, which the pandericht still had (we will return to the functions of the gill cover).

Along with the gill cover, the rigid connection between the girdle of the forelimbs and the skull was also lost. The head was freed from the forelimbs (or limbs from the head). Both became more mobile. Now I could start learning to walk normally. Tiktaalik's ribs are flattened and the vertebral joints are ossified. Due to this, he began to bend worse, but the body acquired stability, which is very important for a walking, rather than a floating, lifestyle.

The loss of the operculum turned out to be extremely useful in the future. The articulation of the remains of the gill cover with the head seems to have become a rudimentary appendage for a terrestrial, lung-breathing creature. But it did not disappear at all, but began a slow and extremely important journey for all terrestrial quadrupeds into the skull, slowly turning into tiny auditory bones. In fact, the auditory ossicles began to form even before the loss of the operculum. As was shown by Per Ahlberg and Martin Brazeau from Uppsala, Pandericht, and not Tiktaalik, was the first to adapt one of the bones of the articulation of the gill apparatus with the skull (hyomandibula) for a primitive stapes (auditory ossicle). This bone was thin and elegant, it was adjacent to the respiratory opening (the first gill slit), which widened in pandericht. In fish, this opening is called the squirter; it is present in tetrapod embryos, but as the embryo develops it becomes the cavity of the middle ear and the Eustachian tube. Pandericht breathed air, the pumping apparatus of the gill cover (see below) weakened, and its parts were reoriented to perform another function - the perception of sound. All tetrapods, including us, inherited the stapes from fish close to panderichthus ( Brazeau, Ahlberg, 2006).

In Ventastega, which in terms of the structure of the skull occupies an intermediate position between Tiktaalik and Acanthostega, unfortunately, almost nothing has been preserved from the limbs. The wide shape of the snout and the structure of the skull make Ventastega similar to Acanthostega, and the shape and proportions of the integumentary bones of the skull are similar to Tiktaalik.

The ventastegi's wide jaw was lined with small, sharp teeth. On a long body - about a meter and a half in length - there were two pairs of short limbs with fingers (how many fingers are unknown) and a tail with a fin, which was supported by fin rays about 7 cm long. Ventastega lived in brackish shallow coastal areas and, having an impressive size, hunted fish. Like Pandericht and Tiktaalik, in Ventastegi part of the former articulation of the gill apparatus with the skull was transformed into an auditory ossicle - the stapes. Where to classify Ventastega - as a fish or already as a tetrapod, that is, the oldest amphibians - is a formal question, but if it really had fingers, then, perhaps, the second option is preferable.

Next in the series are Acanthostega and Ichthyostega, which are considered to be “real” tetrapods. Their limbs represent just another small step towards differentiation of the distal parts of the skeleton (foot and hand). They had three large limb bones that had finally formed (in the front paw these were the shoulder, ulna and radius bones), while the numerous bones of the hand were still small and changeable. But Acanthostega definitely already had real fingers, and it did not have the fin rays that Tiktaalik still retained. At the same time, the number of fingers in Acanthostega varied from five to eight: the feature has not yet “stabilized.” It is also variable in Ichthyostega. A more stable number of fingers is characteristic of Tulerpeton (six fingers); he also had completely “tetrapod” ulna, radius and metacarpal bones. The number of fingers of the Greererpeton has stabilized - now there will always be five of them.

In addition to the limbs, access to land required a profound transformation of the respiratory and circulatory systems. Again, it seems that the transition from breathing water to breathing air is impossible with small improvements in the design - some kind of large-scale transformation of both systems is needed at once. But this, as it turns out, is not so. Similar to the gradual transformation of the limbs, the transition from gill to pulmonary respiration also gradually occurred. And this can be seen in the same series of fossil four-legged fish.

So, you need to form lungs and, accordingly, two circles of blood circulation and a three-chambered heart. Judging by paleontological data, almost all Devonian lobe-finned fish had such a structure of the circulatory and respiratory systems. It has also been preserved in modern lungfishes. Lobe-finned animals began to use a special invagination of the anterior part of the esophagus as an organ of air respiration. The fish swallows air, which is sent into this bubble, entwined with blood vessels - this is the simplest lung.

In lungfishes, the bladder performs two functions: a primitive lung and a primitive hydrostatic organ. In most (but not all) bony fishes, the swollen protrusion of the esophagus has specialized for a second function and developed into a “true” swim bladder. But in Devonian fish, this outgrowth of the esophagus combined the functions of a lung and a buoyancy regulator.

In Devonian reservoirs, the acquisition of an additional respiratory organ was more than justified: the oxygen content in the atmosphere was reduced, and the proportion of carbon dioxide, on the contrary, increased compared to modern times. Therefore, in water, especially in stagnant water, it was difficult for fish to breathe with their gills alone. And they acquired an additional organ for breathing atmospheric air. So, for reaching land, “preparations” were already underway gradually - the lungs for breathing air and the modified circulatory system attached to them were already in store in the Devonian lobe-finned fish. Of course, selection supported them as adaptations for life in oxygen-poor water. Neither the fish nor the selection knew what they would be useful for later.

The advent of the respiratory bladder, a primitive lung, created the preconditions for another innovation: the efficient air pump. In bony fishes, water is driven through the gills by the operculum: the fish closes its mouth and raises the middle part of the operculum, increasing the volume of the gill cavity (its soft rim is pressed tightly to the body). Accordingly, the pressure in the gill cavity decreases and when the fish opens its mouth, water rushes there. Then the fish closes its mouth, the operculum lowers, the rim moves away from the edge and water flows out, washing the gills.

All this is possible in a dense aquatic environment, when it is necessary to ensure a through flow of water through the gills. In a rarefied air environment, such a pump will be extremely ineffective, because in addition to the different density of the medium, the breathing pump has other tasks. It is necessary not to drive the air through a through flow, but to ventilate the blind air bag. But the first tetrapods (amphibians) never really solved this problem. Only reptiles succeeded in this. And in amphibians, the principle of forcing air into the lungs is approximately the same as in air-breathing fish. They use the transformed apparatus of the gill arches to expand the pharyngeal cavity and, accordingly, to inhale. The spent air from the lungs is exhaled due to contraction of the lung muscles (and in lungfish it is released due to the higher pressure in the water compared to the air surface above the water, where the fish puts its head to inhale and exhale).

The features of the gill apparatus, adapted for inhalation and exhalation due to the movement of the pharynx rather than the gill cavity, are clearly reflected in the skeleton of both modern and fossil Devonian lobe-finned fish. To more effectively remove carbon dioxide from the body, lobe-finned animals use gills, and amphibians that have lost gills use skin (“skin respiration”). This creates a lot of restrictions, due to which amphibians were never able to move far from the water.

But in the air, the operculum still did not work as an air pump, so it had to be abandoned, and this opened up additional possibilities for land inhabitants - the bones of the operculum went inside the skull. And they began to form the middle ear. The operculum was partially reduced in Tiktaalik. This means that the method of breathing that other fish continued to use was no longer needed by him. Tiktaalik breathed mostly air through its lungs and possibly through its skin.

