The rise of eukaryotes on Earth began about 1 billion years ago, although the first of them appeared much earlier (perhaps 2.5 billion years ago). The origin of eukaryotes could be associated with the forced evolution of prokaryotic organisms in an atmosphere that began to contain oxygen.

Symbiogenesis - the main hypothesis of the origin of eukaryotes

There are several hypotheses about the origins of eukaryotic cells. The most popular - symbiotic hypothesis (symbiogenesis). According to it, eukaryotes arose as a result of the union of different prokaryotes in one cell, which first entered into symbiosis, and then, increasingly specializing, became organelles of a single organism-cell. At a minimum, mitochondria and chloroplasts (plastids in general) have a symbiotic origin. They originated from bacterial symbionts.

The host cell could be a relatively large anaerobic heterotrophic prokaryote, similar to an amoeba. Unlike others, it could acquire the ability to feed by phagocytosis and pinocytosis, which allowed it to capture other prokaryotes. They were not all digested, but supplied the owner with the products of their vital activity). In turn, they received nutrients from it.

Mitochondria originated from aerobic bacteria and allowed the host cell to switch to aerobic respiration, which is not only much more efficient, but also makes it easier to live in an atmosphere containing a fairly large amount of oxygen. In such an environment, aerobic organisms gain an advantage over anaerobic ones.

Later, ancient prokaryotes similar to living blue-green algae (cyanobacteria) settled in some cells. They became chloroplasts, giving rise to the evolutionary branch of plants.

In addition to mitochondria and plastids, flagella of eukaryotes can have a symbiotic origin. They became symbiont bacteria, like modern spirochetes with a flagellum. It is believed that centrioles, such important structures for the mechanism of cell division in eukaryotes, subsequently emerged from the basal bodies of flagella.

The endoplasmic reticulum, Golgi complex, vesicles and vacuoles may have originated from the outer membrane of the nuclear envelope. From another point of view, some of the listed organelles could have arisen by simplifying mitochondria or plastids.

The question of the origin of the nucleus remains largely unclear. Could it also have formed from a prokaryotic symbiont? The amount of DNA in the nucleus of modern eukaryotes is many times greater than that in mitochondria and chloroplasts. Perhaps part of the genetic information of the latter moved to the nucleus over time. Also, during the process of evolution, there was a further increase in the size of the nuclear genome.

In addition, in the symbiotic hypothesis of the origin of eukaryotes, not everything is so simple with the host cell. They might not be just one type of prokaryote. Using genome comparison methods, scientists conclude that the host cell is close to archaea, while combining the characteristics of archaea and a number of unrelated groups of bacteria. From this we can conclude that the emergence of eukaryotes occurred in a complex community of prokaryotes. In this case, the process most likely began with methanogenic archaea, which entered into symbiosis with other prokaryotes, which was caused by the need to live in an oxygen environment. The appearance of phagocytosis promoted the influx of foreign genes, and the nucleus was formed to protect the genetic material.

Molecular analysis has shown that different eukaryotic proteins come from different groups of prokaryotes.

Evidence for symbiogenesis

The symbiotic origin of eukaryotes is supported by the fact that mitochondria and chloroplasts have their own DNA, which is circular and not associated with proteins (this is also the case in prokaryotes). However, mitochondrial and plastid genes have introns, which prokaryotes do not.

Plastids and mitochondria are not reproduced by the cell from scratch. They are formed from pre-existing similar organelles through their division and subsequent growth.

Currently, there are amoebas that do not have mitochondria, but instead have symbiont bacteria. There are also protozoa that cohabit with unicellular algae, which act as chloroplasts in the host cell.

Invagination hypothesis of the origin of eukaryotes

In addition to symbiogenesis, there are other views on the origin of eukaryotes. For example, intussusception hypothesis. According to it, the ancestor of the eukaryotic cell was not an anaerobic, but an aerobic prokaryote. Other prokaryotes could attach to such a cell. Then their genomes were combined.

The nucleus, mitochondria and plastids arose through the invagination and detachment of sections of the cell membrane. Foreign DNA entered these structures.

The complexity of the genome occurred in the process of further evolution.

The invagination hypothesis of the origin of eukaryotes well explains the presence of a double membrane in organelles. However, it does not explain why the protein biosynthesis system in chloroplasts and mitochondria is similar to the prokaryotic one, while that in the nuclear-cytoplasmic complex has key differences.

Reasons for the evolution of eukaryotes

All the diversity of life on Earth (from protozoa to angiosperms to mammals) gave rise to eukaryotic, not prokaryotic, cells. The question arises, why? Obviously, a number of features that arose in eukaryotes significantly increased their evolutionary capabilities.

First, eukaryotes have a nuclear genome that is many times larger than that of prokaryotes. At the same time, eukaryotic cells are diploid; in addition, in each haploid set, certain genes are repeated many times. All this provides, on the one hand, a large scale for mutational variability, and on the other hand, it reduces the threat of a sharp decrease in viability as a result of a harmful mutation. Thus, eukaryotes, unlike prokaryotes, have a reserve of hereditary variability.

Eukaryotic cells have a more complex mechanism for regulating life activity; they have significantly more different regulatory genes. In addition, DNA molecules formed complexes with proteins, which allowed hereditary material to be packaged and unpacked. All together, this made it possible to read information in parts, in different combinations and quantities, at different times. (If in prokaryotic cells almost all the genome information is transcribed, then in eukaryotic cells usually less than half.) Thanks to this, eukaryotes could specialize and adapt better.

Eukaryotes developed mitosis and then meiosis. Mitosis allows the reproduction of genetically similar cells, and meiosis greatly increases combinative variation, which speeds up evolution.

Aerobic respiration, acquired by their ancestor, played a major role in the prosperity of eukaryotes (although many prokaryotes also have it).

At the dawn of their evolution, eukaryotes acquired an elastic membrane, which provided the possibility of phagocytosis, and flagella, which allowed them to move. This made it possible to eat more efficiently.

At present, the theory of the symbiotic origin of the eukaryotic cell can be considered almost universally accepted (although some voices continue to be heard against it: see, for example, Cavalier-Smith, 2002). Convincing data have been collected proving that mitochondria originate from symbiotic aerobic eubacteria (alpha-proteobacteria), plastids - from symbiotic photosynthetic eubacteria (cyanobacteria) (Kuznetsov, Lebkova, 2002). Unfortunately, there are practically no other firmly established facts. Disagreement remains over the nature of the host cell and the origin of the cytoplasm and nucleus. Most experts are inclined to believe that the host cell belonged to archaeobacteria (Margulis and Bermudes, 1985; Vellai and Vida, 1999). This is evidenced by the great similarity in the genome structure (in particular, its exon-intron organization) in archaeobacteria and eukaryotes, the similarity of the mechanisms of replication, repair, transcription and translation, as well as other molecular data (Cavalier-Smith, 2002; Slesarev et al., 1998).

During the formation of a symbiotic organism, a chimeric (archaeobacterial - eubacterial) genome was formed, in which many metabolic systems were duplicated (Gupta, 1997). The redundant elements were subsequently reduced or changed function (Martin and Schnarrenberger, 1997). In particular, of the two mechanisms of membrane formation (in archaeobacteria, the membranes are based on isoprenoid esters, and in eubacteria, on fatty acid esters), only one was “selected” and preserved – the eubacterial one. According to one hypothesis, the formation of a nuclear membrane could be a by-product of the expression of eubacterial genes responsible for membrane synthesis in the archaeobacterial genetic environment (Martin and Russel, 2003). There are many other hypotheses, even very extravagant ones, such as, for example, the assumption of the formation of a cell nucleus in an archaeobacterium as a result of a viral infection (Takemura, 2001).

A serious problem is the absence in prokaryotes of obvious analogues of such an integral element of the eukaryotic cell as the cytoskeleton, consisting of microtubules (mitotic spindle, flagella, etc.), and the associated ability for phagocytosis. An attractive hypothesis is the origin of microtubular cytoskeletal structures (in combination with the nuclear membrane) as a result of the symbiosis of an archaeobacterium with a motile eubacterium. Although this hypothesis is not universally accepted, it continues to develop and finds new supporters (Dolan et al., 2002). Other hypotheses are also proposed, for example, about the existence in the past of a special group of prokaryotes - “chronocytes”, which were neither bacteria nor archaea; chronocytes had a cytoskeleton and were capable of phagocytosis; they ingested various bacteria and archaea and gave rise to eukaryotes (Hartman and Fedorov, 2002).

Eukaryotes appear to be a monophyletic group, i.e. The “successful fusion” of archaeobacteria with eubacteria into a single organism, giving rise to all eukaryotes, occurred only once in the history of the Earth (Gupta, 1997). Analysis of mitochondrial genomes also showed the monophyletic origin of mitochondria of all modern eukaryotes (Litoshenko, 2002).

The symbiotic nature of eukaryotes has led some researchers to seriously think that perhaps the phenomena of symbiogenesis play a much larger role in evolution than is commonly believed; that the emergence of eukaryotes, lichens and reef-building corals with their zooxanthellae are by no means curiosities or exceptions, but perhaps simply the most obvious manifestations of a general law that guided many other macroevolutionary events, albeit in a less obvious form (Margulis, Bermudes, 1985).

Hypothetical community of prokaryotes – the “collective ancestor” of the eukaryotic cell

Some data on the hypothetical primary biotope of eukaryotes can be obtained by considering the ecology of those bacterial communities in which the first eukaryotic organisms could have arisen.

It is not entirely correct to consider several individual species of prokaryotes united into a symbiotic organism as the ancestors of the first eukaryotic organisms. Following a systemic understanding of life and biological evolution, it would be more accurate to say, paradoxical as it may sound, that the ancestor of eukaryotes was a community of prokaryotic organisms, which included at least three components: 1) anaerobic heterotrophs, in all likelihood, representatives of archaeobacteria with an exon-intron genome organization, received energy through oxygen-free fermentation of carbohydrates (glycolysis); 2) aerobic heterotrophs - eubacteria that received a large amount of energy due to the oxygen oxidation of low molecular weight carbohydrates (in particular, pyruvate, lactic acid or ethanol, which were the end products of energy metabolism of the first component of the community); 3) anaerobic autotrophs - photosynthetics (cyanobacteria), which provided the first component of the community with high molecular weight carbohydrates, and the second with oxygen.

