Theory of symbiogenesis. The origin of eukaryotes What explains the symbiosis hypothesis of the appearance of eukaryotes

Scheme of the evolution of eukaryotic cells.
1 - formation of a double nuclear membrane,
2 - acquisition of mitochondria,
3 - acquisition of plastids,
4 - introduction of the resulting photosynthetic eukaryotic cell into a non-photosynthetic one (for example, during the evolution of cryptophyte algae),
5 - the introduction of the resulting cell back into the non-photosynthetic one (for example, during the symbiosis of these algae with ciliates).
The color indicates the genome
ancestors of eukaryotes , mitochondria And plastid .

Story

The theory of the endosymbiotic origin of chloroplasts was first proposed in 1883 by Andreas Schimper, who showed their self-replication inside the cell. Its emergence was preceded by the conclusion of A. S. Famintsyn and O. V. Baranetsky about the dual nature of lichens - a symbiotic complex of fungus and algae (1867). K. S. Merezhkovsky in 1905 proposed the very name “symbiogenesis”, for the first time formulated the theory in detail and even created a new system of the organic world on its basis. Famintsyn in 1907, based on the work of Schimper, also came to the conclusion that chloroplasts are symbionts, like algae in lichens.

As a result of studying the sequence of bases in mitochondrial DNA, very convincing arguments were obtained in favor of the fact that mitochondria are the descendants of aerobic bacteria (prokaryotes) related to rickettsia, which once settled in the ancestral eukaryotic cell and “learned” to live in it as symbionts (organisms that participating in the symbiote). Now mitochondria are present in almost all eukaryotic cells; they are no longer capable of reproducing outside the cell.

There is evidence that the original endosymbiotic ancestors of mitochondria could neither import proteins nor export ATP. They probably initially received pyruvate from the host cell, and the benefit for the host was the neutralization of oxygen toxic to the nucleocytoplasm by aerobic symbionts.

Proof

have two completely closed membranes. In this case, the outer one is similar to the membranes of vacuoles, the inner one is similar to bacteria.
reproduce by binary fission (and sometimes divide independently of cell division), are never formed by synthesis from other organelles, such as the lysosome, formed from the Golgi complex, and it, in turn, from the ER.
genetic material - circular DNA not associated with histones (The proportion of mitochondrial and plastid DNA is closer to bacterial DNA than to eukaryotic nuclear DNA)
have their own protein synthesis apparatus - ribosomes, etc.
ribosomes of the prokaryotic type - with a sedimentation constant of 70S. The structure of 16s rRNA is similar to that of bacteria.
Some proteins of these organelles are similar in their primary structure to similar proteins of bacteria and are not similar to the corresponding proteins of the cytoplasm.

Problems

The DNA of mitochondria and plastids, unlike the DNA of most prokaryotes, contains introns.
Only part of their proteins are encoded in the own DNA of mitochondria and chloroplasts, while the rest are encoded in the DNA of the cell nucleus. During evolution, part of the genetic material “flowed” from the genome of mitochondria and chloroplasts into the nuclear genome. This explains the fact that neither chloroplasts nor mitochondria can no longer exist (reproduce) independently.
The question of the origin of the nuclear-cytoplasmic component (NCC), which captured proto-mitochondria, has not been resolved. Neither bacteria nor archaea are capable of phagocytosis, feeding exclusively osmotrophically. Molecular biological and biochemical studies indicate the chimeric archaeal-bacterial nature of JCC. How the fusion of organisms from two domains occurred is also not clear.

Examples of endosymbioses

Nowadays, there are a number of organisms that contain other cells inside their cells as endosymbionts. They, however, are not the primary eukaryotes that have survived to this day, in which the symbionts have not yet integrated into a single whole and have not lost their individuality. Nevertheless, they clearly and convincingly demonstrate the possibility of symbiogenesis.

