More than a billion years have passed from the appearance of unicellular organisms to the "invention" of the cell nucleus and the birth of a number of other innovations. Only then did the road open to the first multicellular beings, which gave rise to the three kingdoms of animals, plants and fungi. European scientists have put forward a new explanation for this transformation, which runs counter to the ideas that have existed so far.

Prokaryotes (pre-nuclear unicellular) were born approximately 3.8 billion years ago. More advanced organisms - eukaryotes (their cells contain a nucleus) - arose more than two billion years ago. And from them, about one billion years ago, the evolution of multicellular creatures already started.

Now two such creatures – Nick Lane of University College London (UCL) and William Martin of the Institute of Botany at the University of Düsseldorf – have developed an original theory. According to it, it turns out that the key to the emergence of eukaryotes was not the invention of the nucleus (as scientists argued for 70 years), but the emergence of mitochondria.

It is generally accepted that at first more perfect nuclear cells were born from prokaryotes, relying on the old energy mechanisms, and only later recruits acquired mitochondria. The latter were assigned an important role in the further evolution of eukaryotes, but not the role of the cornerstone that lies at its very foundation.

"We have shown that the first option will not work. To develop the complexity of the cell, it needs mitochondria," explains Martin. "Our hypothesis refutes the traditional view that the transition to eukaryotic cells required only the proper mutations," Lane echoes him.

They co-evolved, with the endosymbiont gradually honing one skill, ATP synthesis. The inner cell decreased in size and transferred some of its secondary genes to the nucleus. So the mitochondria retained only that part of the original DNA that they needed to work as a "living power plant".

The appearance of mitochondria in terms of energy can be compared with the invention of a rocket after a cart, because nuclear cells are on average a thousand times larger in volume than cells without a nucleus.

The latter, it would seem, can also grow in size and complexity of the device (there are isolated striking examples here). But on this path, tiny creatures have a catch: as they grow geometrically, the ratio of surface area to volume rapidly decreases.

Meanwhile, simple cells generate energy with the help of a membrane that covers them. So in a large prokaryotic cell there may be plenty of room for new genes, but it simply does not have enough energy to synthesize proteins according to these "instructions".

A simple increase in the folds of the outer membrane does not particularly save the situation (although such cells are known). With this method of increasing power, the number of errors in operation also increases. energy system. Unwanted molecules accumulate in the cell that can destroy it.

Mitochondria are a brilliant invention of nature. By increasing their number, it is possible to increase the energy potential of the cell without growing its outer surface. Moreover, each mitochondrion also has built-in control and repair mechanisms.

And another plus of innovation: mitochondrial DNA is small and very economical. It does not require a lot of resources to copy it. But bacteria, in order to increase their energy capabilities, can only create many copies of their entire genome. But such a development quickly leads to an energetic impasse.

The authors of the work calculated that the average eukaryotic cell could theoretically carry 200,000 times more genes than the average bacterium. Eukaryotes can be thought of as a library with a large number of shelves - fill it with books to your heart's content. Well, a more extended genome is the basis for further improvement of the structure of the cell and its metabolism, the emergence of new regulatory circuits.

According to Lane and Martin's calculations, for each gene in their hereditary code, eukaryotes have four to five orders of magnitude more energy than bacteria. From this point of view, bacteria are at the bottom of an energetic abyss from which they cannot escape.

The transition of cells to energy production with the help of mitochondria can be compared to the industrial revolution. Instead of linearly increasing the size of the manufactory, the cells made a qualitative change: they built a "factory" and put rows of specialized "machines" in it.

Therefore, despite billions of years of existence, prokaryotes have remained relatively simple creatures to this day, and eukaryotes have long ago invented new means of transmitting signals between cells and stepped towards multicellular life forms. Us with you.

The theory of European scientists, by the way, can also be useful in assessing the likelihood of the existence of complex life forms on other worlds.