The benefits of such a triple symbiosis are quite obvious: each of the three components of the community directly benefits from cohabitation with the other two. Cyanobacteria and archaeobacteria get rid of excess oxygen, which is toxic to them, as well as the end products of oxygen-free energy metabolism; archaeobacteria and aerobic eubacteria receive the organic matter they need for nutrition, and the latter also receive oxygen as a powerful oxidizing agent, allowing them to utilize organic compounds with high efficiency.

In the surface millimeter layer of cyanobacterial mats, widespread already in the Archean, the oxygen concentration was sufficient for the existence of aerobic organisms, despite the fact that there was almost no oxygen in the atmosphere at that time. Oxygen, a byproduct of photosynthesis, is inherently toxic to cyanobacteria, and they have evolved a number of biochemical defenses against this aggressive metabolite (Paerl, 1996). One of these methods was probably aerobic respiration. Interestingly, the most important part of the mechanism of cellular respiration - the electron transport chain - appeared as a result of modification of one of the enzyme systems of photosynthesis (Nakamura, Hase, 1990-91). Modern cyanobacteria prefer to live in communities with microorganisms that actively absorb oxygen, and the rates of oxygen release and consumption in such communities are strictly coordinated (Paerl, 1996). Apparently, it was the surface layer of cyanobacterial mats with a high content of high-molecular carbohydrates and oxygen that was the “cradle” in which first communities of heterotrophic (aerobic and anaerobic) prokaryotes, and then the first eukaryotes, arose (Rozanov and Fedonkin, 1994).

It can be noted that a hypothetical bacterial community, including these three components, already had great advantages compared to a “pure” cyanobacterial mat, even without the association of these components under a common cell membrane, i.e. without the formation of eukaryotic cells.

Most likely, the formation of a eukaryotic cell was preceded by a long period of coevolution of the components of such a community, during which interaction mechanisms were formed and improved, gradually turning the bacterial biocenosis into a single quasi-organism.

Communities of microorganisms can achieve an unusually high degree of integration. Widespread horizontal gene exchange among prokaryotes makes microbial biocenoses, including different types of bacteria, in many ways similar to populations of higher organisms. Other integrating mechanisms include systems of chemical signals that prokaryotes exchange among themselves to coordinate the behavior (i.e., phenotypic expression) of community elements; some eukaryotes have learned to “imitate” such signals and thus regulate the life activity of prokaryotes in their environment (Rice et al., 1999). It is also necessary to note the development in bacteria of the mechanism of apoptosis - programmed cell death (Endelberg-Kulka, Glaser, 1999); The ability for “self-sacrifice” of individual organisms for the benefit of the community, in our opinion, is a sign of a high degree of integrity (individualization) of microbial communities.

Therefore, it is logical to assume that many integrating mechanisms that ensure the integrity and coordinated operation of parts of a eukaryotic cell (primarily signaling and regulatory cascades) began to develop long before these parts actually united under one cell membrane. Note also that the mechanism of horizontal gene exchange could serve as an important pre-adaptation for the subsequent transfer of most mitochondrial and plastid genes into the nucleus.

The hypothetical "ancestral community" of eukaryotes was probably something like an "improved" cyanobacterial mat, including heterotrophic anaerobic archaeobacteria and heterotrophic aerobic eubacteria as symbionts. In such a community, compared to a “pure” cyanobacterial mat, fluctuations in the concentrations of various substances should have been noticeably reduced; excess oxygen harmful to cyanobacteria was removed; a certain amount of carbon dioxide necessary for photosynthesis was produced (which could be very significant due to the sharp decrease in its concentration in the atmosphere at the turn of the Archean and Proterozoic); excess organic matter was utilized; perhaps the numbers of cyanobacteria were controlled to some extent by predation, etc. In general, it can be assumed that such communities were more stable, which ensured their spread during the crisis period under consideration. Modern prokaryotic communities are known, including various cyanobacteria and heterotrophic bacteria, which can to some extent be considered as analogues of the “ancestral communities” of eukaryotes. These bacterial mats are found in various extreme habitats and are characterized by increased ecological tolerance and a high degree of integration of their components, in particular, their complex spatial distribution along gradients of oxygen, pH and other important parameters (Paerl et al., 2000).

Microbial communities involving a variety of true eukaryotes should have acquired even greater stability. Perhaps it was the emergence of such communities that led to a new outbreak of development of stromatolite formations after a minimum observed in the interval 2.5-2.3 billion years ago, and the stromatolites themselves became more massive than in the Archean (Semikhatov et al., 1999). Otherwise, it is difficult to explain the fact that there were more stromatolites in the Proterozoic compared to the Archean, although abiotic conditions clearly became less favorable for cyanobacteria due to a decrease in temperature and carbon dioxide concentration.

The obvious advantages of a eukaryotic cell compared to a community consisting of its free-living components include, first of all, the unified, centralized regulation of the genomes of all symbionts. A eukaryotic cell is actually a small, compact community with centralized regulation of biochemically complementary components. It was mentioned above that eukaryotes developed a more advanced system for maintaining DNA stability and repair, which resulted in a decrease in the frequency of mutations. Perhaps the first effective DNA defense and repair systems evolved in archaeobacteria that lived in extreme conditions (Grogan, 1998). These could be hyperthermophiles (note that at the end of the Archean, a strong warming probably occurred (Sorokhtin, Ushakov, 2002)), or inhabitants of the surface layers of water, where hard ultraviolet radiation made it difficult for eubacteria to exist. The transition to intracellular symbiosis made it possible for eubacteria (future plastids and mitochondria) to transfer their genomes “under the protection” of the repair systems of the archaeobacterial host cell. Perhaps it was this need that stimulated the rapid transition of most mitochondrial and plastid genes into the nucleus.

The early development of systems for regulation, stabilization and protection of the genome in eukaryotes (and partly in their archaeobacterial ancestors) gives reason to believe that the “ancestral” microbial communities discussed above, as well as the first mixed prokaryotic-eukaryotic communities, initially developed in conditions that were not entirely favorable for “ordinary” Archaean cyanobacterial mats. Perhaps this occurred in relatively shallow-water habitats, where environmental conditions were unstable and ultraviolet radiation was more intense. These factors could give microorganisms with a more effective system of defense and genome regulation a selective advantage. This point of view contradicts the previously expressed hypothesis about the deep-sea origin of eukaryotes (Bernhard et al., 2000) (this hypothesis was based, in fact, simply on the fact that it is generally more difficult to live under conditions of hard ultraviolet radiation). The assumption of more or less extreme conditions in the ancestral biotope of eukaryotes is consistent with the opinion expressed by a number of authors that the archaeobacteria that became the basis of the symbiotic organism were acidothermophilic (Dolan et al., 2002). In this regard, it should be recalled that at the end of the Archean, according to the model of Sorokhtin and Ushakov (2002), the water in the ocean could have a temperature of up to 60 o C and a pH of the order of 3-5. Modern cyanobacterial mats, which include a variety of heterotrophic symbionts and live mainly in extreme habitats (Paerl et al., 2000), can be considered ecologically and structurally as analogues of the “ancestral eukaryotic community.”

Fifty years ago, in 1967, Lynn Margulis published an extensive account of the symbiogenetic theory, according to which eukaryotes (organisms with cell nuclei) arose as a result of a series of associations of different cells with each other. A modern amendment to this theory states that the formation of eukaryotes, apparently, was not a general trend that spanned many evolutionary branches (as Margulis assumed), but a unique event that led to the fusion of archaeal and proteobacterial cells. As a result, a complex cell with mitochondria was formed, which became the first eukaryote. Further symbiogenetic events - for example, the capture of algae that became chloroplasts - did occur many times, but they are not associated with the emergence of eukaryotes as such.

More than fifty years ago, in March 1967, the international Journal of Theoretical Biology published an article “On the origin of cells dividing by mitosis” (L. Sagan, 1967. On the origin of mitosing cells). The author of the article was named Lynn Sagan, but this remarkable woman later became much better known as Lynn Margulis. She bore the surname Sagan because she was briefly married to Carl Edward Sagan, an astronomer and writer.

The publication of an article by Lynn Margulis in 1967 (we will call her that for convenience) became the beginning of a renewal of biological concepts, which many authors regarded as a paradigm shift - that is, in other words, as a real scientific revolution (I.M. Mirabdullaev, 1991. Endosymbiotic theory - from fiction to paradigm). The essence of the intrigue here is simple. Since the time of Charles Darwin, biologists have been convinced that the main method of evolution is divergence - the divergence of branches. Lynn Margulis was the first to truly explain to the scientific community that the mechanism of some major evolutionary events was likely fundamentally different. Margulis' interests focused on the problem of the origin of eukaryotes - organisms whose cells have a complex internal structure with a nucleus. Eukaryotes include animals, plants, fungi and many single-celled organisms - amoebas, flagellates, ciliates and others. Margulis showed that the early evolution of eukaryotes was not at all reduced to divergence - it included the merging of evolutionary branches, and more than once. The fact is that at least two types of eukaryotic organelles - mitochondria, thanks to which we can breathe oxygen, and chloroplasts, which carry out photosynthesis - do not come from the same ancestor as the main part of the eukaryotic cell (Fig. 1). Both mitochondria and chloroplasts are former bacteria, initially completely unrelated to eukaryotes (proteobacteria in the case of mitochondria and cyanobacteria in the case of chloroplasts). These bacteria were absorbed by the cell of an ancient eukaryote (or the ancestor of eukaryotes) and continued to live inside it, retaining their own genetic apparatus for the time being.

Thus, a eukaryotic cell is, as Margulis puts it, multigenome system. And it arose as a result of symbiosis, that is, the mutually beneficial cohabitation of different organisms (more precisely, endosymbiosis, one of the participants of which lives inside the other). The corresponding evolutionary branches, of course, merged. This view of evolution is called the theory of symbiogenesis.

Now the theory of symbiogenesis is generally accepted. It has been confirmed as strictly as any theory concerning large-scale evolution can be confirmed. But scientific concepts, unlike religious dogmas, never remain static. Naturally, the overall picture of symbiogenesis now looks to us not quite the same (and in some places not at all the same) as Lynn Margulis imagined it half a century ago.

Logic classic

On the fiftieth anniversary of the publication of the famous article on symbiogenesis Journal of Theoretical Biology prepared a special issue entirely dedicated to the creative legacy of Lynn Margulis. This issue includes a comprehensive article by renowned British biochemist and science popularizer Nick Lane, which compares the current state of the issue of the origin of eukaryotes with classical ideas on the topic. Lane has no doubt that Margulis was right in the main statements (concerning the origin of mitochondria and chloroplasts); in our time, it seems, none of the serious scientists doubt this, because the data of molecular biology on this matter are unambiguous. But the devil, as we know, lives in the details. In this case, by diving into the details, we can find a lot of new and interesting things there, and most importantly, make sure that the topic of the origin of eukaryotes is far from exhausted.