Mixotricha paradoxa- the most interesting organism from this point of view. It uses more than 250,000 bacteria to move Treponema spirochetes, attached to the surface of its cell. The mitochondria of this organism are secondarily lost, but inside its cell there are spherical aerobic bacteria that replace these organelles.
Amoeba genus Pelomyxa also do not contain mitochondria and form symbiosis with bacteria.
Ciliates of the genus Paramecium algae are constantly contained inside the cells, in particular, Paramecium bursaria forms endosymbiosis with green algae of the genus Chlorella ( Chlorella).
Unicellular flagellated algae Cyanophora paradoxa contains cyanella - organelles that resemble typical chloroplasts of red algae, but differ from them in the presence of a thin cell wall containing peptidoglycan (the genome size of cyanella is the same as that of typical chloroplasts, and many times smaller than that of cyanobacteria).

Hypotheses for the endosymbiotic origin of other organelles

Endosymbiosis is the most widely accepted version of the origin of mitochondria and plastids. But attempts to explain the origin of other organelles and cell structures in a similar way do not find sufficient evidence and encounter justified criticism.

Cell nucleus, nucleocytoplasm

The mixing in eukaryotes of many properties characteristic of archaea and bacteria allowed us to assume the symbiotic origin of the nucleus from a methanogenic archaebacterium that invaded the myxobacterium cell. Histones, for example, are found in eukaryotes and some archaea, and the genes encoding them are very similar. Another hypothesis explaining the combination of molecular characteristics of archaea and eubacteria in eukaryotes is that at some stage of evolution, the archaeal-like ancestors of the nucleocytoplasmic component of eukaryotes acquired the ability to enhance the exchange of genes with eubacteria through horizontal gene transfer.

In the last decade, the hypothesis of viral eukaryogenesis has also been formed. It is based on a number of similarities in the structure of the genetic apparatus of eukaryotes and viruses: the linear structure of DNA, its close interaction with proteins, etc. The similarity of the DNA polymerase of eukaryotes and poxyviruses was shown, which made their ancestors the main candidates for the role of the nucleus.

Flagella and cilia

Notes

Schimper A.E.W. Uber die Entwickelung der Chlorophyllkorner und Farbkorper // Bot. Ztschr. Bd. - 1883. - T. Bot. Ztschr. Bd 41. S. 105-114.. Archived from the original on February 8, 2012.
Faminitsyn A.S. On the role of symbiosis in the evolution of organisms // Notes of Imp. AN. - 1907. - T. 20, No. 3, issue. 8 .
Merezhkovsky K.S. Theria of two plasmas as the basis of symbiogenesis, a new doctrine of the origin of organisms // Uch. zap. Kazan University. - 1909. - T. 76.

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 way 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: lizards (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 the 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.

(1867). K. S. Merezhkovsky in 1905 proposed the very name “symbiogenesis”, for the first time formulated the theory in detail and even created a new system of the organic world on its basis. Famintzin in 1907, based on the work of Schimper, also came to the conclusion that chloroplasts are symbionts, like algae in lichens.

Symbiotic origin of mitochondria and plastids

As a result of studying the sequence of bases in mitochondrial DNA, very convincing arguments were obtained in favor of the fact that mitochondria are the descendants of aerobic bacteria (prokaryotes) related to rickettsia, which once settled in the ancestral eukaryotic cell and “learned” to live in it as symbionts. Now mitochondria are present in almost all eukaryotic cells; they are no longer capable of reproducing outside the cell.

Proof

have two completely closed membranes. In this case, the outer one is similar to the membranes of vacuoles, the inner one is similar to bacteria.
reproduce by binary fission (and sometimes divide independently of cell division), are never synthesized de novo.
genetic material - circular DNA not associated with histones (The proportion of mitochondrial and plastid DNA is closer to bacterial DNA than to eukaryotic nuclear DNA)
have their own protein synthesis apparatus - ribosomes, etc.
ribosomes of the prokaryotic type - with a sedimentation constant of 70S. The structure of 16s rRNA is similar to that of bacteria.
Some proteins of these organelles are similar in their primary structure to similar proteins of bacteria and are not similar to the corresponding proteins of the cytoplasm.