Since Tiktaalik had a reduced operculum, this means that it had to learn to swallow without the use of a suction gill pump - fish draw in food particles with a current of water. Tiktaalik swallowed only by moving the throat and head. In Acanthostega, the successor of Tiktaalik, internal gills were still preserved, but in Ichthyostega they were already reduced (external gills are still preserved in amphibian larvae today). Consequently, Tiktaalik followers also swallowed through movements of the head and jaws. Subsequently, as the remains of the gill apparatus were reduced in ancient amphibians, the cervical region gradually developed.

Thus, there were no sharp jumps in the structure of the respiratory and circulatory systems during the transition from fish to tetrapods, and most of the important changes occurred at the “fish” stage. Each innovation in breathing and blood circulation was not created from scratch, but only improved the required functions based on the development of existing devices.

We examined a number of transitional forms linking lobe-finned fishes with primitive tetrapods. It is completely unclear where in this series the sought-after great “macrotransition” is located. Acquisition of outgrowths of the esophagus into which air can be drawn? No, this is simply an adaptation of fish to poorly aerated reservoirs. Similar organs of air respiration (for example, in the form of modifications of the oral cavity) arose in later times in different fish that found themselves in similar conditions. Acquisition of limb girdle bones from eusthenopteron? Hardly, because this was also a purely “fish” adaptation to greater mobility of fins. Maybe we should set this milestone for the acquisition of a movable joint at the paws, i.e. somewhere between eustenopteron and Tiktaalik? But this is simply an adaptation of fish to crawling along the bottom of small puddles and crawling from one dry body of water to another, which also developed over a long period of time and gradually. Another option is to assign a “macro” title to Tiktaalik, whose forelimbs separated from the skull and became free paws... although this is just an improved adaptation for moving along the bottom in shallow water. Loss of fin rays and gain of fingers? If you really want to, you can, of course, declare this small modification of the distal parts of the limbs to be the epoch-making “transformation of a fish into an amphibian.” Indeed, it is precisely at this point in the continuous series of transitional forms that experts draw a formal line between rhipidistia and tetrapods. But isn’t it better to honestly admit that paleontological data simply show a long series of gradual small changes, each of which in itself does not in any way amount to a “macro event”?

Everywhere in this series we see successive adaptations that served their owners faithfully in specific conditions. None of them alone deserve the status of “macro change.” Evolutionary transformations of comparable scale are taking place in modern nature around us, in laboratory experiments, and in the daily work of breeders. And of course, if the first tetrapods had died out without giving rise to a large and diverse group of animals, intelligent octopuses would never have singled out Ichthyostega, Acanthostega and their relatives into a special class - they would at most have considered them a separate dead-end family of lobe-finneds. And they would be right.

In disorganized rows - into the land future

Concluding the conversation about transitional forms between fish and tetrapods, it is necessary to clarify that by arranging the finds in one row, we, of course, greatly simplify reality. In paleontology, this is a typical situation: while few transitional forms have been found, it is convenient to arrange them in a row, but the more finds there are, the more bushy the evolutionary scheme becomes. In general, evolution is more often like a bush than a thread, and numerous parallelisms are its most typical feature (we talked about parallel evolution and its genetic basis in Chapter 4).

At first glance, the successive changes from fish to tetrapods correlate well with the order of position of transitional forms in the time series: creatures with a predominance of fish characteristics lived first, then over time the fish traits were lost, replaced by tetrapods. Panderichts lived 385–383 million years ago, Tiktaalik - 380–375, Ventastega - 374–365, Acanthostega and Ichthyostega - 370–365, Greererpeton - 359 million years ago.

However, other “four-legged fish” have been found that violate this harmonious picture. For example, Metaxygnathus, from which only the lower jaw was found. Its age is 374 million years. It should, judging by its geological age, have a morphology close to Tiktaalik, but its morphological place is after Ventastegi. Obviously, Metaxygnathus The development of the jaw apparatus proceeded faster than in other forms that developed in parallel. At the same time, his other organs could change more slowly (although we can only guess about this for now). The unequal rate of evolution of different organ systems is characteristic of the starting period of many large-scale evolutionary transformations (the emergence of fish on land, the origin of arthropods, birds, mammals, flowering plants, etc.). At this stage, forms may coexist in which, for example, the respiratory system is already advanced, but there are no legs yet, or there may be those in which the legs are already fully walking, and the lungs are still like a fish.

The different rates of evolution of morphological structures in different lineages create a mosaic of advanced and primitive characters that cannot be sorted into one neat line. One can only see the general direction of change: how intermediate forms with a predominance of fishy features are slowly dying out, but surviving with a predominance of tetrapods.

In the Late Devonian (385-359 million years ago), there were representatives of different lines of fish, rushing in their adaptive race to a semi-aquatic, semi-terrestrial lifestyle. Late Devonian fossils help reconstruct the general direction of this race, but not the route of each line. What paleontologists know - meager fragments of an apparently rich fauna - dottedly draws the course of different evolutionary lines, but does not show the connections between them, their beginnings and ends. Therefore, it cannot be said that they all appeared and died out in the Late Devonian, because we know only some of the dotted lines.

Most likely, the history of four-legged fish began not in the Late, but in the Middle Devonian (397-385 million years ago) or even earlier. In this regard, one can expect the discovery of transitional forms with fish characteristics, slightly diluted with tetrapod characteristics, in the layers of the Middle Devonian. The fact that vertebrates may have begun to explore land earlier than the Late Devonian is evidenced by fossilized traces left by some animal that walked on land. These traces were found in Poland in a layer 395 million years old. There seems to be no reason to doubt the age of the layer in which these traces were found. The layer formed in a coastal marine environment. Perhaps it was a lagoon or a flood plain. Even pits from raindrops and drying cracks are visible on the surface of the layer. The tracks themselves vary in size and morphology; There are chains of traces, and there are also single prints. The largest footprint is 26 cm wide; in comparison, the Ichthyostega, which was about 2.5 m long, left a 15 cm imprint. In the chain of tracks, the prints of the front and hind paws differ - the front ones are smaller than the hind ones.

Reconstruction of the stride from these tracks suggests that the animal moved by bending its body from side to side, much like a salamander. There are no traces of tail dragging. This means that the sacrum and girdle of the hind limbs were already formed, raising the body above the ground. All these are signs of four-legged walking ( Nied Wiedzki et al.., 2010). Experts argue about the interpretation of these signs - some fish also like to go out into the air and leave traces on land: let us remember, for example, the wonderful fish - the mudskipper, which has perfectly adapted to walks in the fresh air. But even if doubts about the tetrapod origin of those ancient traces finally disappear, this will only indicate the mosaic nature of evolution in the early stages of the formation of tetrapods. We will then be convinced that in one of the lines of rhipidistia, quadrupedal walking improved faster than in others. This may have been at the expense of a more balanced morphology (see “Selection for Evolutionary Prospect” in Chapter 4).

Modern experimenters: mudskipper and anglerfish

Some existing bony fish already in the Cenozoic began to make new “attempts” to develop land, sometimes very successful. These modern fish have much fewer prerequisites for entering land than their Devonian predecessors. Their former lungs have already been “spent” on the swim bladder, and the hind limbs (pelvic fins) have become unsuitable for turning into legs. Nevertheless, the mudskipper deftly walks on land, relying on its pectoral fins and helping itself with its tail, and breathes air with the help of a new, rather “artisanal” and ineffective air breathing organ that has formed in its mouth. These amazing fish are able to feed on land and even breed on the shore in clay towers, at the bottom of which they create a puddle where eggs develop.