Let's start with the fact that some of Margulis' private assumptions turned out to be incorrect. This is normal: given the enormous speed of development of biology, it is simply incredible that absolutely everything was accurately guessed in an article published half a century ago. New facts that could not have been known to the author at the time will certainly make some adjustments. This is what happened here too. First of all, Margulis insisted on the symbiotic origin not only of mitochondria and chloroplasts, but also of eukaryotic flagella. She believed that the ancestors of flagella were long, spirally twisted, motile bacteria attached to a eukaryotic cell, similar to modern spirochetes (see Fig. 1). Alas, this hypothesis did not receive any molecular biological confirmation, and now no one supports it anymore.

In some moments, Margulis could have been right (this is not prohibited either by the laws of nature or by the internal logic of her own theory), but nevertheless, for reasons beyond her control, she missed. For example, she believed that since mitochondria are descendants of bacteria, sooner or later biologists will learn to cultivate them in a nutrient medium outside eukaryotic cells - well, like ordinary microbes. If this were possible, it would be ideal proof of the theory of symbiogenesis. Alas, in fact, modern mitochondria are fundamentally incapable of independent survival, because most of their genes, during evolution, migrated to the cell nucleus and were integrated there into the genome of the eukaryotic “host”. Now the protein products of these genes are synthesized outside the mitochondrion, and then transported into it using special transport systems belonging to the eukaryotic cell. The genes remaining in the mitochondria itself are always few in number - they are not enough to support life. In 1967, no one knew this yet.

However, by and large, all this is particular. Lynn Margulis's thinking was synthetic: she was not limited to explanations of individual facts, but sought to combine them into an integral system that described the evolution of living organisms in the context of the history of the Earth (Fig. 2). Modern scientific knowledge makes it possible to test this system of ideas for strength.

Tree and network

It all started with oxygen. There was no molecular oxygen (O 2) in the Earth's ancient atmosphere. Then cyanobacteria, which were the first to master oxygen photosynthesis, began to release this gas into the atmosphere (for them it was simply an unnecessary by-product). Meanwhile, pure oxygen is a very toxic substance for anyone who does not have special biochemical means of protection against it. Unsurprisingly, oxygen emissions from cyanobacteria poisoned the Earth's atmosphere and led to a mass extinction. The “oxygen holocaust” began (L. Margulis, D. Sagan, 1997. Microcosmos: four billion years of microbial evolution).

An amendment is already needed here. Many modern researchers believe that the transition from an oxygen-free biosphere to an oxygen one was in fact much more gradual and less destructive than speculation about the “oxygen holocaust” suggests (see, for example: “The Great Oxygen Event” at the turn of the Archean and Proterozoic was not great, not an event, “Elements”, 03/02/2014). Moreover, it is possible that the appearance of free oxygen is even more likely increased diversity of microorganisms, because the oxidation of a number of minerals by atmospheric oxygen has enriched the chemical composition of the environment and created new ecological niches (M. Mentel, W. Martin, 2008. Energy metabolism among eukaryotic anaerobes in light of Proterozoic ocean chemistry). In general, the idea of ​​the appearance of oxygen in the atmosphere as a one-time grandiose catastrophe that divided the entire history of the Earth into “before” and “after” now seems to be outdated.

One way or another, there is no doubt that alpha-proteobacteria benefited most from the enrichment of our planet with oxygen. They have learned to directly use oxygen to produce energy - and with great efficiency. But the single-celled ancestors of eukaryotes did not have such an ability. They were anaerobic, that is, they could not breathe oxygen. But they were predators who learned to absorb smaller cells through phagocytosis. And this gave them an excellent opportunity: to capture some bacteria, not digesting them, but “enslaving” them and appropriating the products of their metabolism. Having absorbed the alpha-proteobacterium, primitive eukaryotes were able to breathe oxygen - this is how mitochondria were formed. And having absorbed the cyanobacterium, it was able to photosynthesize - this is how chloroplasts were formed. Margulis believed that such events occurred many times, following a general trend that emerged. This is the so-called script serial endosymbiosis.

So, Margulis turns out that at a certain stage in the development of life, endosymbiosis became almost a universal pattern. Then, at the base of the evolutionary tree of eukaryotes there should be literally a whole network of evolutionary branches, intersecting with each other due to endosymbiotic events and “growing” in approximately the same direction - in the one that was dictated by the combination of the then external conditions with the structural features of the cells (Fig. 3, A ).

It must be said that by the end of the 20th century in evolutionary biology (and especially in paleontology), the idea that most major evolutionary events have a natural and systemic character had already gained some popularity. Such an event covers many evolutionary branches at once, in which, under the influence of common heredity, approximately the same characteristics arise in parallel (see, for example: A. G. Ponomarenko, 2004. Arthropodization and its ecological consequences). Examples of such events were called mammalization (the origin of mammals), angiospermization (the origin of flowering plants), arthropodization (the origin of arthropods), tetrapodization (the origin of terrestrial vertebrates), ornithization (the origin of birds) and much more. It seemed that the formation of eukaryotes - eukaryotization - fits perfectly into this series.

For example, Kirill Eskov in his wonderful book “The History of the Earth and Life on It” (written in the 1990s) says the following: “Most likely, different variants of eukaryoticity, that is, intracellular colonies, arose many times (for example, there is reason to believe that red algae, which differ sharply from all other plants in many key characteristics, are the result of such “independent eukaryotization” of cyanobacteria)” (K. Yu. Eskov, 2000. History of the Earth and life on it).

Alas, in relation to eukaryotes (we are not discussing other examples of “-ations” now), modern data cast doubt on this beautiful scenario.

Mitochondria problem

Let's start with the fact that the hypothesis about red algae discussed by Eskov is now outdated. Molecular studies show that the evolutionary lineage of red algae lies deep within the eukaryotic tree (they are fairly close relatives of green plants), and their independent eukaryoticization is extremely unlikely.

But something else is much more serious. If symbiogenesis was a natural, long, multi-stage process, and even proceeded in parallel in different evolutionary branches, then we would expect that we would see a spectrum of quite diverse transition states between eukaryotes and non-eukaryotes. That's exactly what Margulis thought. The fact that these transitional states are not noticeable, she (as far as can be judged) considered a purely technical problem associated with a lack of knowledge and imperfect methods. Is this true now that we know immeasurably more about living cells than we knew fifty years ago?

Let's speculate. The supposed serial endosymbiosis was supposed to proceed, firstly, gradually, and secondly, slightly differently in different evolutionary lines (since there are no exact repetitions in evolution). Based on this, Margulis predicted that sooner or later eukaryotes would be discovered that had chloroplasts, but never had mitochondria; eukaryotes that have retained bacterial flagella (which differ sharply in structure from the flagella of eukaryotes); and finally, primarily anaerobic eukaryotes, in whose cells there are no traces of adaptation to an oxygen atmosphere. None of these predictions were confirmed. None of the eukaryotes have even a hint of bacterial-type flagella - their means of movement are completely different. None of the known eukaryotes can be called a primary anaerobe - all of them, without exception, went through the “oxygen phase” at some point in their evolution. Finally, all eukaryotes have either active mitochondria, or their remnants that have lost a significant part of their functions (hydrogenosomes, mitosomes), or - at worst - mitochondrial genes that have managed to move into the nucleus.

At the end of the 20th century, there was a popular hypothesis that some modern unicellular eukaryotes do not and never have had mitochondria. It was proposed to allocate such primarily non-mitochondrial eukaryotes into a special kingdom Archezoa. Margulis accepted this hypothesis quite early and was faithful to it to the last - even when many other scientists had already rejected it (L. Margulis et al., 2005. “Imperfections and oddities” in the origin of the nucleus). She considered it quite likely that primarily non-mitochondrial eukaryotes (“archaeoprotists”) still live in some inaccessible oxygen-free habitats, where they are very difficult to detect. Alas, no “archeprotists” have yet been found, but any number of remains of mitochondria have been found in those single-celled organisms that were previously classified as Archezoa. At the moment, only one eukaryote is known that has no traces of mitochondria at all - the flagellate Monocercomonoides, but the position of this creature on the evolutionary tree leaves no doubt that it once had mitochondria (A. Karnkowska et al., 2016. A eukaryote without a mitochondrial organelle). In general, at the moment, without exception, all cases of the absence of mitochondria in eukaryotes must be considered secondary. This means that there was no ancient non-mitochondrial stage in the history of eukaryotes - at least their modern groups -.

Margulis believed (quite reasonably for her time) that at a certain period in the history of life, eukaryotization was a broad trend - a “trend”, as they say now. Based on this, it would be possible to assume that different eukaryotes have different ancestors: for example, that eukaryotic algae evolved from cyanobacteria, animals from predatory bacteria, and fungi from osmotrophic bacteria that absorb nutrients through the cell surface. This hypothesis does not contradict any fundamental laws of biology. But, unfortunately, it strikingly contradicts the facts. Molecular systematics shows that the common ancestor of plants, animals and fungi was not a transitional form, but a true eukaryote, “fully fledged,” as Nick Lane puts it. We can safely say that the common ancestor of all modern eukaryotes was already a full-fledged eukaryotic cell: it had a nucleus, endoplasmic reticulum, Golgi apparatus, microtubules, microfilaments, mitochondria and flagella. In general, a complete set of eukaryotic characteristics.

Please note that this set of characteristics does not include chloroplasts. They did not appear in all eukaryotes and not immediately. In addition, chloroplasts were certainly acquired more than once, and in different ways in different evolutionary branches. Chloroplasts are like primary(when a eukaryote invades a cyanobacterium), and secondary(when a eukaryote captures another eukaryote with a cyanobacterium inside) and even tertiary(when one eukaryote captures a second eukaryote, inside of which lives a third eukaryote, and inside that one - a cyanobacterium). Here evolution, as they say, is in full swing. With mitochondria, the situation is completely different: based on their presence, we do not see any special diversity and no transitional stages (except for numerous facts of secondary loss, but such facts say absolutely nothing about the origin of eukaryotes). If Margulis's scenario were completely correct, then the situation with mitochondria and flagella would be approximately the same as with chloroplasts - but this is not the case.