Problems

The DNA of mitochondria and plastids, unlike the DNA of most prokaryotes, contains introns.
Only part of their proteins are encoded in the own DNA of mitochondria and chloroplasts, while the rest are encoded in the DNA of the cell nucleus. During evolution, part of the genetic material “flowed” from the genome of mitochondria and chloroplasts into the nuclear genome. This explains the fact that neither chloroplasts nor mitochondria can no longer exist (reproduce) independently.
The question of the origin of the nuclear-cytoplasmic component (NCC), which captured proto-mitochondria, has not been resolved. Neither bacteria nor archaea are capable of phagocytosis, feeding exclusively osmotrophically. Molecular biological and biochemical studies indicate the chimeric archaeal-bacterial nature of JCC. How the fusion of organisms from two domains occurred is also not clear.

Examples of endosymbioses

Mixotricha paradoxa- the most interesting organism from this point of view. It uses more than 250,000 bacteria to move Treponema spirochetes, attached to the surface of its cell. The mitochondria of this organism are secondarily lost, but inside its cell there are spherical aerobic bacteria that replace these organelles.
Amoeba genus Pelomyxa also do not contain mitochondria and form symbiosis with bacteria.
Ciliates of the genus Paramecium algae are constantly contained inside the cells, in particular, Paramecium bursaria forms endosymbiosis with green algae of the genus Chlorella ( Chlorella).
Unicellular flagellated algae Cyanophora paradoxa contains cyanella - organelles that resemble typical chloroplasts of red algae, but differ from them in the presence of a thin cell wall containing peptidoglycan (the genome size of cyanella is the same as that of typical chloroplasts, and many times smaller than that of cyanobacteria).

Hypotheses for the endosymbiotic origin of other organelles

Cell nucleus, nucleocytoplasm

In the last decade, the hypothesis of viral eukaryogenesis has also emerged. viral eukaryogenesis). It is based on a number of similarities in the structure of the genetic apparatus of eukaryotes and viruses: the linear structure of DNA, its close interaction with proteins, etc. The similarity of the DNA polymerase of eukaryotes and poxyviruses was shown, which made their ancestors the main candidates for the role of the nucleus.

Flagella and cilia

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Notes

Literature

Kulaev I. S.// Soros Educational Journal, 1998, No. 5, p. 17-22.