We don't know whether mudskippers will usher in a new wave of land-based fish expansion. Maybe they will, although now they have a lot of competitors on land. Devonian four-legged fish had no competitors on the shore, but food (terrestrial and soil invertebrates) was already abundant. If, nevertheless, a new large land group arises from the brave mudskippers, zoologists of the future will give it a high rank, and its ancestors - the current mudskippers - will be looked at with completely different eyes: not as an extravagant outsider, but as a far-sighted experimenter.

Mudskippers are far from the only fish that today, to one degree or another, “reproduce” that long-standing macroevolutionary event - reaching land. Another example is the shallow-water anglerfish Antennariidae. They do not go onto land, but their pectoral fins have acquired a striking resemblance to the paws of quadrupeds. They even have something like fingers!

Anglerfish crawl along the bottom with their paws and hold on to stones so that they are not tossed around by the waves. As we can see, paws can also be useful under water (the first tetrapods also most likely spent most of their lives in water). Although the legs of anglerfish are very similar in appearance and in their movements to the legs of tetrapods, their skeleton is different. Such similarity is called analogous, that is, it appeared independently on a different basis, as opposed to homologous similarity that is demonstrated by related species and genera.

Dinosaurs master the air

The modern world is replete with flying creatures - insects, birds, bats; there are others who, although not real flyers, are no longer quite land dwellers - tree frogs, squirrels, woolly wings, lizards - “flying dragons” ... There is even aeroplankton - the smallest animals and bacteria, adapted to be transported over long distances by air by the masses. Man also does not lag behind them - he rises into the air with the help of technical means. Thus, many groups of animals show a tendency to develop air space.

Flight requires coordinated and multiple rearrangements of the body. The more balanced and deeper they are, the more skillful the resulting flyer. In this section we will look at how birds took flight, because their evolutionary history has become remarkably clear in recent years thanks to a series of brilliant discoveries of various transitional forms.

Shortly after the publication of The Origin of Species, the first skeleton of Archeopteryx, the famous intermediate form between reptiles and birds, was discovered. It was buried in the Late Jurassic sediments of Germany, whose age, according to the latest data, is just under 150 million years. Archeopteryx had developed plumage (a typical bird feature), and its skeletal structure differed little from small theropod dinosaurs. It had claws on its forelimbs, teeth and a long tail. There were few characteristic “bird-like” features of the skeleton (hooked processes on the ribs, fork). Archeopteryx is a classic example of a transitional form. But this fossil was just a lonely sign along the long road that leads to birds. It is clear that with the gradual development of avian features, such intermediate forms were much more diverse. But have they survived? Will paleontologists be able to detect them?

Managed. Further searches in the Jurassic and Cretaceous layers revealed a rich fauna of primitive transitional half-dinosaurs, half-birds. Below is a phylogenetic diagram reflecting the relationships of most of the transitional forms known today that combine the characteristics of birds and dinosaurs.

Due to the rules of modern cladistic taxonomy, the fact of the origin of birds from dinosaurs (and not from common ancestors with dinosaurs) requires considering birds as a subgroup of dinosaurs, and a cumbersome term is now used for dinosaurs “actually” non-avian dinosaurs(non-avian dinosaurs).

The beginning of the formation of birds should be considered the Jurassic era, when many predatory dinosaurs began to evolve “in the avian direction.” Representatives of the line of dinosaurs in which this parallel “opticalization” took place are united under the common name Paraves. Modern birds themselves are represented in the diagram by four species - chicken ( Gallus) and gokko ( Crax), duck ( Anas) and palamedea ( Chauna). They occupy a modest place on one of the branches among many fossil species, forming together with them an extensive group Avialae, “sister” to another large clade of feathered dinosaurs - deinonychosaurs. Deinonychosaurs are divided into troodontids and dromaeosaurids. Archeopteryx took a completely logical place in the diagram as one of the most primitive representatives of the entire diverse group of Mesozoic flyers. At the end of the Cretaceous period, 65.5 million years ago, they all went extinct except for a small group that survived the Cretaceous crisis and gave rise to modern birds.

The most primitive representative Paraves today it is considered epidexipteryx. It is close to the common ancestor of the first flying dinosaurs. It was found in 2008 in Jurassic (from 168 to 152 million years ago) deposits of Inner Mongolia (China). This animal is the size of a pigeon. The researchers estimate that Epidexipteryx weighed about 160 grams, less than other bird-like dinosaurs. It differed from its relatives primarily in its short tail: it has only 16 caudal vertebrae, with the last ten forming something reminiscent of the pygostyle of primitive birds, although these vertebrae do not merge into a single whole. The length of the tail is 70% of the length of the body, while its closest relative, Epidendrosaurus, had a tail three times longer than the body.

The name "epidexypteryx" (from the Greek . epidexi- “show” and pteryx- “feather”, “wing”) the dinosaur received for a reason. Attached to the caudal vertebrae were four unusually long feathers with a central shaft and a fan of unbranched barbs. The body, including the limbs, was covered with small feathers without a shaft, similar to the down of other dinosaurs and primitive birds. Epidexipteryx did not have feathers suitable for flight, and it probably could not fly.

These long tail feathers served exclusively “for beauty,” that is, to attract a mate. The males of many modern birds show off long tail feathers in front of the females, which are no longer good for anything and only interfere with flight. The costume of Epidexpteryx proves that the beauty of mating costumes was important for the “avian” branch of dinosaurs already at the very beginning of their history ( Zhang et al., 2008).

Feathers are needed for beauty, warmth and flight. Apparently, at first they performed the first two tasks and only later were useful for the third. And the flight did not begin immediately: the flight feathers developed gradually. Increasing the load-bearing surface of the forelimbs increased the stability of a running or jumping dinosaur. The initial stages of the evolution of flight feathers may have been associated with the stabilization of fast running on uneven surfaces. At the same time, large feathers gradually shifted to the rear edge of the limb; the structure of the feathers themselves and their attachment points was gradually improved.

The feathered primitive ancestors of birds gave rise to the group of dromaeosaurids. These animals said goodbye to the surface of the earth, preferring trees. Recent studies have shown that even the famous velociraptor did not so much run across the plains as climb trees, and its structure was adapted precisely to this way of life. It turned out that velociraptors were covered in feathers, which implies they were warm-blooded ( Turner et al., 2007). Previously, they were considered (this version was immortalized by the film “Jurassic Park”) as high-speed plains pack predators, endowed with terrible weapons - toothy jaws and four giant claws (one on each of the four limbs). But it turned out that the structure and geometry of the claws on the hind legs of a velociraptor are comparable to the claws of some tree-climbing birds and mammals. This comparison definitely moved Velociraptor from the ground to the tree. Velociraptors used their colossal claws on their feet for more than just killing thick-skinned prey. When climbing the trunk, they relied on this claw. It had to be large in order to hold a rather massive animal on a vertical trunk - these dinosaurs weighed about 15 kg.