What Margulis was right about was that eukaryotes in general are quite predisposed to taking over endosymbionts. Here we can give a variety of examples, including the acquisition of bacterial symbionts by some deep-sea worms, on which these worms actually live (V.V. Malakhov, 1997. Vestimentiferans are autotrophic animals). The rapid evolution of chloroplasts is the most striking manifestation of this trend. Only the “actors” who acquired them apparently already had by that time a full set of eukaryotic characteristics, including mitochondria. The configuration of the evolutionary tree of eukaryotes, as we now know it, simply does not allow other versions.

To this, Lane adds that the basic structure of cells differs surprisingly little among different eukaryotes, depending on their lifestyle (although the lifestyle itself can vary greatly). All the characteristic components of a cell that make it eukaryotic are generally arranged in the same way in plants, animals, fungi, flagellates, and amoebae... “We now know that almost all differences between eukaryotes reflect secondary adaptations.” , Lane writes in the article under discussion. The uniformity of the structure of the eukaryotic cell means that the first stages of its formation left practically no traces in the modern diversity of eukaryotes.

Unique event

The conclusions that Lane draws today can no longer be called new or unexpected. Modern data are most compatible with the assumption that the formation of the eukaryotic cell was single event, completed (in the time scale available to us) very quickly. It is likely that the ancestors of eukaryotes went through a kind of bottleneck at this stage (in one earlier article, Lane suggested that it was a small, unstable, short-lived population in which all the major changes took place; N. Lane, 2011. Energetics and genetics across the prokaryote-eukaryote divide). As a result, the first “fully fledged” eukaryote arose, whose descendants dispersed into different ecological niches - but the fundamental structure of the cell no longer changed. Thus, there was no parallel eukaryotization. In any case, modern biology does not find evidence confirming it.

Data from comparative genomics suggest that the threshold event that separated eukaryotes from the rest of living nature was the union of two cells - an archaeal one (probably belonging to one of the Lokiarchaeota) and a bacterial one (probably belonging to one of the Proteobacteria). The resulting superorganism became the first eukaryote (Fig. 3, B). The modern “mainstream” point of view identifies this event with the acquisition of mitochondria (the so-called “early mitochondrial” scenario; see, for example: N. Yutin et al., 2009. The origins of phagocytosis and eukaryogenesis). Indeed, mitochondria are indisputable descendants of proteobacteria, and they certainly penetrated as symbionts into the cell of an archaea (or a primitive eukaryote not too far removed from the archaea). However, to the question of how exactly they got there, Lane gives a rather unexpected answer. Namely: “We don’t know.”

What's the matter? According to the classical theory, all internal symbionts were acquired by eukaryotic cells through phagocytosis, that is, capture by pseudopods with isolation of the captured object and its subsequent digestion (in this case, failed). This is apparently true for chloroplasts, but very doubtful for mitochondria. The assumption that phagocytosis appeared earlier than mitochondria does not fit well with bioinformatics data. A comparative analysis of protein sequences shows that the actin microfilaments that form the internal framework of any pseudopods were most likely immobile at first - proteins that also allow them to contract appeared much later (E.V. Kunin, 2014. Logic of the case). This means that the evolution of eukaryotes could not begin directly with phagocytosis - mitochondria were acquired in some other way.

But it must be emphasized that all this is still just speculation. The mystery of the origin of mitochondria, not to mention the origin of the nucleus, has still not been solved.

Chance and Necessity

So, is the serial endosymbiosis hypothesis correct? Yes - in the sense that symbiotic events have indeed occurred many times in the history of eukaryotes. This is best illustrated by the long, rich and now well-studied history of chloroplasts (P. Keeling et al., 2013. The number, speed, and impact of plastid endosymbioses in eukaryotic evolution). No - in the sense that serial endosymbiosis was not a prerequisite for the emergence of eukaryotes as a group. The endosymbiotic event that led to the emergence of eukaryotes was, as far as we can now judge, unique.

Thus, the “parallel eukaryotization” scenario is not confirmed. This does not mean that evolutionary events of this type do not happen at all: some of them are described in detail by paleontologists (for example, the mammalization of animal-like reptiles, which acquire the characteristics of mammals in parallel in several evolutionary branches). Moreover, the list of such “parallel scenarios” has even been growing recently. “Elements” has written more than once about the hypothesis of the independent emergence of the nervous system in two completely different branches of multicellular animals (see The discussion about the role of ctenophores in evolution continues, “Elements”, 09.18.2015). But the emergence of eukaryotes is one of the most unique events in the entire history of life on Earth. This is probably why it falls out of this series.

In modern scientific literature there is such a concept as rare earth hypothesis(see Rare Earth hypothesis). Proponents of this hypothesis admit that relatively simple life (at the bacterial level of organization) can exist on many planets and be quite common in the Universe. But relatively complex life (eukaryotic or comparable) arises only under the rarest combination of circumstances; it is possible that there is only one planet with such life in the Galaxy. If the rare Earth hypothesis is correct, then the emergence of eukaryotes is most likely the milestone event separating “simple” life (widespread) from “complex” (unlikely) life.

The author of the famous book “The Origin of Life,” Mikhail Nikitin, recently (and completely independently) came to similar conclusions. “We don’t even know yet how natural the emergence of eukaryotes was. If for other stages of the development of life, such as the transition from the RNA world to the RNA-protein world, the separation of prokaryotic cells from the pre-cellular “world of viruses” or the emergence of photosynthesis, we can confidently say that they are natural and almost inevitable, since life has already appeared , then the appearance of eukaryotes in the prokaryotic biosphere could be very unlikely. It is possible that in our Galaxy there are billions of planets with life at the bacterial level, but only on Earth did eukaryotes appear, on the basis of which multicellular animals and then intelligent beings appeared” (M. Nikitin, 2014. A new hypothesis of the origin of the eukaryotic cell has been put forward). Perhaps this is why it is so difficult for us to understand the details of the origin of eukaryotes: this is a unique (on a planetary scale) event, to which it is very difficult to apply the principle of uniformitarianism, which requires “by default” to proceed from the uniformity of factors and processes at all points in time. But this is precisely why the mystery of the origin of eukaryotes is one of the most fascinating in all of biology. There are still many unresolved issues in this area; not all of them are mentioned here (as in the article discussed by Nick Lane).

ANSWER:

In most biology courses, one of the main features of the difference between prokaryotes and eukaryotes is the presence of double-membrane organelles (mitochondria and plastids) in the latter. These organelles, in addition to the double membrane, have a number of characteristic features that distinguish them from other cellular membrane formations. The question of their origin is inextricably linked with the question of the origin of eukaryotes. The answer to this question is provided by the theory of symbiogenesis.

So, according to this theory, mitochondria and chloroplasts originated from symbiotic prokaryotic organisms captured by a protoeukaryote through phagocytosis. This protoeukaryote, apparently, was an amoeboid heterotrophic, anaerobic organism with already developed eukaryotic characteristics.

This becomes understandable if we take into account the circumstances of the existence of life at that time. The first probable remains of eukaryotes are about 1.5 billion years old. The oxygen content in the atmosphere then was less than 0.1% of what it is today. At some point in biological evolution (when is not known exactly) photosynthesis arose. Photosynthetics were, of course, prokaryotes: cyanobacteria and other groups of phototrophic bacteria. Stromatolites - stones from deposited layers of lime, evidence of phototrophic bacterial communities, appeared more than 2 billion years ago (they are similar to modern ones, which in some places form cyanobacteria). Until this time, the atmosphere was oxygen-free; at some point oxygen began to accumulate. Its accumulation has created big problems. It is chemically active and essentially poisonous. I had to invent protection methods, incl. biochemical (perhaps one of them is bioluminescence). Many prokaryotes have learned to neutralize it (although a significant part of them have remained strict anaerobes - for them even now oxygen is poison). But some went further - they began to use this poison to oxidize substrates to produce energy. Aerobic metabolism emerged.

Wednesday! There are almost no strict anaerobes in eukaryotes. But this is not their merit: having become biochemically stupid due to predation, they stole the invention of prokaryotes. They did this by enslaving the prokaryotes themselves - turning them into their intracellular symbionts.

Even 25-30 years ago in our country the theory of symbiogenesis was ridiculed and considered a heresy. But today it can be considered generally accepted, although even today it faces a number of difficulties.

2. Theory of symbiogenesis: history of the issue

The idea that some cell organelles can be symbiotic organisms arose at the beginning of the century on domestic soil. Its author is the curator of the Zoological Cabinet of Kazan University K.S. Merezhkovsky. This was preceded by the establishment by Famintsyn and Baranovsky of the symbiotic nature of lichens (1867). The fact that lichens are a product of symbiosis was not recognized by some botanists even 50 years later! It is very unusual that such a “familiar”, dear organism is not “on its own”, but a fusion of two other organisms. The same situation occurred with Merezhkovsky’s ideas. Chloroplasts are not parts of a cell, but independent organisms?! Our cells are stuffed with bacteria - mitochondria?! And it’s not us who breathe, but them?! This theory was also not recognized for 50 years. However, then followers appeared - already in America; priority was, as happened more than once, lost.

Why did the theory win? “Omnipotent because she is faithful”?.. In fact, because new data has accumulated.

3. Theory of symbiogenesis: evidence.

The point of view of mitochondria and chloroplasts as symbiotic bacteria acquired by the cell is confirmed by a number of features of the structure and physiology of these organelles:

1) They have all the signs of an “elementary cell”: “a completely closed membrane;

Genetic material - DNA;

Its protein synthesis apparatus - ribosomes, etc.;

They reproduce by division (and sometimes divide independently of cell division).

2) They have signs of similarity to bacteria:

The DNA is usually circular and not associated with histones;

Prokaryotic ribosomes - 70S-rana and smaller. There is no 5.8S-pRNA characteristic of eukaryotes;

Ribosomes are sensitive to the same antibiotics as bacterial ones.

4. Theory of symbiogenesis: difficulties

Chloroplasts and mitochondria do not have the cell wall characteristic of putative ancestral groups. But it is absent or almost absent in many modern endosymbionts. Apparently, it is lost to facilitate the exchange between the symbiont and the host. This is just an easy problem. In addition, the algae Cyanophora paradoxa found

“intermediate forms” - the so-called cyanelles. These organelles (symbionts?) have a reduced cell wall, which is why they are considered cyanobacteria. At the same time, they have a genome size 10 times smaller than that of bacteria (which is typical for chloroplasts) and do not reproduce outside the host cell. It is noteworthy that Cyanophora is a flagellated algae, and its cyanelles most closely resemble the chloroplasts of red algae, which always lack flagella.