An excerpt characterizing Symbiogenesis

Before dawn, Count Orlov, who had dozed off, was awakened. They brought a defector from the French camp. This was a Polish non-commissioned officer of Poniatowski's corps. This non-commissioned officer explained in Polish that he had defected because he had been wronged in his service, that he should have been an officer long ago, that he was braver than everyone else and therefore abandoned them and wanted to punish them. He said that Murat was spending the night a mile away from them and that if they gave him a hundred men as an escort, he would take him alive. Count Orlov Denisov consulted with his comrades. The offer was too flattering to refuse. Everyone volunteered to go, everyone advised me to try. After many disputes and considerations, Major General Grekov with two Cossack regiments decided to go with a non-commissioned officer.
“Well, remember,” Count Orlov Denisov said to the non-commissioned officer, releasing him, “if you lied, I’ll have you hanged like a dog, but the truth is a hundred ducats.”
The non-commissioned officer with a decisive look did not answer these words, sat on horseback and rode off with Grekov, who had quickly gathered. They disappeared into the forest. Count Orlov, shaking from the freshness of the morning that was beginning to break, excited by what he had started on his own responsibility, having seen Grekov off, came out of the forest and began to look around the enemy camp, which was now visible deceptively in the light of the beginning of the morning and the dying fires. To the right of Count Orlov Denisov, along the open slope, our columns should have appeared. Count Orlov looked there; but despite the fact that they would have been noticeable from afar, these columns were not visible. In the French camp, as it seemed to Count Orlov Denisov, and especially according to his very vigilant adjutant, they began to stir.
“Oh, really, it’s late,” said Count Orlov, looking at the camp. Suddenly, as often happens, after the person we trust is no longer in front of his eyes, it suddenly became completely clear and obvious to him that the non-commissioned officer is a deceiver, that he lied and will only ruin the whole attack by the absence of these two regiments, whom he will lead God knows where. Is it possible to snatch the commander-in-chief from such a mass of troops?
“Really, he’s lying, this scoundrel,” said the count.
“We can turn it back,” said one of the retinue, who, like Count Orlov Denisov, felt distrust of the enterprise when he looked at the camp.
- A? Right?..what do you think, or leave it? Or not?
-Would you like to turn it back?
- Turn back, turn back! - Count Orlov suddenly said decisively, looking at his watch, “it will be late, it’s quite light.”
And the adjutant galloped through the forest after Grekov. When Grekov returned, Count Orlov Denisov, excited by this canceled attempt, and by the vain wait for the infantry columns, which still did not show up, and by the proximity of the enemy (all the people of his detachment felt the same), decided to attack.
He commanded in a whisper: “Sit down!” They distributed themselves, crossed themselves...
- With God blessing!
“Hurray!” - there was a rustle through the forest, and, one hundred after another, as if pouring out of a bag, the Cossacks flew cheerfully with their darts at the ready, across the stream to the camp.
One desperate, frightened cry from the first Frenchman who saw the Cossacks - and everyone in the camp, unclothed and sleepy, abandoned their cannons, rifles, horses and ran anywhere.
If the Cossacks had pursued the French, not paying attention to what was behind and around them, they would have taken Murat and everything that was there. The bosses wanted this. But it was impossible to move the Cossacks from their place when they got to the booty and prisoners. Nobody listened to the commands. One thousand five hundred prisoners, thirty-eight guns, banners and, most importantly for the Cossacks, horses, saddles, blankets and various items were immediately taken. All this had to be dealt with, the prisoners and guns had to be seized, the booty had to be divided, shouting, even fighting among themselves: the Cossacks did all this.
The French, no longer being pursued, began to gradually come to their senses, gathered in teams and began to shoot. Orlov Denisov expected all the columns and did not advance further.
Meanwhile, according to the disposition: “die erste Colonne marschiert” [the first column is coming (German)], etc., the infantry troops of the late columns, commanded by Bennigsen and controlled by Toll, set out as they should and, as always happens, arrived somewhere , but not where they were assigned. As always happens, people who had gone out cheerfully began to stop; Displeasure was heard, a sense of confusion was heard, and we moved somewhere back. The adjutants and generals who rode by shouted, got angry, quarreled, said that they were completely in the wrong place and were late, scolded someone, etc., and finally, everyone gave up and went off only to go somewhere else. “We’ll come somewhere!” And indeed, they came, but not to the right place, and some went there, but were so late that they came without any benefit, only to be shot at. Toll, who in this battle played the role of Weyrother at Austerlitz, diligently galloped from place to place and everywhere found everything topsy-turvy. So he galloped towards Baggovut’s corps in the forest, when it was already quite daylight, and this corps should have been there long ago, with Orlov Denisov. Excited, upset by the failure and believing that someone was to blame for this, Tol galloped up to the corps commander and sternly began to reproach him, saying that he should be shot for this. Baggovut, an old, militant, calm general, also exhausted by all the stops, confusions, contradictions, to the surprise of everyone, completely contrary to his character, flew into a rage and said unpleasant things to Tolya.
“I don’t want to take lessons from anyone, but I know how to die with my soldiers no worse than anyone else,” he said and went forward with one division.
Having entered the field under French shots, the excited and brave Baggovut, not realizing whether his entry into the matter now was useful or useless, and with one division, went straight and led his troops under the shots. Danger, cannonballs, bullets were exactly what he needed in his angry mood. One of the first bullets killed him, the next bullets killed many soldiers. And his division stood for some time under fire without benefit.