Most likely, velociraptors had a “bird-like” mechanism for fixing their claws. When a bird sits on a branch, its talons wrap around the branch and lock into position without expending any energy. This allows the bird to sleep while sitting on a tree without falling. The support for the arboreal predator when landing on the trunk was also provided by a hard tail, composed of fused vertebrae and ossified tendons. Feathers, an arboreal lifestyle, and even in a flock - why not a bird!

Anchiornis(Anchiornis huxleyi) belongs to another part of the phylogenetic tree - to the basal (i.e., primitive, early) troodontids. Anchiornis lived before Archeopteryx - 155–151 million years ago. When he was found and described (and this happened in 2009), he immediately took pride of place among the most amazing and unexpected animals. This flying creature, about the size of a crow, had contoured flight feathers on both its front and hind limbs. Excellent prints unmistakably convey the structure of the skeleton and feathers, so there is no doubt in the interpretations - Anchiornis had large feathers on its legs, suitable for flight.

This is not the first four-winged dinosaur found - such were known before, for example microraptor, who lived 125 million years ago. At first, many experts interpreted four-wingedness as a kind of curiosity, an extravagant but insignificant twist in evolution Paraves.

In 2005, another dinosaur with large feathers on its legs was described - pedopenna(which translates as “featherfoot”), which lived at the same time as Archeopteryx or perhaps a little earlier. It is known only from the remains of feathered hind limbs. Pedopenna was interpreted as a basal representative Paraves(it is not shown in the diagram)

Judging by the distribution of four-winged forms along the evolutionary tree, four-wingedness was characteristic of the first representatives of all three branches Paraves: troodontids, dromaeosaurids and Avialae. It follows that modern birds evolved from four-winged ancestors. By the way, among primitive Avialae, including Archeopteryx, did have contour feathers on their legs, although not as large.

Unlike those modern birds that have more or less developed plumage on their legs, in Microraptor, Pedopenna and Anchiornis the contour feathers of the hind limbs were arranged in an orderly manner and formed large flat surfaces, which undoubtedly influenced the aerodynamic properties of the animal. Exactly how these additional planes were used - for planning, maneuvering or, say, braking - is not known exactly. It is unlikely that four-winged dinosaurs could actively flap their “hind wings” - the skeleton of their hind limbs is not designed for such movements - but they could rotate their legs in such a way that the feathers helped to glide. Four-wing flight is not very efficient anyway.

Anchiornis, judging by the skeleton, was not a very good flyer (Microraptor flew better). The hind limbs of Anchiornis, if you forget about the feathers, are more like the legs of a professional runner. But the large feathers on his legs hardly helped him run fast - rather, on the contrary, they should have gotten in the way. Modern fast-running birds tend to lose feathering on their legs. Perhaps Anchiornis has not yet had time to “decide” what is more important for him - flying or running, and the structure of his hind legs is the result of a compromise between multidirectional selection vectors. During the further evolution of early birds, wings and legs more clearly divided their functions: the former for flight, the latter for running.

Anchiornis is similar in many skeletal features to troodontids, to which it was classified, but some features (for example, very long forelimbs) bring it closer to dromaeosaurids and Avialae. As a result, formal diagnoses (lists of distinguishing characteristics) of the three groups Paraves become more vague. This is a typical situation, unpleasant for taxonomists, but absolutely inevitable when studying the basal representatives of large taxa (we talked about this when discussing Cambrian types). It is clear that the closer we get to the common ancestor of all Paraves, the less clear the differences become between the three evolutionary lines that make up this group. "Transitional Forms" blur these differences. Darwin was right when he talked about the blurring of boundaries between taxonomic groups.

In addition, the mosaic of avian and dinosaur features so widely represented in Paraves, once again reminds us of parallel evolution. The beginning of the evolution of each large group, as a rule, is represented by several lines, each of which, in its own way, with its own characteristics and at its own speed, adapted to a new environment or a new way of life. In the case of the “opticalization” of dinosaurs, these are the lines of dromaeosaurids, troodontids and Avialae.

Birds, dinosaurs and their genes

It is psychologically quite difficult to admit that birds are dinosaurs - and this is precisely what modern taxonomy requires. In addition, there are “non-dinosaur” hypotheses of the origin of birds. True, today they have almost no supporters left (we talked about one of them in the book “The Birth of Complexity”). Genetic analysis could help, but dinosaur DNA has not been preserved (at least not yet isolated and read). But scientists were still able to get close to the genes of dinosaurs ( Organ et al., 2009). They indirectly estimated the size of the genomes of various dinosaurs. To do this, they took advantage of the fact that in fossil bones, if they are well preserved, small cavities can be visible on sections, in which bone tissue cells - osteocytes - were located during the life of the animal.

Genome size is known to be positively correlated with cell size in many groups of living things. Bone sections from 26 species of modern tetrapods taken for the study revealed a linear relationship between genome size and average osteocyte volume. The found relationship allowed scientists to estimate with acceptable accuracy the size of the genomes of 31 species of dinosaurs and fossil birds.

Modern birds have an unusually small genome size, ranging from 0.97 to 2.16 billion base pairs, with an average of 1.45. For comparison, a toad has 6.00, a crocodile has 3.21, a cow has 3.7, a cat has 2.9, a mouse has 3.3, and a human has 3.5. What about dinosaurs? The genomes of ornithischian dinosaurs (despite their name not related to the avian lineage) had an average size of about 2.5 billion base pairs, which is comparable to modern reptiles. The genomes of theropods (carnivorous bipedal dinosaurs), including the most ancient ones that lived long before the appearance of birds, were much smaller - on average 1.78 billion bp. Of the nine theropod species studied, only one (Oviraptor) had a genome size outside the range typical of modern birds.

Thus, the common ancestor of all dinosaurs most likely had a large genome typical of land vertebrates. This condition was preserved in ornithischian dinosaurs, as well as in reptiles that have survived to this day. At the dawn of their history (in the Triassic), lizard-hipped dinosaurs (which include theropods) experienced a radical reduction in their genome. Birds thus inherited a small genome from their theropod dinosaur ancestors, rather than acquiring it later as an adaptation for flight.

Nevertheless, there is still a connection between genome size and flight. This is evidenced by two circumstances. First, flightless birds (such as ostriches) have larger genomes than flying birds. Second, bats have smaller genomes than other mammals.

Another genetic study that shed light on the early evolution of birds was performed in 2011.

From the point of view of embryology and comparative anatomy, the three wing fingers in birds correspond to fingers II, III and IV of the original five-fingered limb. This is contrary to paleontological evidence: fossil evidence shows the gradual loss of digits IV and V in the evolutionary lineage leading from the first Triassic dinosaurs to Archeopteryx and other extinct lizard-tailed birds. The three remaining digits they have are digits I, II and III. This means that if modern (fan-tailed) birds are descendants of lizard-tailed birds, then their wing fingers should be fingers I-II-III, and not II-III-IV. It turns out that fingers are just fingers! - do not agree to descend from dinosaurs. And this is enough to cast doubt on the validity of the dinosaur theory. But through the efforts of geneticists, this conflict between embryology and paleontology was resolved ( Wang et al., 2011).