But here is a difficult difficulty. Many proteins of mitochondria and chloroplasts are encoded by nuclear genes, synthesized on ribosomes in the cytoplasm, and only then delivered through two membranes to the organelle! How could this happen? The only explanation within the framework of symbiogenesis is that some of the genes of the organelles moved to the nucleus. Even twenty years ago it seemed that this was pure nonsense. Then data on mobile genetic elements accumulated (one of the stages in which the scientific community in our country became acquainted with them was the book by R.B. Khegin “Genome Instability”). Genes change places in chromosomes, viruses are integrated into the genomes of bacteria and eukaryotes, etc... The process of moving genes into the nucleus began to seem more likely, but nothing has been proven. However, later evidence appeared that such a process apparently actually took place. Proteins from several amino acid chains helped. One of these mitochondrial proteins, proton ATP synthetase, consists of 8 subunits (peptide chains). And it turned out that in yeast, 4 are encoded in the mitochondria, and 4 in the nucleus. This in itself is suspicious! And in humans, all 8 circuits are encoded in the nucleus. This means that during the evolution of eukaryotes from the common ancestors of yeast and humans, genes moved to the nucleus - which means this is possible in principle - hurray!

Using the theory of symbiogenesis, many features of mitochondria and chloroplasts have been predicted and/or explained. Some of the predictions discussed below are now more like evidence: they have been confirmed. First of all, the theory of symbiogenesis explains the presence of a double membrane and its properties. The acquisition of a double membrane is the result of phagocytosis; the outer membrane is the former membrane of the digestive vacuole and thus belongs to the host and not to the endosymbiont. Although now this membrane reproduces together with the organelle, oddly enough, in terms of lipid composition it is more similar to the membrane of the endoplasmic reticulum of the cell than to the inner membrane of the organelle itself.

Our theory also explains the differences in the metabolism of the cytoplasm and organelles. Anaerobe - protoeukaryote acquired bacteria that have already become aerobic (mitochondria); heterotroph acquired phototrophs (chloroplasts). The theory of symbiogenesis predicts homology (similarity) of the DNA sequences of organelles and bacteria. This prediction was brilliantly confirmed with the advent of sequencing methods. For example, according to the nucleotide sequences of 168-ribosomal RNA, chloroplasts are most similar to lanobacteria, and mitochondria are closest to purple bacteria. Both rRNAs differ sharply from the rRNA of eukaryotic ribosomes in the host cytoplasm. Finally, the theory of symbiogenesis predicts the possibility of multiple (repeated) acquisition of symbionts and the likelihood of finding several different free-living bacteria similar to their ancestors. Apparently, this possibility was realized in the case of chloroplasts.

It should be noted here that most books on cytology limit themselves to describing the chloroplasts of green plants: this is what we imagine when we talk about these organelles. The chloroplasts of green algae have a similar structure. But in other algae they can differ significantly both in the structure of the membrane parts and in the set of pigments.

In red algae, chloroplasts contain chlorophyll a and fncobilins - protein pigments collected in special bodies - phycobilisomes; membrane sacs - lamellae - are located in them individually. According to these characteristics, they are most similar (of all chloroplasts) to cyanobacteria, of which they apparently are direct descendants. Green algae and higher plants have chlorophylls a and b; no phycobilins; lamellae are collected in stacks - grana. They differ from typical cyanobacteria in these characteristics. And so in the 70s. of our century, a remarkable prokaryotic photoautotroph, Prochloron, was described in detail. It was known earlier - it is a symbiont of didemnid ascidians, in which it lives not inside cells, but in the cloacal cavity. (Didemnids are bag-shaped, transparent colonial creatures that live on coral reefs. Because of the symbionts, they have a bright green color. They can move slowly, choosing illuminated areas. The symbionts receive protection and the necessary growth substances from the host, and in return share products with him photosynthesis and, apparently, amino acids - prochloron is capable of nitrogen fixation.) It turned out that prochloron is most likely a cyanobacterium (although some scientists allocate it to a special division of Prochlorophyta, along with the later discovered free-living filamentous bacterium Prochlorothryx), but... it does not have phycobilins; there are chlorophylls a, b; there are stacks of lamellae. Thus, this is a “model” of the ancestor of chloroplasts in higher plants!

“Models” of chloroplasts in other algae (for example, brown and golden algae) may still be discovered. So the theory of symbiogenesis has also found confirmation.

Question 9 General information about the geochronology of the Earth. Main paths and stages? evolution of plants and animals. ANSWER:

The evolution of the organic world of the Earth is inextricably linked with the evolution of the lithosphere. The history of the development of the Earth's lithosphere is divided into geological eras: Katarchean, Archean, Proterozoic, Paleozoic, Mesozoic, Cenozoic. Each era is divided into periods and epochs. Geological eras, periods and epochs correspond to certain stages in the development of life on Earth.

Katarchean, Archean and Proterozoic are united in the Cryptozoic - “the era of hidden life”. Fossil remains of the Cryptozoic are represented by individual fragments that are not always identifiable. The Paleozoic, Mesozoic and Cenozoic are combined into the Phanerozoic - the “era of manifest life”. The beginning of the Phanerozoic is characterized by the appearance of skeletal-forming animals that are well preserved in fossil form: foraminifera, shell mollusks, and ancient arthropods.

Early stages of development of the organic world

The predecessors of modern organisms (archaebionts) were characterized by the presence of the main components of the cell: plasmalemma, cytoplasm and genetic apparatus. There were metabolic systems (electron transport chains) and systems for the reproduction, transmission and implementation of hereditary information (replication of nucleic acids and protein biosynthesis based on the genetic code).

Further development of the organic world includes the evolution of individual groups of organisms within ecosystems. An ecosystem must include at least three components: producers, consumers and decomposers. Thus, in the early stages of the development of the organic world, the main modes of nutrition should have been formed: photoautotrophic (holophytic), heterotrophic holozoic and heterotrophic saprotrophic. The photoautotrophic (holophytic) type of nutrition involves the absorption of inorganic substances by the body surface and subsequent chemosynthesis or photosynthesis. With the heterotrophic saprotrophic type of nutrition, dissolved organic substances are absorbed by the entire surface of the body, and with the heterotrophic holozoic type of nutrition, large food particles are captured and digested. In conditions of excess of ready-made organic substances, the heterotrophic (saprotrophic) method of nutrition is primary. Most archaebionts specialize in heterotrophic saprotrophic nutrition. They develop complex enzyme systems. This led to an increase in the volume of genetic information, the appearance of a nuclear membrane, various intracellular membranes and organelles of movement. Some heterotrophs undergo a transition from saprotrophic to holozoic nutrition. Subsequently, histone proteins appeared, which made possible the appearance of real chromosomes and perfect methods of cell division: mitosis and meiosis. Thus, there is a transition from the prokaryotic type of cell organization to the eukaryotic one.

Another part of the archaebionts specializes in autotrophic nutrition. The oldest method of autotrophic nutrition is chemosynthesis. On the basis of enzyme-transport systems of chemosynthesis, photosynthesis arises - a set of metabolic processes based on the absorption of light energy with the help of various photosynthetic pigments (bacteriochlorophyll, chlorophylls a. b, c, d and others). The excess of carbohydrates formed during CO2 fixation allowed the synthesis of various polysaccharides.

All of the listed characteristics in heterotrophs and autotrophs are major aromorphoses.

Probably, in the early stages of the evolution of the organic world of the Earth, there was widespread gene exchange between

completely different organisms (gene transfer by transduction, interspecific hybridization and intracellular

symbiosis). During synthesis, the properties of heterotrophic and photoautotrophic organisms were combined in one cell.

This led to the formation of various divisions of algae - the first true plants.

Main stages of plant evolution

Algae are a large heterogeneous group of primary aquatic photoautotrophic organisms. In the fossil state, algae are known from the Precambrian (over 570 million years ago), and in the Proterozoic and early Mesozoic all the currently known divisions already existed. None of the modern divisions of algae can be considered the ancestor of another division, which indicates the reticulate nature of the evolution of algae.

In the Silurian 3, the ocean became shallower and water desalinated. This created the prerequisites for the settlement of the littoral and supratitorial zones (littoral is the part of the coast that is flooded during tides; the littoral occupies an intermediate position between the aquatic and land-air habitats; supratitorial is the part of the coast above the tide level, moistened by splashes; in essence, the supratitorial is part of the terrestrial - air habitat).

The oxygen content in the atmosphere before the appearance of land plants was significantly lower than the modern level: Proterozoic - 0.001 from the modern level, Cambrian - 0.01, Silurian - 0.1. When there is a deficiency of oxygen, the limiting factor in the atmosphere is ultraviolet radiation. The emergence of plants onto land was accompanied by the development of the metabolism of phenolic compounds (tannins, flavonoids, anthocyanins), which are involved in protective reactions, including those against mutagenic factors (ultraviolet, ionizing radiation, some chemicals). The movement of plants onto land is associated with the appearance of a number of aromorphoses:

The appearance of differentiated tissues: integumentary, conductive, mechanical, photosynthetic. The appearance of differentiated tissues is inextricably linked with the appearance of meristems and main parenchyma. _? The appearance of differentiated organs: shoot (organ of carbon nutrition) and root (organ of mineral nutrition).

Multicellular gametangia appear: antheridia and archegonia.? Significant changes occur in metabolism.

The ancestors of Higher plants are considered to be organisms similar to modern Characeae algae. The oldest known land plant is Cooksonia. Cooksonia was discovered in 1937 (W. Lang) in the Silurian sandstones of Scotland: (age about 415 million years). This plant was an algae-like bush of twigs bearing sporangia. Attached to the substrate using rhizoids.

The further evolution of higher plants was divided into two lines: gametophytic and sporophytic

Representatives of the gametophytic line are modern Bryophytes. These are avascular plants that lack

specialized conductive and mechanical fabrics.

Another line of evolution led to the emergence of vascular plants, in which the sporophyte dominates in the life cycle, and all the tissues of higher plants are present (educational, integumentary, conductive, main parenchyma and its derivatives). Thanks to the appearance of all types of tissues, the plant body differentiates into roots and shoots. The oldest of the vascular plants are the now extinct Rhineaceae (psilophytes). During the Devonian, modern groups of spore plants (mosses, horsetails, ferns) were formed. However, spore plants lack a seed, and the sporophyte develops from an undifferentiated embryo.