The fact is that examples of absorption of other cells by bacteria are extremely rare. This means that, once having arisen, life can linger for many eons at a simple unicellular stage. Until a lucky break helps her invent intracellular energy factories. "The basic principles are universal. Even aliens need mitochondria," concludes Lane.

General information

prokaryotes(lat. Procaryota, from lat. pro- "before", "before" and Greek. karyon- "core"), or non-nuclear- unicellular living organisms that do not have (unlike) a formalized cell nucleus.

Prokaryotic cells are characterized by the absence of a nuclear envelope, DNA is packaged without the participation of histones.

The genetic material of prokaryotes is represented by one DNA molecule closed in a ring, there is only one replicon. There are no organelles in cells that have a membrane structure.

Characteristic features of prokaryotes

• Lack of a formalized core
• The presence of flagella, plasmids and gas vacuoles
• Structures where photosynthesis takes place - chlorosomes
• Forms of reproduction- asexual way, there is a pseudosexual process, as a result of which only the exchange of genetic information occurs, without an increase in the number of cells.
• Ribosome size- 70s.

Evolution of prokaryotes

According to another theory, as such, a common ancestor did not exist, and the first protozoa that lived at that time, with the help of horizontal gene transfer between themselves, constantly evolved. It is assumed that at the earliest stages of evolution there was some kind of common genetic "communal economy". The picture of evolutionary connections in the world of ancestral prokaryotes was not so much a tree as a kind of mycelium with an intertwined network of horizontal transfers in the most diverse and unexpected directions. As organisms became more complex and the mechanisms of sexual reproduction and reproductive isolation developed, horizontal transfer became more rare. At the same time, thanks to bacteriophage viruses, bacteria also have a simple immune system.

Unlike a eukaryotic cell, a prokaryotic cell generates energy not with the help of mitochondria (which she lacks), but with a membrane covering them. As a result, the prokaryotic cell not enough energy for protein synthesis. A simple increase in the folds of the outer membrane does not particularly save the situation (although such cells are known). With this method of increasing power, the number of errors in the operation of the energy system also increases. Unwanted molecules accumulate in the cell that can destroy it. All this led to the fact that prokaryotic cells remained thousands of times smaller than eukaryotic ones and their genomic material is several times smaller than more perfect eukaryotes.

Division of classification of prokaryotes:

Sub-empire:
Kingdom:	prokaryotes
Kingdom:	bacteria	Archaea

Evolution of cellular organisms

The appearance of the first cellular organisms: over 4 billion years ago

The first simplest unicellular organisms (prokaryotes) appeared more than 4 billion years ago.Recently, traces of complex cellular structures dating back to at least 3.86 billion years have been found in the oldest Archean sedimentary rocks on Earth, found in southwestern Greenland.

According to one of the theories, about 4.1 - 3.6 billion years ago during the Eoarchean period, from the diversity of unicellular living beings (prokaryotes) that existed at that time (Fig. 1), our first common ancestor living then was divided into several branches, which subsequently in turn, they were divided into the currently existing kingdoms (animals, plants, fungi, protists, chromists, bacteria, archaea and viruses). Over time, the rest of the inhabitants of that period could not stand the competition with them and disappeared from the face of the Earth.

According to another theory, as such, a common ancestor did not exist, and the first protozoa that lived at that time, with the help of horizontal gene transfer between themselves, constantly evolved. It is assumed that at the earliest stages of evolution there was some kind of common genetic "communal economy". The picture of evolutionary connections in the world of ancestral prokaryotes was not so much a tree as a kind of mycelium with an intertwined network of horizontal transfers in the most diverse and unexpected directions. As organisms became more complex and the mechanisms of sexual reproduction and reproductive isolation developed, horizontal transfer became more rare (Fig. 2). At the same time, thanks to bacteriophage viruses, the simplest immune system appears in bacteria.

At the same time, symbiogenesis occurred - mitochondria and plastids, in the form of independent unicellular organisms that existed at that time, became part of a larger cell becoming endosymbionts. Gradually, they lost the ability to exist independently and turned into organelles . R developing together, the endosymbiont gradually honed one skill - synthesis ATP . The inner cell decreased in size and transferred some of its secondary genes to the nucleus. So the mitochondria retained only that part of the original DNA that they needed to work as a "living power plant".