They measured gene activity in the digit buds of the front and hind limbs of the chick embryo. In total, the activity of 14,692 genes was recorded in the finger primordia. The analysis was carried out separately for each finger at two stages of bud development (early and late). It turned out that, based on the nature of gene expression, the first (innermost) fingers of the wings and toes are clearly different from the remaining (outer) fingers, but at the same time they are similar to each other. The first digits differ from the other digits in the expression level of 556 genes. This is a strong argument in favor of the fact that the first digit of the wing is actually digit I, and not II, as follows from embryological data.

Obviously, it is the work of these 556 genes that determines the “identity” of the first fingers from a molecular genetic point of view. As for the second and third fingers of the wing, their developmental programs seem to have been formed anew from fragments of the developmental programs of fingers II, III and IV. Therefore, it is impossible to say definitely which toes they are homologous from a molecular genetic point of view. But if we take into account the entire complex of available data (molecular genetic homology of the first fingers, the relative position of the wing fingers, paleontological data), it still turns out that the three fingers of a bird’s wing come from fingers I-II-III, and not II-III- IV. This means that the conflict between the data of embryology and paleontology has been removed.

The results obtained confirmed the previously stated hypothesis, according to which in the evolution of theropod dinosaurs there was a shift in the areas of work of the development programs of digits I-II-III by one embryonic position in the distal direction (from the body), so that digit I began to develop where it was supposed to develop finger II ( Wagner, Gauthier, 1999).

“Nothing is particularly difficult if you divide the work into parts”

We owe this famous saying to Henry Ford. Guided by him, he achieved success. Evolution, following this rule, has repeatedly done difficult work. And she also achieved tremendous success. We see perfect designs of ideal flyers - petrels, swifts; we see superbly designed swift runners - horses, cheetahs; we know eagles with almost magical visual acuity; we are surprised at the accuracy of the hinge joints of robotic insects; We praise the brainy super thinker - the man. They all have complex organs perfectly suited for their purpose: perfect running legs, highly technical joints, efficient muscles, eyes and brains. If they were designed a little differently, if you slightly misaligned their parts, the leg would flounder, the knee wouldn’t bend, the eye wouldn’t see, and the wing wouldn’t fly. How did natural selection cope with the creation of organs that would seem to be useful only when fully assembled? The correct answer is gradually. And at every step of this process, the design turned out to be useful to its owner in one way or another. This could be a consistent improvement of the structure, during which the desired organ served its owner better and better. But it also happened differently: the “unfinished” organ was used completely differently from how today’s happy owner uses it.

Let's look at one striking example. The journey of the ossicles of the first and second gill arches - characteristic of all gnathostomes - into the skull and their transformation into auditory ossicles is like a magic trick. We see in primitive fishes several composite gill (visceral) arches, among which the first one stands out - in gnathostomes it became the upper and lower jaws. There is a jaw joint between the upper and lower jaws. The remaining gill arches are more or less the same. Then the second gill arch acquires its own special role. Connected to the skull, it serves as a pendant for the gill cover. The functions of the first two arches in fish are obvious: the first is the jaws, the second is the respiratory pump. Then, with the transition to air breathing, the need for the gill cover disappears and it is reduced. And the bone of the joint, with the help of which the gill cover was attached to the skull, is sent into exile in the middle ear to transmit sound signals. This is how the first auditory bone appeared - the stapes. And this happened, as we remember, even among the Devonian “four-legged fish”. Both the bone itself and its joint were useful to four-legged fish in a new way.

The first gill arch - fish jaws - is also transformed with the transition to terrestrial life. Its ossified part in the form of a jaw joint still somehow served in its place, but then in mammals it also remained out of use - the joint was renewed at the expense of other bones. This worn-out joint of the former first arch received a new purpose - it also went to the middle ear. In the middle ear, therefore, a company of three bones has gathered, which once fed their owners and helped them breathe, but were displaced from their positions by elements and joints more suitable in the new conditions. These three bones - the malleus, the anvil and the stirrup - did not disappear, but took on new responsibilities for the perception and transmission of sound vibrations. Moreover, they have become one of the main skeletal features by which mammals can be distinguished from all other terrestrial tetrapods. This is the evolutionary route for the auditory ossicles: from the gill arches to the middle ear. If you do not know the intermediate stages of this route, then it seems that such a path is incredible and impossible. Fortunately, the wonderful transformations of the visceral arches can be traced both from embryology and comparative anatomy, and from fossil material.

Fossil primitive mammals show transitions from a reptilian organization with a single auditory ossicle (stapes) to forms that have all three, but two of them (the malleus and incus) still remain attached to the lower jaw. This is a memory of the past when they worked as a jaw joint. The separation of the malleus and incus from the jaw also occurred in stages. This could be guessed based on embryological data. In modern mammals, during embryonic development, the auditory ossicles are first separated from the lateral surface of the lower jaw, but remain connected to it anteriorly through Meckel's cartilage, a remnant of the primary lower jaw. At the second stage, this anterior connection also disappears: Meckel’s cartilage in adult mammals is resorbed. And now such fossil species have been found in which, in adulthood, a partial connection of the auditory ossicles with the lower jaw is preserved. Morganucodon ( Morganucodon) - a primitive representative of the “mammaliaforms” (immediate ancestors of mammals), who lived in the Late Triassic about 205 million years ago. Morganucodon already has all three bones in its middle ear. But the malleus and incus are still connected to the lower jaw both in front and on the sides. And recently, two more ancient animals were found in which the process of separating the auditory ossicles had advanced a little further. This is Yanoconodon ( Yanoconodon allini) and Lyaoconodon ( Liaoconodon hui), primitive mammals that lived 125-120 million years ago (in the Early Cretaceous era) ( Luo et al., 2007; Meng et al., 2011). What remained of them were skeletons in excellent preservation, in which the bones of the middle ear were also preserved.

Yanoconodon and Laoconodon, like most Mesozoic mammals, were small, inconspicuous nocturnal animals. Like any nocturnal animal, they would benefit from keen hearing. Most likely, the nocturnal lifestyle of the first mammals contributed to the evolution of hearing organs. In Yanoconodon and Laoconodon, the auditory ossicles show the stage when the malleus and incus have already separated from the lower jaw laterally, but are still attached to it in front by means of Meckel's cartilage - an ancient fish jaw. Just like at one of the embryonic stages of development of the middle ear in modern mammals. These fossil animals became a real gift to evolutionists - a vivid illustration of the gradual formation of complex organs.

However, it must be borne in mind that three auditory ossicles by themselves do not mean a radical improvement in hearing. In order to further improve this function, it is necessary to add methods for processing sound information to the mechanism for transmitting sound vibrations, and this also did not happen immediately. For example, in one of the primitive mammals Hadrocodium the malleus and incus are already separated from the mandible. But the structure of the inner ear is Hadrocodium still primitive, the same as Morganucodon. Apparently, ancient mammals were not yet first-class listeners. Hearing improved gradually as different elements of the system of perception, transmission and analysis of sound information developed.

First instinct, then philosophy

The brain of mammals (and especially humans) is often compared to a highly complex computer capable of performing many tasks at once. To create such a computer and bring it to such perfection that it works without freezing every minute, and faithfully serves its owner throughout his life... well, how can you do that! However, nature managed to cope with this, gradually increasing capacity and coordinating cognitive tasks. Starting with the neural tube of the first chordates, natural selection gradually reached mammals with large brains and a developed neocortex (new cortex).