At the beginning of the Mesozoic (? 220 million years ago), the first gymnosperms appeared, which dominated the Mesozoic era. The largest aromorphoses of Gymnosperms:

The appearance of ovules; The female gametophyte (endosperm) develops in the ovule.

The appearance of pollen grains; In most species, the pollen grain, when germinated, forms a pollen tube, forming a male gametophyte.

The appearance of a seed, which includes a differentiated embryo.

However, gymnosperms retain a number of primitive characteristics: the ovules are located openly on the seed scales (megasporangiophores), pollination occurs only with the help of the wind (anemophily), the endosperm is haploid (female gametophyte), primitive conducting tissues (xylem includes tracheids). In the Cenozoic, Gymnosperms yielded dominance to Angiosperms.

The first angiosperms (flowering) plants probably appeared in the Jurassic period, and their adaptive radiation began in the Cretaceous period. Currently, angiosperms are in a state of biological progress, which is facilitated by a number of aromorphoses:? The appearance of a pistil - a closed carpel with ovules.

The appearance of perianth, which made possible the transition to entomophily (pollination by insects).? The appearance of the embryo sac and double fertilization.

Currently, angiosperms are represented by many life forms: trees, shrubs, vines, annual and perennial grasses, and aquatic plants. The structure of the flower achieves particular diversity, which contributes to the accuracy of pollination and ensures intensive speciation - about 250 thousand plant species belong to Angiosperms.

Main stages of animal evolution

In lower worms (Flat and Roundworms), a third germ layer appears - the mesoderm. This is a major aromorphosis, due to which differentiated tissues and organ systems appear. J3aTeM the molar tree of animals branches into Protostomes and Deuterostomes. Among Protostomes, Annelids form a secondary body cavity (coelom). This is a major aromorphosis, thanks to which it becomes possible to divide the body into sections.

Annelids have primitive limbs (parapodia) and homonomic (equivalent) body segmentation. But at the beginning of the Cambrian, arthropods appeared, in which parapodia were transformed into articulated limbs. U V I

In arthropods, heteronomous (unequal) segmentation of the body appears. They have a chitinous exoskeleton, which contributes to the appearance of differentiated muscle bundles. The listed features of Arthropods are aromorphoses.

The most primitive arthropods - Trilobites - dominated the Paleozoic seas. Modern gill-breathing primary aquatic arthropods are represented by Crustaceans. However, at the beginning of the Devonian (after plants reached land and the formation of terrestrial ecosystems), Arachnids and Insects reached land. Arachnids came to land thanks to numerous allomorphoses (idioadaptations):? Impermeability of covers to water.

Loss of larval stages of development (with the exception of ticks, but the nymph of ticks is not fundamentally different from adult animals).

Formation of a compact, weakly dissected body.

Formation of respiratory and excretory organs corresponding to new living conditions. Insects are most adapted to life on land, thanks to the appearance of large aromorphoses:? The presence of germinal membranes - serous and amniotic.? Presence of wings.? Plasticity of the oral apparatus.

With the appearance of flowering plants in the Cretaceous period, the joint evolution of Insects and Flowers (coevolution) begins, and joint adaptations (coadaptations) are formed in them. In the Cenozoic era, insects, like flowering plants, are in a state of biological progress.

Among Deuterostome animals, chordates reach their highest peak, in which a number of large aromorphoses appear: notochord, neural tube, abdominal aorta (and then the heart).

The origin of the notochord has not yet been precisely established. It is known that strands of vacuolated cells are present in lower invertebrates. For example, in the ciliated worm Coelogynopora, the branch of the intestine, located above the nerve ganglia at the anterior end of the body, consists of vacuolated cells, so that an elastic rod appears inside the body, which helps to dig into the sandy soil. In the North American ciliated worm Nematoplana nigrocapitula, in addition to the described foregut, the entire dorsal side of the intestine is transformed into a cord consisting of vacuolated cells. This organ was called the intestinal chord (chorda intestinalis). It is possible that the dorsal chord (chorda dorsalis) of endomesodermal origin arose directly from the vacuolated cells of the dorsal side of the intestine.

Or primitive chordates in the Silurian the first Vertebrates (Jawless) occur. In vertebrates, the axial and visceral skeleton is formed, in particular, the braincase and the jaw region of the skull, which is also 1 aromorphosis. Lower gnathostome vertebrates are represented by a variety of fish. Modern classes of fish (Cartilaginous and Bony) were formed at the end of the Paleozoic - the beginning of the Mesozoic).

Some of the Bony fish (Flesh-footed fish), thanks to two aromorphoses - light breathing and the appearance of real limbs - gave rise to the first Quadrupeds - Amphibians (Amphibians). The first amphibians came onto land in the Devonian period, but their heyday occurred in the Carboniferous period (numerous stegocephals). Modern amphibians appear at the end of the Jurassic period.

In parallel, among the Quadrupeds, organisms with embryonic membranes appear - Amniotes. The presence of embryonic membranes is a major aromorphosis that first appears in Reptiles. Thanks to the embryonic membranes, as well as a number of other features (keratinizing epithelium, pelvic buds, appearance of the cerebral cortex), Reptiles have completely lost their dependence on water. The appearance of the first primitive reptiles - cotylosaurs - dates back to the end of the Stone Age period. In the Permian, various groups of reptiles appeared: beast-toothed, proto-lizards and others. At the beginning of the Mesozoic, branches of turtles, plesiosaurs, and ichchosaurs were formed. Reptiles begin to flourish. Two branches of evolutionary development are separated from groups close to the proto-lizards. One branch at the beginning of the Mesozoic gave rise to a large group of pseudosuchians. Pseudosuchia belongs to several groups: crocodiles, pterosaurs, the ancestors of birds and dinosaurs, represented by two branches: saurischians (Brontosaurus, Diplodocus) and ornithischians (only herbivorous species - Stegosaurus, Triceratops). The second branch at the beginning of the Cretaceous period led to the emergence of a subclass of squamates (lizards, chameleons and snakes).

However, Reptiles could not lose their dependence on low temperatures: warm-bloodedness is impossible for them due to incomplete separation of the blood circulation. At the end of the Mesozoic, with climate change, a mass extinction of reptiles occurred.

Only in some pseudosuchians in the Jurassic period does a complete septum between the ventricles appear, the left aortic arch is reduced, a complete separation of the circulatory circles occurs, and warm-bloodedness becomes possible. Subsequently, these animals acquired a number of adaptations to flight and gave rise to the Bird class.

In the Jurassic deposits of the Mesozoic era (? 150 million years ago), prints of the First Birds were discovered: Archeopteryx and Archaeornis (three skeletons and one feather). They were probably arboreal climbing animals that could glide but were not capable of active flight. Even earlier (at the end of the Triassic,? 225 million years ago) protoavis existed (two skeletons were discovered in 1986 in Texas). The skeleton of Protoavis differed significantly from the skeleton of reptiles; the cerebral hemispheres and cerebellum were increased in size. During the Cretaceous period, there were two groups of fossil birds: Ichthyornis and Hesperornis. Modern groups of birds appear only at the beginning of the Cenozoic era. A significant aromorphosis in the evolution of birds can be considered the appearance of a four-chambered heart in combination with a reduction of the left aortic arch. There was a complete separation of arterial and venous blood, which made possible further development of the brain and a sharp increase in the level of metabolism. The flourishing of Birds in the Cenozoic era is associated with a number of major idioadaptations (the appearance of feathers, specialization of the musculoskeletal system, development of the nervous system, caring for offspring and the ability to fly), as well as with a number of signs of partial degeneration (for example, loss of teeth).

At the beginning of the Mesozoic era, the first Mammals appeared, which arose due to a number of aromorphoses: enlarged hemispheres of the forebrain with a developed cortex, a four-chambered heart, reduction of the right aortic arch, transformation of the suspension, quadrate and articular bones into auditory ossicles, the appearance of fur, mammary glands, differentiated teeth in the alveoli, preoral cavity. The ancestors of Mammals were primitive Permian Reptiles, which retained a number of characteristics of Amphibians (for example, skin glands were well developed). In the Jurassic period of the Mesozoic era, Mammals were represented by at least five classes (Multitubercles, Tritubercles, Tricodonts, Symmetrodonts, Panthotheriums). One of these classes probably gave rise to modern Protobeasts, and the other - to Marsupials and Placentals. Placental mammals, thanks to the appearance of the placenta and true viviparity, enter a state of biological progress in the Cenozoic era. The original order of Placentals are Insectivores. Early on, the Insectivores separated from the Incomplete Teeth, Rodents, Primates and the now extinct group of Creodonts - primitive predators. Two branches separated from the Creodonts. One of these branches gave rise to modern Carnivores, from which Pinnipeds and Cetaceans separated. The other branch gave rise to primitive ungulates (Condylarthra), and then to the Odd-toed, Artiodactyl and related orders.

The final differentiation of modern groups of Mammals was completed during the era of great glaciations - in the Pleistocene. The modern species composition of Mammals is significantly influenced by the anthropogenic factor. In historical times, the following species were exterminated: aurochs, Steller's cow, tarpan and other species.

At the end of the Cenozoic era, some Primates experienced a special type of aromorphosis - overdevelopment of the cerebral cortex. As a result, a completely new species of organisms arises - Homo sapiens.

10. basic methods for studying the evolutionary process:

1) paleontological;

2) comparative anatomical;

3) embryological;

4) biogeographical;

5) genetic data;

6) biochemical data;

7) molecular biology data; Paleontological methods

1. Fossil transitional forms - forms of organisms that combine I admit! older and younger groups. The transitional forms from fish to terrestrial vertebrates are lobe-finned fish.

2. Paleontological series - series of fossil forms related to each other in the process of evolution and reflecting the course of phylogenesis; the phylogenetic series should consist of intermediate forms, similar in basic and particular structural details and genealogically related to each other in the process of evolution.

3. Sequence of fossil forms. Under favorable conditions, all extinct forms are preserved in the same territory in a fossil state.

groups. When analyzing sediments, it is possible to determine the sequence of appearance and changes in forms, the real speed of the evolutionary process.

Comparative anatomical method

This method is based on establishing similarities in the structure of modern organisms of various systematic groups. Organs that correspond to each other in structure and origin of independently performed functions are called homologous (scales on the rhizome, stem scales of horsetail, bud scales). Embryological methods

1. Identification of germinal similarity. In the 19th century, Karl Baer formulated the “law of germinal similarity”: the earlier stages of individual development are studied, the more similarities are found between different organisms.