This led to the appearance in the Paleoproterozoic era (more than 2 billion years ago) of the first eukaryotes with a nucleus and being the ancestors of modern animals, plants, protists and chromists.

For the next almost 1.5 billion years, unicellular organisms impeccably reigned on our planet, until the first multicellular creatures appeared in the Edicarian period about 630 million years ago. Initially, the protozoan choanoflagellates, which are believed to be on the verge between unicellularity and multicellularity, were combined into multicellular structures, forming embryonic colonies only with the help of bacterial lipid, which is obtained from eaten bacteria. The next step was the appearance in the same period of the first real multicellular macroorganisms - these organisms appeared on Earth immediately after the Marinoan glaciation - one of the stages of global glaciation, when our planet was completely covered with ice for many millions of years. Such unusual forms will never appear in nature. Basically, these are soft-bodied organisms, consisting of individual fractals. Their body sizes ranged from one centimeter to one meter. They looked so unusual that for a long time scientists argued to which kingdom - plants or animals they can be attributed.

About 480-460 million years ago, in the Silurian period, the first plants appeared on land (according to other sources, this happened in the Upper Cambrian 499-488 million years ago), and 50 million years later, in the Devonian period, the first animals (although there is some data showing that the first land animals lived in the Silurian (Fig. 3) or even the Vendian periods). After that, the rapid development of all kinds of living beings began, the descendants of which we are.

Division classification:

Where can one see life as it was at the time of its birth? Renowned film director James Cameron is convinced that this can be done by sinking to the bottom of the Mariana Trench. The ecosystems that the brave traveler discovered there are reminiscent of those that existed on our planet over three billion years ago.

James Cameron, as part of his new work, made an unexpected discovery: at the bottom of the Mariana Trench, at a depth of 10.9 kilometers, microbial mats live for themselves - biofilms that feed on substances that they extract from bottom sediments. Similar habitats and processes taking place in them, researchers believe, in ancient times gave rise to a chemical reaction, as a result of which on Earth, and possibly in other places solar system the first living organisms appeared.

“We think that this chemical reaction may underlie metabolism,” says Kevin Hand, an astrobiologist at the California Jet Propulsion Laboratory (JPL). “It may be the driving force that led to the emergence of life. Perhaps not only here, but also in worlds such as Europa (Jupiter's icy moon)."

The Cameron Deepsea Challenger mission made a number of dives, including one manned, into the Mariana Trench between January 31 and April 3 this year. Cameron plunged into the depths of the sea personally. Having descended to the bottom, the director not only admired the surrounding landscape: Cameron took soil samples and took a number of pictures. Going upstairs, Cameron told reporters that it was quite gloomy down there, and the bottom looked like the surface of the moon. However, unlike the lifeless satellite of the Earth, life still lurks in the cold depths of the ocean.

The bacterial mats found by the researchers represent a fairly common ecosystem of prokaryotes since ancient times. Although some researchers consider it an analogue of the multicellular organism, the bacteria that make up the "rug" are painfully coordinated. As a rule, the mat unites several groups of "narrow" specialists: some, for example, decompose only hydrogen sulfide, others prefer sulfides, others prefer sulfates, etc. Thus, the mat "works", using almost all the resources in the form of chemical compounds that are around, and the members of this colony share with each other organic matter resulting from this diverse chemosynthesis.

It is also interesting that often the "waste" of some bacteria that make up the mat is useful resource For others. This can be easily demonstrated by the example of the cohabitation of two groups of bacteria - hydrogen sulfide photosynthetics and sulfate reducers. The first of them can photosynthesize using not oxygen, like higher plants, but hydrogen sulfide. However, a by-product of their activity is sulfur oxides, which, once in water, immediately form sulfuric acid, and then sulfates. These sulfates are desirable food for sulfate reducers, who reduce them with hydrogen. But the by-product of this process is hydrogen sulfide, which is used by the first group of bacteria.