The mammalian brain is radically different from the reptile brain not only in size, but also in structure. In particular, in mammals, the olfactory bulbs and parts of the cortex associated with smell, as well as the cerebellum, have sharply increased.

We are especially interested in the process of formation of the neocortex, which allows us, readers, to disassemble and perceive letters, put them into words, analyze information, and then, remembering it, participate in further debates. So how did we mammals get this amazingly complex organ - the neocortex, the cerebral cortex?

The immediate ancestors of the first mammals belong to the group mammaliaform, which, in turn, originate from cynodonts. Paleontologists have reconstructed in detail the stages of evolutionary transformations of the teeth and skeleton of cynodonts as they “mammalized” - a gradual evolutionary movement towards mammals. Much less is known about the evolution of the brain. Meanwhile, it is obvious that it was the development of the brain that largely predetermined the evolutionary success of mammals.

The study of the brain of mammaliaforms and the first mammals was complicated, firstly, by the fact that it was rarely possible to find a well-preserved skull, and secondly, by the fact that to study the endocrane (a cast of the brain cavity, from which one can judge the size and shape of the brain), the skull, as the rule had to be broken.

American paleontologists managed to fill this annoying gap to some extent ( Rowe et al., 2011). Using computed x-ray tomography, without destroying the precious skulls, they obtained detailed three-dimensional images of the endocranes of two mammaliaforms that lived at the beginning of the Early Jurassic (200-190 million years ago) in what is now China.

Mammaliaforms studied Morganucodon oehleri(the already mentioned animal with ready-made middle ear bones) and Hadrocodium wui- the closest relatives of the first mammals. In terms of their skeletal structure, they represent classic transitional forms between “still reptiles” and “already mammals.” Wherein Morganucodon stands closer to primitive cynodonts, and Hadrocodium came as close to mammals as possible while remaining formally outside the group. The study showed that in terms of brain structure, these animals also occupy an intermediate position between typical cynodonts and their descendants - mammals.

The endocranes of early cynodonts were previously studied Thrinaxodon And Diademodon. It turned out that their brain was still quite reptilian in size and structure.

The relative size of the brain in vertebrates is estimated using the "encephalization coefficient" (EQ), which is calculated using the empirically derived formula EQ = EV / (0.055 0.74), where EV is the volume of the brain cavity in milliliters, Wt is body weight in grams.

In basal cynodonts Thrinaxodon And Diademodon EQ varies from 0.16 to 0.23. For comparison, the rat has EQ = 0.53. Their olfactory bulbs were small, and there were no ossified turbinates in the nose, which indicates the poor development of the olfactory epithelium. The forebrain was small and narrow, not divided into sections, without signs of the presence of a neocortex. The midbrain and pineal gland (parietal eye) were not covered from above by the forebrain hemispheres. The cerebellum was wider than the forebrain, the spinal cord was thin. These and other “reptilian” features of the brain and skull of cynodonts indicate that, compared to mammals, they had a weak sense of smell and less than perfect hearing, touch, and coordination of movements.

Brain Morganucodon, as it turns out, was more like a mammal's brain. It is one and a half times larger in volume than the brain of basal cynodonts (EQ = 0.32). The olfactory bulb and olfactory cortex increased the most. This clearly indicates a more developed sense of smell. The forebrain hemispheres became convex due to the development of the neocortex. They cover the midbrain and pineal gland on top, like in mammals. The forebrain of Morganucodon is wider than the cerebellum, although the cerebellum has also grown markedly compared to basal cynodonts. Enlargement of the cerebellum indicates improved coordination of movements. This is also indicated by the thicker spinal cord than in basal cynodonts.

The development of the neocortex in ancient mammals was primarily associated with the improvement of somatosensory functions. A significant part of the neocortex in primitive mammals such as the opossum is the somatosensory cortex, which is responsible for collecting and analyzing signals from the numerous mechanoreceptors scattered throughout the body. Especially many of these receptors are confined to hair follicles.

According to many paleontologists, hair first performed a tactile (tactile) function, and began to be used for thermoregulation later, when the ancestors of mammals began to develop warm-bloodedness. U Morganucodon And Hadrocodium no reliable remains of hair have yet been discovered, but their close relative is the beaver-like mammaliaform Castorocauda(translated from Latin - “with a tail like a beaver”) - was covered with thick fur, consisting of axial hair and undercoat. This suggests that Morganucodon And Hadrocodium were also covered with fur. Probably, the appearance of the neocortex in mammaliaforms was also associated with the development of hair and touch.

Thus, the brain of the primitive Morganucodon illustrates the first stage of progressive brain evolution during the development of mammals. At this stage, the enlargement of the brain was due to the development of the sense of smell, touch and coordination of movements.

Hadrocodium, a more advanced member of the mammaliaforms and the closest relative of “true” mammals, illustrates the second stage of brain development. Encephalization coefficient Hadrocodium is equal to 0.5, i.e. The brain has increased another one and a half times compared to Morganucodon and reached sizes characteristic of some true mammals. The brain grew mainly through the olfactory bulbs and olfactory cortex. Thus, the second stage of the progressive evolution of the brain during mammalization was also associated with the development of smell.

The third stage corresponds to the transition from higher mammalia forms, such as Hadrocodium, to real mammals. At this stage, the sense of smell becomes even more subtle, as evidenced by changes in the ethmoid bone: nasal conchae form on it, supporting the expanded olfactory epithelium.

These data indicate that the need for fine sense was probably the main driver of brain development during the development of mammals. Mammals, as is known, have a much better developed sense of smell than all other terrestrial vertebrates. Most likely, this was initially due to adaptation to a nocturnal lifestyle, when it makes no sense to rely on vision (we talked about this in Chapter 5). In such a situation, it was beneficial to improve the existing ancient olfactory system: to diversify the olfactory receptors and strengthen the brain power for their analysis. Well, even if it was the sense of smell, but as a by-product, mammals received a cortex with a mass of neurons and eventually began to use it to comprehend the changes that had occurred.

Because different parts of the fitness landscape have different “passability”: some are like flat plateaus (where neutral evolution occurs quickly), others are like a labyrinth of narrow paths over abysses (where neutral changes accumulate slowly). See below for more details.

The answer is: \(q = \frac(F^x×k)(1+F^xk) \), where q is the frequency of the A 2 allele after X generations, F is the relative fitness of the A 2 allele compared to the competing allele A 1 (in our case F = 21/20 = 1.05), \(k=\frac(q_(0))(1-q_(0)) \), where q 0 is the frequency of allele A 2 at the initial moment of time. In an infinitely large population, the dynamics of q must exactly correspond to this formula. In small populations the match will be imprecise due to genetic drift.

The slow elimination of mildly harmful mutations can benefit the population. After all, some mutations that are harmful here and now may turn out to be useful in the future. The main thing is that this potential advantage has time to be realized before selection eliminates the mutation. We will look at such collisions in the following chapters.

This corresponds to Ernst Haeckel's famous "biogenetic law", which states that individual development follows evolution. In its rigid and absolutized form, the biogenetic law is incorrect (in biology it is generally dangerous to absolutize anything), but in a softened version it is quite fair.