2. The principle of recapitulation. The study of germinal similarity made it possible

4. Darwin and E. Haeckel to conclude that in the process of ontogenesis, many structural features of ancestral forms seem to be repeated (recapitulated): at the early stages of development, the characteristics of more distant ancestors are repeated, and at later stages, close ancestors (or more related modern forms) are repeated. Biogeographical methods

1. Comparison of flora and fauna. Accumulated materials on the originality, similarities and differences of the flora and fauna of continents and individual regions

2. Relics - individual species or small groups of species and complexes of characteristics characteristic of long-extinct groups of past eras.

3. Intermittent propagation. There are cases when organisms were unable to adapt to the pace of environmental change and disappeared in most of their former range, and survived only in areas with conditions close to the previous ones.

Study of island forms. The uniqueness of the fauna and flora of the islands depends on the duration of isolation from the main entity

Question 11. The doctrine of microevolution and its basic provisions ANSWER:

In order to distinguish between the mechanisms of adaptation genesis and the formation of higher taxa, Yuri Aleksandrovich Filnchenko (1927) introduced the terms “microevolution” and “macroevolution”.

Microevolution is the totality of evolutionary processes within species. The essence of microevolutionary transformations is a change in the genetic structure of populations. As a result of the action of elementary evolutionary factors, new alleles appear, and as a result of selection, new adaptations are formed. In this case, one allele is replaced by another allele, one isotype of a protein (enzyme) is replaced by another isotype. Populations are open genetic systems. Therefore, at the microevolutionary level, lateral gene transfer occurs - the exchange of genetic information between populations. This means that an adaptive trait that originated in one population can move to another population. Therefore, microevolution can be viewed as the evolution of open genetic systems capable of exchanging genetic material. Macroevolution is a set of evolutionary transformations occurring at the level of supraspecific taxa. Supraspecific taxa (genera, families, orders, classes) are closed genetic systems. [To designate the mechanisms of formation of higher taxa (divisions, types), J. Simpson introduced the term “megaevolution.”] The transfer of genes from one closed system to another is impossible or unlikely. Thus, an adaptive trait that arose in one closed taxon cannot be transferred to another closed taxon. Therefore, in the course of macroevolution, significant differences arise between groups of organisms. Therefore, macroevolution can be viewed as the evolution of closed genetic systems that are unable to exchange genes under natural conditions. Thus, the doctrine of macroevolution includes, on the one hand, the doctrine of the related relationships of taxa, and, on the other hand, the doctrine of evolutionary (phylogenetic) transformations of the characteristics of these taxa. Proponents of STE believe that “since evolution is a change in the genetic composition of populations, the mechanisms of evolution are problems of population genetics” (Dobzhansky, 1937). Large morphological changes observed throughout evolutionary history can then be explained by the accumulation of small genetic changes. Thus, “microevolution gives macroevolution.”

The connection between microevolution and macroevolution is reflected in the law of homological series. N.I. Vavilov created the doctrine of species as a system. In this species theory, intraspecific variation is completely separated from taxonomic differences (this attempt was first made by J. Ray).

However, opponents of STE believe that the synthetic theory of evolution explains the survival of the fittest, but not their emergence. For example, Richard Goldschmidt (“Material Foundations of Evolution”, 1940) believes that the accumulation and selection of small mutations cannot explain the appearance of the following characteristics:? alternation of generations in a wide variety of organisms;? appearance of mollusk shells;

The appearance of fur in mammals and feathers in birds;? the emergence of segmentation in arthropods and vertebrates;

Transformations of the aortic arches in vertebrates (together with muscles, nerves and gill slits);? appearance of vertebrate teeth;

The appearance of compound eyes in arthropods and vertebrates.

The appearance of these signs may be due to macromutations in genes that are responsible not for the structure of enzymes, but for the regulation of development. Then macroevolution is an independent phenomenon not related to microevolution. This approach suits opponents of Darwinism, who recognize the natural scientific basis of microevolution, but deny the natural scientific basis of macroevolution.

3, General patterns of evolution

Macroevolution is a generalized picture of evolutionary transformations. Only at the level of macroevolution are general trends, directions and patterns of evolution of the organic world revealed. During the second half of the 19th - first half of the 20th century, based on numerous studies of the laws of the evolutionary process, the basic rules (principles) of evolution were formulated. (These rules are limited in nature, do not have universal meaning for all groups of organisms, and cannot be considered laws.)

1. The rule of irreversibility of evolution, or Dollot’s principle (Louis Dodlo, Belgian paleontologist, 1893): an extinct characteristic cannot reappear in its previous form. For example, secondary aquatic mollusks and aquatic mammals have not restored gill respiration.

2. The rule of descent from unspecialized ancestors, or Cope’s principle (Edward Cope, American paleontologist-zoologist, 1904): a new group of organisms arises from unspecialized ancestral forms. For example, unspecialized insectivores (such as modern tenrecs) gave rise to all modern placental mammals.

3. The rule of progressive specialization, or the Depere principle (C. Depere, paleontologist, 1876): a group that has embarked on the path of specialization will, in its further development, follow the path of ever deeper specialization. Modern specialized mammals (Chiroptera, Pinnipeds, Cetaceans) will most likely evolve towards further specialization.

4. The rule of adaptive radiation, or the Kovalevsky-Osborn principle (V.O. Kovalevsky, Henry Osborne, American paleontologist): a group that has an unconditionally progressive trait or a set of such traits gives rise to many new groups that form many new ecological niches and even going into other habitats. For example, primitive placental mammals gave rise to all modern evolutionary-ecological groups of mammals.

5. The rule of integration of biological systems, or the Shmachhausen principle (I.I. Shmatgauzen): new, evolutionarily young groups of organisms absorb all the evolutionary achievements of precursor groups. For example, mammals used all the evolutionary achievements of ancestral forms: the musculoskeletal system, jaws, paired limbs, the main parts of the central nervous system, embryonic membranes, perfect excretory organs (pelvic kidneys), various derivatives of the epidermis, etc.

6. The rule of phase change, or the Severtsov-Schmalhausen principle (A.N. Severtsov, I.I. Shmalhausen): various mechanisms of evolution naturally replace each other. For example, allomorphoses sooner or later become aromorphoses, and on the basis of aromorphoses new allomorphoses arise.

In addition to the rule for changing phases, J. Simpson introduced a rule for alternating rates of evolution: according to the speed of evolution! transformations, he distinguished three types of evolution: bradytellic (slow pace), horotellic (medium pace) and tachytellic (fast pace).

Question 12. Population structure of the species. ANSWER:

Population structure of the species

A species is actually a much more complex system than just a collection of similar individuals interbreeding. It breaks up into smaller natural groups of individuals - populations representing the population of individual relatively small areas within the entire distribution zone (area) of a given species. Within each population the greatest degree of panmixia occurs; Crossbreeding of individuals originating from different populations occurs relatively less frequently, and the exchange of genetic information between different populations is more limited. This determines a certain independence of genetic processes occurring in different populations of the same species. As a result, each population is characterized by its own specific gene pool with a ratio of frequencies of occurrence of different alleles unique to this population and with corresponding features of the spectrum of variability. These genetic differences between populations can be either random or non-random. The latter is characteristic of relatively large populations (about 500 oba or more) that have existed for a long time in a given geographical area.

Natural conditions in different parts of the species' range are usually more or less different. As a result, selection has different directions for populations of the same species inhabiting different areas. The consequence of this is the emergence of relatively stable differences in the gene pools of different populations. The characteristics of population gene pools, due to the action of selection, acquire an adaptive character: the selection of alleles in a particular gene pool, which determines a specific pattern of combinative and modification variability in a given population, becomes optimal for the living conditions of this population.

The frequency of occurrence of different alleles in a population is determined by the frequency of direct and reverse mutations, selection pressure, and the exchange of hereditary information with other populations as a result of emigration and immigration of individuals. With relative stability of conditions in a sufficiently large population, all these processes come to a state of relative equilibrium, the specific nature of which is determined, on the one hand, by the specific conditions, and on the other, by the genetic system of a given species. As a result, such fairly large and stable populations acquire a balanced and selectively optimized gene pool, the features of which are adaptive in nature and determine the specific features of a given population (ecological, behavioral, and in many cases, quite specific morphophysiological indicators).

Differences between populations inhabiting remote or relatively isolated areas become more pronounced due to a decrease in the exchange of genetic information between them. The result of sufficiently long isolation is the formation of subspecies, which are understood as populations of a given species that inhabit different parts of the species' range (i.e., having an allopatric distribution) and are characterized by a stable complex of morphological, physiological and ecological characteristics fixed hereditarily. However, the subspecies fully retain their bonding with each other, and if contact between them expands again, an intergradation zone arises in which, as a result of hybridization, individuals have an intermediate state of characteristics. The presence within a species of several subspecific forms that are consistently different from each other is designated by the term polytypicity of the species.

If the ranges of individual subspecies are large enough, the subspecies break up into populations of a smaller scale - ecologists distinguish several levels of such territorial (allopatric) groupings. Thus, within a species there is a complex hierarchical system of territorial populations, which is an adaptation to the optimal use of the entire diversity of conditions in different areas of the species' range.

Since populations have a specific gene pool under the control of natural selection, it is obvious that these natural groupings of individuals must play a critical role in the evolutionary transformations of the species. All processes leading to any changes in a species - to its division into daughter species (speciation) or to a directed change in the entire species as a whole (phyletic evolution) begin at the level of species populations. These processes of transformation of population gene pools are usually called microevolution. According to the definition of N.V. Timofeev-Resovsky, N.N. Vorontsov and A.V. Yablokov, populations are elementary structural units of the evolutionary process, and vectorized (directed) changes in gene pools of populations are elementary evolutionary phenomena.

Mitochondria are faithful companions of eukaryotes. According to the theory of symbiogenesis, it was the acquisition of mitochondria that provoked the formation of nuclear organisms. One of the proofs of this theory was the discovery of mitochondria or similar organelles in all, even the simplest, eukaryotes. But in May 2016, a team of Czech scientists described the first nuclear organism in history that did not contain even indirect signs of mitochondria. Could this discovery shake current ideas about the early evolution of eukaryotes?