Thus, if two groups of these bacteria live within the same mat, then they form a completely self-sufficient ecosystem. And if we add to them methane-oxidizing bacteria as hydrogen donors (they oxidize methane with the formation of carbon dioxide and molecular hydrogen) and methonogenic bacteria, which, using carbon dioxide and the molecular hydrogen produced by the methane oxidizers is obtained as a by-product of the very methane that the first group so badly needs, then the "economic activity" will become even more balanced. Then there is no need to go far for hydrogen, it can be supplied by other members of the colony. In a word, the mat is a practically waste-free plant, which people have not yet been able to create, well, nature gave birth to it over three billion years ago!

In the Mariana Trench, as the results of the expedition showed, not only microbial "rugs" live - several other representatives of the animal world previously unknown to science were also noticed there. For example, giant 17 cm amphipod crustaceans ( Amphipoda), they are called amphipods in Russia, outwardly they are very similar to shrimp. A study of these crustaceans has shown that their body contains compounds that help tissues work more efficiently under extremely high pressure.

"One of these compounds is scilloinositol, which is identical in composition to a drug currently being tested to destroy the amyloid plaques that have been linked to Alzheimer's disease," said Doug Bartlett, a microbiologist at the Scripps Institution of Oceanography at the University of California, San Diego. Another 20 thousand microbes taken from the Mariana Trench are waiting for their turn to the researchers.

Another "newcomer" was found at a depth of 8.2 kilometers in the New British Trench off the coast of Papua New Guinea. It turned out to be a representative of sea cucumbers, or holothurians ( Holothurioidea) - funny creatures from the group of echinoderms ( Echinodermata). "They've existed in these depths in the past, but they haven't been captured on film. We saw one of them and we think it's a new species," says Bartlett. And the walls of the gutter are adorned with a huge number of acorn worms, deep-sea invertebrates that cover the bottom of the cavity with their spiraling excrement. "If you've never thought of worms with love, then after watching this video, you would love them," says Bartlett.

Cameron's video shows not only deep-sea inhabitants, but also the oldest seabed on the planet. One hundred and eighty million years ago, when dinosaurs still walked the Earth, the rocks at the bottom of the Mariana Trench were red-hot lava. And the footage filmed by the director in the New England Trench may well be a record for the depth of the lava pillows, says marine geologist Patty Fryer from the University of Hawaii at Honolulu.

Altered rocks that feed microbial mats are part of young tectonic plates that lie on top of an ancient seafloor Pacific Ocean. The Mariana Trench is a subduction zone where two tectonic plates collided and one of them crawled over the other. Water seeping through piles of rocks changes the composition of rocks through serpentinization. During this process, sulfur, methane and hydrogen are formed, which gives the bacteria food.

AT last years scientists are inclined to believe that early life on Earth originated about four billion years ago in subduction zones like the Mariana Trench. Temperatures were cooler in these troughs, and the serpentinized rocks provided the necessary impetus for the chemical reaction that led to the birth of life.

"Those trenches could be where life began," says Cameron. "This mystery needs to be solved. I hope we're still diving." So far, no new dives are planned, but, according to the director, submersibles and descent deep-sea vehicles are in working order and are now stored on the territory of his mansion.

Has a long history. It all started about 4 billion years ago. The Earth's atmosphere does not yet have an ozone layer, the concentration of oxygen in the air is very low and nothing is heard on the surface of the planet, except for erupting volcanoes and wind noise. Scientists believe that this is what our planet looked like when life began to appear on it. It is very difficult to confirm or deny this. Rocks that could give more information to people collapsed a long time ago, thanks to the geological processes of the planet. So, the main stages of the evolution of life on Earth.

The evolution of life on earth. unicellular organisms.