Sometimes these arches are called the second and third, respectively, since the very first arch - the labial - is absent in modern fish or remains in the form of a rudiment.

Preface
Why is life wonderful?

The amazing complexity of living beings, their fantastic diversity, their almost perfect adaptability to the environment, to each other, to their “place in the economy of nature” are remarkable facts that require explanation. In the past they amazed the imagination no less than now. However, in the pre-scientific era, the explanations were, frankly speaking, simpler: almost any aesthetically balanced invention was suitable for this role.

As science developed, the attitude of literate people towards traditional mythological “explanations” became cooler. “It is in vain that many people think that everything, as we see, was created from the beginning by the Creator... Such reasoning is very harmful to the growth of all sciences, and therefore to natural knowledge of the globe. Although it is easy for these clever people to be philosophers, having learned three words by heart: God created this way, and giving this in response instead of all reasons,” wrote M. V. Lomonosov.

But how can we explain the amazing harmony of living nature without invoking hypotheses about the supernatural? Despite the attempts of many extraordinary minds - from Empedocles to Lamarck - to offer a rational explanation, until 1859 the generally accepted answer to this question remained a resounding “no way.” The complexity and adaptability of living organisms were considered almost the most visual and irrefutable evidence of the divine creation of the world. The "Book of Nature" was called the second Scripture, its study - "natural theology." We read, for example, from the same Lomonosov: “The Creator gave the human race two books. In one he showed his majesty, in the other his will. The first is this visible world, created by him, so that man, looking at the enormity, beauty and harmony of its buildings, would recognize the divine omnipotence of the concept given to himself. The second book is Holy Scripture. It shows the Creator’s favor for our salvation.”

It seemed that the more new facts we discovered, the more clearly we would comprehend the highest plan.

Everything went wrong after the publication of Darwin's book On the Origin of Species by Means of Natural Selection (1859). Before Darwin, humanity knew only one reliable way to create complex, purposefully designed objects: intelligent design. Early evolutionary hypotheses, such as Lamarck's in his Philosophy of Zoology (1809), offered only untestable and incomplete alternatives. For example, Lamarck's idea of ​​inheritance of the results of exercise and disuse of organs offered a rational explanation (albeit incorrect, as we now know) for the increase or decrease of existing structures, but did not explain the origin of new organs. Therefore, in addition to the exercise, Lamarck had to postulate a special driving force that forces organisms to develop along the path of complexity - the mysterious “striving for perfection.” It was little better than God's providence.

Darwin proposed another, much simpler, more elegant and obvious way for the spontaneous improvement of living beings: natural selection of random hereditary changes. Darwin's hypothesis did not postulate any unknown forces or new laws of nature and, in general, seemed to lie on the surface. If objects are able to reproduce, if they pass on their individual traits to their descendants, if these traits sometimes change randomly and, finally, if at least some of these changes increase the efficiency of reproduction, then such objects simply must - and will! - by themselves, without any reasonable intervention, become more and more perfect over generations. In this case, perfection means fitness, aka the efficiency of reproduction.

The scientific world was shocked. True, Laplace (in response to Napoleon’s question, where is God in his model of the solar system) half a century ago declared that he “does not need this hypothesis.” But Laplace was talking about physics. It seemed to almost everyone that there was nowhere in biology without “this hypothesis.”

Of course, expressing a beautiful guess is not enough; to justify it logically is also not enough; the guess must still be correct (and verifiable). Darwin's theory may not have been confirmed in the course of further development of science. But he had a special gift for putting forward correct hypotheses based on incomplete data. Without knowing genetics, without knowing the nature of heredity, long before the discovery of DNA, Darwin was able to correctly formulate the main law of life.

For “natural theology,” Darwin’s book was the beginning of the end. This is precisely what orthodox and fundamentalists will never forgive Darwin (the notorious “man descended from apes” is, in general, a trifle, a special case). The vector of development of biology, and indeed the entire scientific understanding of the world, has changed to the opposite. New discoveries have proven Darwin over creation over and over again. Natural selection, the blind force of nature, triumphed over “intelligent design.” Darwin overturned the universe as it had been previously imagined, replacing a beautiful fairy tale with an equally beautiful, but, alas, more difficult to understand scientific theory.

Darwin's model of selection of small hereditary changes seems to be simple - but its simplicity is only apparent. It is no coincidence that this model, which now underlies biology, entered science so late - in the second half of the 19th century. In other sciences - mathematics, physics, astronomy - theoretical breakthroughs comparable to it in their significance and level began one or two centuries earlier. Even today, there are frequent cases when not the most stupid people “stumble” on the Darwinian model, cannot understand how it works, how it explains the observed facts (and some even think that these facts do not need explanations).

Perhaps one of the reasons for the misunderstanding is lack of attention to detail. Based on general principles, almost any phenomenon can be interpreted one way or another: philosophical reasoning, as we know, is a good tool for justifying directly opposite conclusions. Evolution is counterintuitive. We are accustomed to the fact that everything will work out as it should only with a clearly defined goal and competent leadership. In this sense, it is more comfortable for us to exist when someone has outlined a development plan for five years in advance, and not in the conditions of the cruel elements of the “free market”. We know very well that it is easier to win if the attack is carried out by an army deployed in planned positions, and not by many disparate individuals pursuing their own personal goals. One way or another, intuition usually tells us that without conscious planning and control, nothing good will come of it, only chaos. Evolutionary biology will forever remain incomprehensible to those whose minds cannot free themselves from the captivity of these intuitive sensations.

The task we set ourselves when starting to work on the book was to try, based on new scientific data, to understand how Darwinian selection works. Why And How From the chaos of random hereditary changes, something new, useful, beautiful, harmonious and complex is born. Here it is important to abandon general reasoning and conversations at the level of “common sense” - they will convince few people now. We must take a closer look at the facts, details, examples, from which the mechanism of the great historical engine - natural selection - will appear in all its complexity and logic.

How new genes, new traits, new adaptation, new species, new types? What is the general biological meaning of these words: new, useful, beautiful, harmonious, complex? After all, all these terms in biology have special shades of meaning. What is considered a real “innovation” - is it the acquisition of a new mutation, a new appearance, a new gene, a new function or a new place of residence? Try to answer such questions on the fly... What is “beauty” from the point of view of a bee or a colored lake fish? It’s probably not the same as for the jury of the Miss World competition. To understand the structure of living nature, in order to understand the meaning of all its components and interrelations, it is necessary first of all to understand their evolutionary context. We want to see evolution up close. We want to unscrew the evolutionary mechanism into all its cogs and gears, study them, understand how they connect, and then screw them back together and make sure that it is still ticking. But this work will give us an idea of ​​the whole device - if it ticks, then we understand its mechanics.

In The Birth of Complexity, our first book in evolutionary biology, the emphasis was on “challenging dogma.” Indeed, many conclusions that seemed absolute 50 years ago now have to be revised. It became clear that life is more complicated than it seemed just recently. In the natural sciences in general, and in biology in particular, there is a problem with absolute truths. There are exceptions to any rule. On the other hand, The Birth of Complexity may create a kind of “dogma-refuting bias” in the minds of readers. One might think that too much of what the classics of evolutionary biology wrote about has not stood the test of time.