Meanwhile, another column was supposed to attack the French from the front, but Kutuzov was with this column. He knew well that nothing but confusion would come out of this battle that had begun against his will, and, as far as it was in his power, he held back the troops. He didn't move.
Kutuzov rode silently on his gray horse, lazily responding to proposals to attack.
“You’re all about attacking, but you don’t see that we don’t know how to do complex maneuvers,” he said to Miloradovich, who asked to go forward.
“They didn’t know how to take Murat alive in the morning and arrive at the place on time: now there’s nothing to do!” - he answered the other.
When Kutuzov was informed that in the rear of the French, where, according to the Cossacks’ reports, there had been no one before, there were now two battalions of Poles, he glanced back at Yermolov (he had not spoken to him since yesterday).
“They ask for an offensive, they propose various projects, but as soon as you get down to business, nothing is ready, and the forewarned enemy takes his own measures.”
Ermolov narrowed his eyes and smiled slightly when he heard these words. He realized that the storm had passed for him and that Kutuzov would limit himself to this hint.
“He’s having fun at my expense,” Ermolov said quietly, nudging Raevsky, who was standing next to him, with his knee.
Soon after this, Ermolov moved forward to Kutuzov and respectfully reported:
- Time has not been lost, your lordship, the enemy has not left. What if you order an attack? Otherwise the guards won’t even see the smoke.
Kutuzov said nothing, but when he was informed that Murat’s troops were retreating, he ordered an offensive; but every hundred steps he stopped for three quarters of an hour.
The whole battle consisted only in what Orlov Denisov’s Cossacks did; the rest of the troops only lost several hundred people in vain.
As a result of this battle, Kutuzov received a diamond badge, Bennigsen also received diamonds and a hundred thousand rubles, others, according to their ranks, also received a lot of pleasant things, and after this battle even new movements were made at headquarters.
“This is how we always do things, everything is topsy-turvy!” - Russian officers and generals said after the Tarutino battle, - exactly the same as they say now, making it feel like someone stupid is doing it this way, inside out, but we wouldn’t do it that way. But people who say this either do not know the matter they are talking about or are deliberately deceiving themselves. Every battle - Tarutino, Borodino, Austerlitz - is not carried out as its managers intended. This is an essential condition.
An innumerable number of free forces (for nowhere is a person freer than during a battle, where it is a matter of life and death) influences the direction of the battle, and this direction can never be known in advance and never coincides with the direction of any one force.
If many, simultaneously and variously directed forces act on some body, then the direction of movement of this body cannot coincide with any of the forces; and there will always be an average, shortest direction, what in mechanics is expressed by the diagonal of a parallelogram of forces.
If in the descriptions of historians, especially French ones, we find that their wars and battles are carried out according to a certain plan in advance, then the only conclusion that we can draw from this is that these descriptions are not true.
The Tarutino battle, obviously, did not achieve the goal that Tol had in mind: in order to bring troops into action according to disposition, and the one that Count Orlov could have had; to capture Murat, or the goals of instantly exterminating the entire corps, which Bennigsen and other persons could have, or the goals of an officer who wanted to get involved and distinguish himself, or a Cossack who wanted to acquire more booty than he acquired, etc. But , if the goal was what actually happened, and what was a common desire for all Russian people then (the expulsion of the French from Russia and the extermination of their army), then it will be completely clear that the Tarutino battle, precisely because of its inconsistencies, was the same , which was needed during that period of the campaign. It is difficult and impossible to imagine any outcome of this battle that would be more expedient than the one it had. With the least tension, with the greatest confusion and with the most insignificant loss, the greatest results of the entire campaign were achieved, the transition from retreat to offensive was made, the weakness of the French was exposed and the impetus that Napoleon’s army had only been waiting for to begin their flight was given.

Napoleon enters Moscow after a brilliant victory de la Moskowa; there can be no doubt about victory, since the battlefield remains with the French. The Russians retreat and give up the capital. Moscow, filled with provisions, weapons, shells and untold riches, is in the hands of Napoleon. The Russian army, twice as weak as the French, did not make a single attack attempt for a month. Napoleon's position is most brilliant. In order to fall with double forces on the remnants of the Russian army and destroy it, in order to negotiate an advantageous peace or, in case of refusal, to make a threatening move towards St. Petersburg, in order to even, in case of failure, return to Smolensk or Vilna , or stay in Moscow - in order, in a word, to maintain the brilliant position in which the French army was at that time, it would seem that no special genius is needed. To do this, it was necessary to do the simplest and easiest thing: to prevent the troops from looting, to prepare winter clothes, which would be enough in Moscow for the entire army, and to properly collect the provisions that were in Moscow for more than six months (according to French historians) for the entire army. Napoleon, this most brilliant of geniuses and who had the power to control the army, as historians say, did nothing of this.

Theory of symbiogenesis.

New theories have never won;

The supporters of the old ones simply died out.

If the ancestors of euglenoids fed on cyanobacteria or similar prokaryotes - photosynthetics, then symbionts - chloroplasts - could eventually form from the ingested but not digested bacteria. This idea arose in the 19th century and was called the theory of symbiogenesis.