Life got its start with the advent of the simplest forms of life - single-celled organisms. The first unicellular organisms were prokaryotes. These organisms first appeared after the Earth became suitable for the beginning of life. would not allow even the simplest forms of life to appear on its surface and in the atmosphere. This organism did not require oxygen for its existence. The concentration of oxygen in the atmosphere increased, which led to the appearance of eukaryotes. For these organisms, oxygen became the main thing for life, in an environment where the oxygen concentration was low, they did not survive.

The first organisms capable of photosynthesis appeared 1 billion years after the appearance of life. These photosynthetic organisms were anaerobic bacteria. Life gradually began to develop, and after the content of nitrogenous organic compounds fell, new living organisms appeared that could use nitrogen from the Earth's atmosphere. Such creatures were blue-green algae. The evolution of unicellular organisms took place after terrible events in the life of the planet and all stages of evolution were protected under magnetic field earth.

Over time, the simplest organisms began to develop and improve their genetic apparatus and develop methods of their reproduction. Then, in the life of unicellular organisms, there was a transition to the division of their generative cells into male and female.

The evolution of life on earth. multicellular organisms.

After the emergence of unicellular organisms, more complex forms of life appeared - multicellular organisms. The evolution of life on planet Earth has acquired more complex organisms, characterized by a more complex structure and complex transitional stages of life.

The first stage of life Colonial unicellular stage. The transition from unicellular organisms to multicellular organisms, the structure of organisms and the genetic apparatus becomes more complicated. This stage is considered the simplest in the life of multicellular organisms.

Second stage of life Primary differentiated stage. A more complex stage is characterized by the beginning of the principle of "division of labor" between the organisms of one colony. At this stage, there was a specialization of body functions at the tissue, organ and system-organ levels. Thanks to this, a nervous system began to form in simple multicellular organisms. The system did not yet have a nerve center, but there is a coordination center.

Third stage of life Centralized-differentiated stage. During this stage, the morphophysiological structure of organisms becomes more complicated. The improvement of this structure occurs through the strengthening of tissue specialization. The food, excretory, generative and other systems of multicellular organisms become more complicated. The nervous systems have a well-defined nerve center. The methods of reproduction are improving - from external fertilization to internal.

The conclusion of the third stage of life of multicellular organisms is the appearance of man.

Vegetable world.

The evolutionary tree of the simplest eukaryotes was divided into several branches. Multicellular plants and fungi appeared. Some of these plants could float freely on the surface of the water, while others were attached to the bottom.

psilophytes- plants that first mastered the land. Then other groups of land plants arose: ferns, club mosses and others. These plants reproduced by spores but preferred aquatic habitats.

Plants reached a great diversity in the Carboniferous period. Plants developed and could reach a height of up to 30 meters. In this period, the first gymnosperms appeared. Lycosform and cordaites could boast of the greatest distribution. Cordaites resembled the shape of the trunk coniferous plants and had long leaves. After this period, the surface of the Earth was diverse with various plants that reached 30 meters in height. Later a large number of time, our planet became similar to the one we know now. Now on the planet there is a huge variety of animals and plants, man has appeared. Man, as a rational being, after getting "on his feet" devoted his life to studying. Riddles began to interest a person, as well as the most important thing - where did a person come from and why does he exist. As you know, there are still no answers to these questions, there are only theories that contradict each other.

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

Symbiogenesis - the main hypothesis of the origin of eukaryotes

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

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

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

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

In addition to mitochondria and plastids, eukaryotic flagella can have a symbiotic origin. They turned into symbionts-bacteria like modern spirochetes with a flagellum. It is believed that subsequently centrioles, such important structures for the mechanism of eukaryotic cell division, originated from the basal bodies of the flagella.

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

In many respects, the question of the origin of the nucleus remains unclear. Could it also have formed from a symbiont prokaryote? The amount of DNA in the nucleus of modern eukaryotes is many times greater than its amount in mitochondria and chloroplasts. Perhaps some of the genetic information of the latter eventually moved into the nucleus. Also in the process of evolution there was a further increase in the size of the nuclear genome.