Mitochondria are double-membrane organelles that supply energy to the cells of almost all eukaryotes. It is reliably known that they are related to α-proteobacteria and became part of eukaryotic cells about 1.5 billion years ago. The bacterial origin of mitochondria is evidenced by the presence of two membranes (internal own and external host), their own circular DNA and translation machine, as well as the ability to divide independently. Some even believe that apoptosis is an attempt by the mitochondria to kill the exploiter that has absorbed it.

However, there is still no consensus in the scientific community regarding the role of this symbiosis in the development of eukaryotes (Fig. 1). Supporters symbiogenesis theories argue that the merger of a certain archaea with the ancestor of mitochondria (bacteria) launched a chain of events that led to the formation of modern eukaryotes. Followers archaeozoic theory (hypothesis) On the contrary, they believe that only an already formed nuclear organism, an archezoan, could “tame” the mitochondrion.

Thanks to Carl Woese and George Fox, who in the 70s of the last century compared the 16S rRNA genes of many living creatures, which had long deceived microbiologists with their visual similarity, the two forms of prenuclear organisms (prokaryotes) were separated completely and irrevocably, and even at the highest level: eubacteria were deprived of their rights for “truth” (-eu), archaebacteria have the right to bear the proud name of bacteria, but in the new system of living organisms they were given their own domain (taxon of the highest rank): “ Evolution between a rock and a hard place, or how microbiology saved evolution from being absorbed by molecular biology" And " Carl Woese (1928–2012)". So in 1990, a person was asked to realize that all living beings are phylogenetically divided into three domains: Bacteria, Archaea and Eukaryotes, and bacteria differ from archaea even more than archaea from eukaryotes, and recently they almost doubted the advisability of dividing the latter: " Found the ancestors of all eukaryotes". However, the proposal of almost thirty years ago has still not found understanding among many authors of domestic biology textbooks. And what? What if these scientists change everything again, and they have to rewrite textbooks every twenty years or something? - Ed.

In 1928, the famous biologist of the turn of the 19th-20th centuries, Edmund Wilson, spoke about the hypothesis of the bacterial origin of mitochondria as follows: “Such ideas are too fantastic to be discussed in a decent biological society.”. Today, a similar attitude has formed towards the archaeozoic theory, and the key role of mitochondria in the early evolution of eukaryotes is generally recognized. Opening of the first true non-mitochondrial protozoan makes you think again about the strengths and weaknesses of each theory.

Symbiogenesis theory

One of the advantages of the symbiogenetic theory compared to the archaeozoan theory is that it explains the emergence of the nucleus and the intronic structure of the genome. In prokaryotes, horizontal gene transfer (HGT) is widespread, due to which populations can quickly exchange parts of the gene pool. HGT contributes to the insecurity of the genome - after all, the incoming DNA in this case is in no way separated from the contents of the host cell.

Quite likely attempts by the host cell to destroy the not yet domesticated symbiont led to the release of symbiotic DNA into the cytoplasm. This DNA, being in close proximity to the host genome, could easily integrate into it. Because of HGT, even in eukaryotes that have lost mitochondria, initially mitochondrial genes are found.

Such a fusion of genomes could, firstly, contribute to the development of interdependence between the symbiont and the host. Secondly, abundant HGT could carry not only genes that ensured the interweaving of the metabolism of two organisms, but also selfish retroelements. The invasion of group II introns, escaped from α-proteobacteria, led to the loosening of the initially very dense host genome: up to 80% of the host DNA was now introns. In such a complex situation, the host cell developed several lines of defense for its genome from a barrage of introns: a system of internal membranes and a nucleus, a ubiquitin system for the degradation of damaged proteins, nonsense-mediated RNA decay, and other characteristic features of eukaryotes emerged (Fig. 3).

Figure 3. Formation of the main features of eukaryotes can be explained by the invasion of group II introns into the host cell genome that followed mitochondrial symbiosis.

Another powerful piece of evidence for symbiogenesis is the energy requirements of eukaryotes. Although the energy consumption of pro- and eukaryotes per gram of weight is approximately the same, nucleated cells are much larger than non-nucleated cells, causing them to consume approximately 5000 times more energy (2300 pW/cell versus 0.5 pW/cell). When energy consumption is recalculated for the average gene of a single-celled organism, it turns out that a eukaryotic gene consumes 1000 times more energy. Without mitochondrial energy, it would be impossible not only to create complex, large and actively moving organisms, but even to ensure the functioning of cellular structures typical of eukaryotes.

In giant bacteria, scaling of prokaryotic energy due to mass polyploidization (as in the case Epulopiscium, growing to 0.6 mm and containing 200,000 copies of a genome measuring 3.8 million bp) does not lead to an increase in energy output per gene, and the cell remains typically bacterial. - Auto.

Another important fact supporting the symbiogenetic scenario is the existence of intracellular bacterial symbionts. Cases of endosymbiosis in bacteria are extremely rare in nature, but they still exist and demonstrate how the eukaryotic domain of life could arise.

Archaeozoic theory

Archaeozoans are the putative non-mitochondrial but nuclear ancestors of modern eukaryotes. According to the archaeozoic scenario, mitochondria were domesticated only in the late stages of eukaryotic evolution and did not have a significant impact on this process.

One of the main provisions of symbiogenesis is the hypothesis of initial simplicity. Very little is known about life during the Proterozoic, so there are many often mutually exclusive assumptions about its structure. If, according to the first hypothesis, it is believed that more complex eukaryotes evolved from prokaryotes with very compact genomes, then in the archaeozoic scenario there initially existed cells with confusing and bulky genomes, from which simpler prokaryotes evolved through reduction. Eukaryotes only retained their primary complexity.

The evolution of genomes does not always move from simple to complex. And among eukaryotes there are examples confirming this.

Nevertheless, genome reduction does not necessarily accompany its compaction. Evidence of this can be found in both protozoan and multicellular life forms.

For example, free-living ciliates Paramecium tetraurelia contains 30,000 genes, each of which has an average of 2 kb. This compactness is achieved by reducing the size of introns to the maximum 25 bp. and reducing intergenic distances.

Even vertebrates can have unusually compact genomes: the puffer fish genome is eight times smaller than humans, largely due to its low repeat content (Figure 4).

Figure 4. Puffer fish has an unusually compact genome for a vertebrate. partly due to short introns. The vertical axis is given on a logarithmic scale.

Figure 5. Initial complexity hypothesis implies that prokaryotic branches of life evolved from more complex forms through reduction. The reduction vector of development could have been set by the first predatory archaeozoans, oppressing other organisms.

The above examples show that the simplicity of prokaryotic genomes can arise secondary. If this is true, then LUCA - the last common ancestor of all modern organisms - could have a eukaryotic genome.

The hypothesis of initial complexity is also confirmed by the so-called signature genes (“signatures”) - eukaryotic genes that do not have prokaryotic homologues. Most likely, these genes were contained in LUCA, but were lost by bacteria and archaea.

Unfortunately for the archezoic theory, the list of signatures has thinned out significantly since the beginning of the 21st century. Among the many genomes sequenced since then, their prokaryotic homologues have been found. Thus, every year there are more and more proteins, whose presence in eukaryotes can be explained by the fact that their genes were brought by an archaeal or bacterial ancestor during symbiogenesis.

And at the same time, the discovery of prokaryotic homologues of cellular movement proteins (actins, tubulins and kinesins) indirectly confirms the possibility that archaeozoans could actively move and even be the first predators on Earth capable of phagocytosis. The emergence of predators in the autotrophic-saprotrophic community of the cradle of life should have had a colossal impact on the course of evolution. In the most exciting scenarios, some archezoan prey adapt to rapid division and growth, while others adapt to niches that the archezoan cannot penetrate. As a result, the hypothetical archezoan led the evolution of its contemporaries along a reductive path with an emphasis on metabolic flexibility and the rate of division, during which the prokaryotes known to us were formed (Fig. 5).

But despite the fact that the archaeozoic theory has some strong points or at least delivers pointed blows towards symbiogenesis, it lacks the main thing - it does not explain how or why the nucleus formed.

Unique find

In the 1980s, there were many contenders for the title of modern archaeozoan, but in subsequent years, mitochondria-like organelles (mitosomes and peroxisomes) and marker genes of the mitochondrial past were found in all of them: genes for the assembly of Fe-S proteins, mitochondrial transporters and chaperones, cardiolipin synthetase. In addition, some proteins synthesized in the cytoplasm have import sequences into mitochondria, which can persist in the absence of mitochondria themselves.

With each new “closure” of a potential archaeozoan, the mitochondria-free scenario for the formation of eukaryotes turned out to be less and less likely. And then, in May 2016, a new potential archezoic finally appeared, containing not even a trace of mitochondria.

We are talking about anaerobic oxymonade Monocercomonoides sp. PA203, living in the intestines of insects. Oxymonads lack mitochondria and do not contain genes of mitochondrial origin in their nuclear DNA. They receive energy from glycolysis occurring in the cytoplasm.

Genome Monocercomonoides sp., deciphered by a team of Czech scientists, contains 16,629 genes, among which there are no markers mentioned above. The search for mitochondrial homologs and proteins with import sequences also did not give satisfactory results (Fig. 6).

The only thing that was found were two genes, the products of which were found in a close relative Monocercomonoides sp. may (or may not) be contained in mitochondria, but they lack import sequences.

The authors of the discovery believe that Monocercomonoides did once contain mitochondria, as closely related genera have traces of mitochondria. It is still possible that these protozoa have as yet undetected mitosomes that have become so degraded that there is no evidence of their presence left in the genome.

Anyway, Monocercomonoides sp.- so far a unique case of a truly mitochondrial-free protist in the entire history of biology. And this case proves that eukaryotes can live not only without mitochondria, but also without their genetic inheritance.

In what sequence the ancestors of eukaryotes acquired intracellular belongings and what became the lucky ticket to the evolutionary future, you can find out from the article “ Protein genealogy suggests late acquisition of mitochondria by eukaryotic ancestors» . - Ed.

This discovery, of course, does not deal a crushing blow to the theory of symbiogenesis, but it definitely raises questions about what is necessary and what is excess in eukaryotes.

Literature

1. How mitochondria appeared (a story similar to a fairy tale);
2. Kunin E.V. Logic of case. M.: Tsentrpoligraf, 2014. - 527 pp.;
3. Evolution between a rock and a hard place, or How microbiology saved evolution from being swallowed up by molecular biology;
4. Carl Woese (1928–2012);
5. Kondratenko Y. (2015). "We found the ancestors of all eukaryotes." "Shroedinger `s cat". 6 ;
6. van der Giezen M. (2009).