In 1867, Russian scientists Andrei Sergeevich Famintsyn and Osip Vasilyevich Baranetsky proved that “gonidia” - green cells of lichens - can reproduce independently (in the absence of a fungus). in this case, flagellated cells similar to algae are formed. Scientists have shown that gonidia can live and reproduce on an artificial nutrient medium, which means they are independent organisms.

It was also noticed by scientists that chloroplasts also divide, and not always at the same time as the cell. It was possible to grow chloroplasts on an artificial nutrient medium, although without reproduction.

These facts formed the basis of the theory of symbiosis. Author – K.S. Merezhkovsky, who published his ideas in 1905. Studying the chloroplasts of algae, he wrote: “Chlorophyll grains of grain grow, feed, reproduce, synthesize proteins and carbohydrates, pass on their characteristics, and all this is independent of the nucleus. In a word, they behave as independent organisms and therefore must be considered as such. These are symbionts, not organs.”

A look at the origin of plant cells through symbiosis led Merezhkovsky to another guess: he was the first to propose dividing the organic world into prokaryotes and eukaryotes.

Confirmation of symbiogenesis: chloroplasts reproduce only by division; they have their own DNA, similar to the DNA of bacteria; have their own bacterial-type ribosomes; Chloroplasts synthesize some proteins themselves, and most proteins come from the cytoplasm of the host cell; double membrane. The inner one is the membrane of the prokaryotic cell, the outer one is the membrane of the digestive vacuole of the “host” cell.

Mitochondria have the same origin.

Cells similar to modern single-celled algae - eukaryotes - are known from rocks more than a billion years old. By that time, oxygen began to accumulate in the atmosphere. Oxygen had a destructive effect on many processes occurring in cells. But the cells themselves have learned to protect themselves from the toxic effects of oxygen (except for strict anaerobes). Learning to protect themselves from oxygen, some bacteria began to use it to obtain energy. (due to mitochondria).

The ancestors of eukaryotes acquired phagocytosis. And single-celled eukaryotes became the first true predators. The predatory lifestyle led to larger sizes, faster movement and changes in cell shape (for swallowing prey). Lysosomes appeared in their cells to ensure the digestion of victims. Eukaryotes were lucky that aerobic bacteria - the ancestors of mitochondria - learned to protect themselves from digestion and settled in their cytoplasm. Symbiogenesis has taken place. And approximately 700 - 650 million years ago, eukaryotes “came out of hiding” - multicellular organisms appeared in the geological record.

The theory of symbiogenesis states that chloroplasts and mitochondria are bacteria that have lost their independence - symbionts, which in ancient times settled inside cells - eukaryotes.

Although chloroplast mitochondria cannot reproduce outside the cell, they have retained many features of independence and similarities with bacteria. Mitochondria and chloroplasts reproduce only by division; they are passed on from generation to generation and never arise again. They have their own hereditary material - molecules similar to the heredity molecules of bacteria, and their own protein synthesis apparatus - ribosomes, also similar to bacterial ones.

The ancestors of chloroplasts could be prokaryotes, similar to cyanobacteria, and the ancestors of mitochondria could be aerobic bacteria that learned to use chemical reactions involving oxygen to produce energy. The symbiosis of eukaryotes with mitochondria and chloroplasts arose at least 1 billion years ago. It is likely that different “plant” protists could have independently evolved from different “animal” protists.

Theory of symbiogenesis. The origin of eukaryotes What explains the symbiosis hypothesis of the appearance of eukaryotes

Story

Proof

Problems

Examples of endosymbioses

Hypotheses for the endosymbiotic origin of other organelles

Cell nucleus, nucleocytoplasm

Flagella and cilia

Notes

Logic classic

Tree and network

Mitochondria problem

Unique event

Chance and Necessity

Symbiotic origin of mitochondria and plastids

Proof

Problems

Examples of endosymbioses

Hypotheses for the endosymbiotic origin of other organelles

Cell nucleus, nucleocytoplasm

Flagella and cilia

Write a review about the article "Symbiogenesis"

Notes

see also

Literature

An excerpt characterizing Symbiogenesis