So, correcting this imbalance is another task of the book you are holding in your hands. After all, in fact, classical ideas are not so much refuted by new discoveries as they are concretized, refined and developed. Thus, paleontologists can correct the systematic position of trilobites as much as they like, bringing them closer to crustaceans, then to arachnids, or separating them into a separate subtype - it does not at all follow from this that our knowledge about trilobites is unreliable or that science is marking time, lost in conjectures, - on the contrary, these processes reflect an increasingly complete and correct scientists' understanding of this extinct group of animals, and the most fundamental, classical truths remain unshakable and are only strengthened (for example, the belief that trilobites are representatives of arthropods, which means that the last common ancestor of the trilobite and the fly lived later than the last common ancestor of the trilobite and the sparrow ). Classic ideas are often classic because they have been reliably confirmed from many angles. They allow us to develop and modify ideas about the world without any damage to them. This, of course, is the best version of “classical ideas”: sometimes truly outdated dogmas are successfully disguised as them. Both are boring clichés, but what can you do - these are the ones you encounter every now and then in scientific life. One way or another, those classic ideas that will be discussed in the book are classics in the good sense of the word. We will try to support this statement with the latest scientific data.

In recent years, biologists have received a whole body of new data that has allowed them to better understand how the gears of the evolutionary mechanism turn. Miracles are happening right before our eyes. The skeletons of theories are overgrown with the flesh of real facts. Many beautiful hypotheses and models, which until now could not be verified experimentally, have finally been subjected to such testing. Regularities that until now existed only in the imagination of theorists, we can now examine through a microscope. We can use them! “Applied evolutionary biology” is no longer fiction, but reality. The book is dedicated to such discoveries.

We have to look at specific examples of how heredity, variability, selection, competition, isolation, drift and other components of the great natural machine work, tirelessly creating new types of living beings.

We hope that the reader already has minimal knowledge of biology - and if he does not, he will be able to look up the missing information himself in available sources, for example, in a textbook or on Wikipedia. In the end, you can't each a popular science book to repeat the same information from the school curriculum. It’s a pity for the time, paper and those readers for whom this will not be the first biological book they have picked up. Therefore, we will not retell in detail for the hundredth time what DNA replication and a cell membrane are, but will get straight to the point.

A few terms you can't do without

The hereditary information contained in DNA is heterogeneous and written in several different “languages.” Best learned language protein-coding regions of DNA. The sequence of nucleotides in such a region represents instructions for the synthesis of a protein molecule, written using genetic code- a system of correspondences common to all living things between certain triplets of DNA nucleotides (triplets, or codons) and the amino acids that make up the protein. For example, the triplet of nucleotides AAA codes for the amino acid lysine, and CGG for arginine.

To synthesize a protein based on such instructions, information must first be rewritten from DNA to RNA - a molecule that differs from DNA in some details: for example, instead of the nucleotide T (thymidine), RNA uses U (uridine). Rewriting information from DNA to RNA (RNA synthesis on a DNA template) is called transcription. A gene can be transcribed frequently, and then the cell will produce many molecules of this protein, or rarely, and then there will be little protein. It's called expression level gene. The expression level is controlled by special regulatory proteins.

The resulting RNA molecule is then used to synthesize protein. The molecular “machine” for protein synthesis based on instructions written in RNA is called ribosome, and the process of protein synthesis itself - broadcast.

Rated the book

I like to switch things up sometimes. Biology is not within the scope of my standard interests, therefore, unlike books on history and economics, Markov’s books (and this is the 4th book of his that I have read) you read thoughtfully, slowly, going back every now and then, connecting individual passages into a complete picture.

"Evolution", in fact, is a direct continuation of "The Birth of Complexity", since it talks about the same thing, but from a different angle and taking into account new facts that have accumulated over 5 years from the date of publication of the first book. It is curious that Alexander Markov finally decided to officially list his wife as a co-author of the book (before this, her name was not on the cover, and the sections she wrote were specially noted).

From the very first pages my head began to swell with new and incredibly interesting logical abstractions. Protein universe, fitness landscape, Möller's ratchet, Müllerian mimicry, Dobzhansky-Möller model. Considering that all this was told in an interesting, lively and clear language, the process of mastering new meanings went very well.

I cannot help but mention the rich imaginative thinking of the authors. Mentioning the eyes of calcite trilobites, the authors talk about the “real stone gaze” of these animals. And speaking of flounder, the authors cite a stunning quote from Babel: “with a crumpled, sleepy face on the edge.” Babel, to put it mildly, was not talking about flounder: “And even Kolya Schwartz brought with him his wife in a purple shawl with fringe, a woman fit to be a grenadier and long as the steppe, with a wrinkled, sleepy face on the edge.”

The authors painstakingly lead to the idea that any macro changes consist of many micro changes. And so, as the story progresses, we train long enough on bacteria and worms to then gallop through magnificent chapters on the evolution of color vision and the development of limbs in fish.

Well, in fact, there is a wild temptation to apply many of the discovered patterns not only to biological, but also to social (and even technical) evolution. The repeated emergence of multicellular life (what if not economic formations?), the victory in the arms race of species that are less adapted at the current moment, but with a great potential for “modernization” (combat aircraft?), a mechanism for strengthening small differences when one species diverges into several (the formation of East Slavic peoples?). The conclusion is simple, the book is very good because it makes you think.

Rated the book

Markov's previous books turned out to be very important for me. As some of the most fascinating and vibrant representatives of the genre, they shaped my interest in evolution in general. The author was extremely friendly to ordinary people, explained the topic, as a rule, in a simplified and exciting way, exactly as needed. And sometimes, just sometimes, if something works, it doesn’t need to be improved.
Finally, it was time for the next book and I was looking forward to how the author, in his own words, would tell me about the latest news in the study of evolution. Just then a re-release arrived, which gave me confidence in the book’s fascination.
I bought Markov in paper for the first time, I was so sure that I would want to put it on the shelf. And I stopped reading before the first quarter of the book, but I’ll still keep it and it will stand on the shelf wherever this shelf takes it. I finished reading it after a long break; I had to come to terms with the fact that it was now written differently, too academically.
The text immediately greets you with a warning that you don’t want to believe: “It won’t be easy” (I’m roughly conveying this) the POPULAR science book tells us, completely forgetting what it is and where it is. We are asked to prepare for an abundance of terms and a high barrier to entry, but why this was done remains unclear to me. It is probably assumed that the audience has become smarter, but this is a very naive hope. Gentlemen, authors, we are still as stupid as before and there is an insurmountable wall between us and science, you, being inside the territory of knowledge, climbed this wall in order to tell us what is going on there and got through. But suddenly they decided that our hearing has become better and it’s no longer worth climbing the insurmountable wall and we can just scream on our own, but we haven’t become more sensitive, we just hear less now.
To put it simply, the book is essentially a collection of new research, with a small summary and short conclusions but an overly detailed and detailed description of the process itself. The terminology here unfolds in all its glory, but the structure suffers and does not look like a coherent work. Of course, all parts are individually interesting, but the overall picture is not formed, the book is very unfriendly.
There is no need to do this, work through and adapt the text for ordinary people, even if it is a little further from the essence but much clearer and more exciting, there are more conclusions and lyrical digressions; we ourselves cannot always draw these conclusions.