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

Molecular analysis has shown that various eukaryotic proteins are derived from different groups prokaryotes.

Evidence for symbiogenesis

In favor of the symbiotic origin of eukaryotes is the fact that mitochondria and chloroplasts have their own DNA, moreover, circular and not associated with proteins (this is also the case for prokaryotes). However, the genes of mitochondria and plastids have introns, which prokaryotes do not have.

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

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

Invagination hypothesis of the origin of eukaryotes

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

The nucleus, mitochondria and plastids arose by invagination and lacing of sections of the cell membrane. Alien DNA got into these structures.

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

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

Reasons for the evolution of eukaryotes

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

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

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

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

An important role in the prosperity of eukaryotes was played by aerobic respiration acquired by their ancestor (although many prokaryotes also have it).

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

Russian paleontologists have planted a bomb under traditional views on the origin of life on the planet. The history of the earth must be rewritten.

It is believed that life originated on our planet about 4 billion years ago. And the first inhabitants of the Earth were bacteria. Billions of individual individuals formed colonies that covered the vast expanses of the seabed with a living film. Ancient organisms were able to adapt to the realities of harsh reality. High temperatures and anoxic environments are conditions under which one would rather die than stay alive. But the bacteria survived. The unicellular world was able to adapt to an aggressive environment due to its simplicity. A bacterium is a cell that does not have a nucleus inside. Such organisms are called prokaryotes. The next round of evolution is associated with eukaryotes - cells with a nucleus. The transition of life to the next stage of development occurred, as scientists were convinced until recently, about 1.5 billion years ago. But today the opinions of experts about this date are divided. The reason for this was the sensational statement of researchers from the Paleontological Institute of the Russian Academy of Sciences.

Give me air!

Prokaryotes have played an important role in the history of the evolution of the biosphere. Without them, there would be no life on Earth. But the world of nuclear-free beings was deprived of the opportunity to develop progressively. What prokaryotes were 3.5-4 billion years ago, they have remained almost the same to this day. A prokaryotic cell is unable to create a complex organism. In order for evolution to move on and give rise to more complex forms of life, another, more perfect type of cell was required - a cell with a nucleus.

The appearance of eukaryotes was preceded by one very significant event: Oxygen appeared in the Earth's atmosphere. Cells without nuclei could live in an oxygen-free environment, but eukaryotes no longer could. The first producers of oxygen, most likely, were cyanobacteria, which found effective method photosynthesis. What could he be? If before that bacteria used hydrogen sulfide as an electron donor, then at some point they learned how to get an electron from water.

"The transition to the use of such an almost unlimited resource as water has opened up evolutionary opportunities for cyanobacteria," says Alexander Markov, a researcher at the Paleontological Institute of the Russian Academy of Sciences. Instead of the usual sulfur and sulfates, oxygen began to be released during photosynthesis. And then, as they say, the most interesting began. The appearance of the first organism with a cell nucleus opened up wide opportunities for the evolution of all life on Earth. The development of eukaryotes has led to the emergence of such complex forms as plants, fungi, animals and, of course, humans. All of them have the same type of cell, in the center of which is the nucleus. This component is responsible for the storage and transmission of genetic information. He also influenced the fact that eukaryotic organisms began to reproduce themselves through sexual reproduction.

Biologists and paleontologists have studied the eukaryotic cell in as much detail as possible. They assumed that they also knew the time of origin of the first eukaryotes. Experts called the numbers 1-1.5 billion years ago. But it suddenly turned out that this event happened much earlier.

unexpected find

Back in 1982, paleontologist Boris Timofeev conducted an interesting study and published his results. In the Archean and Lower Proterozoic rocks (2.9-3 billion years) on the territory of Karelia, he discovered unusual fossilized microorganisms about 10 micrometers (0.01 millimeters) in size. Most of the finds had a spherical shape, the surface of which was covered with folds and patterns. Timofeev suggested that he discovered acritarchs - organisms that are classified as representatives of eukaryotes. Previously, paleontologists found similar samples of organic matter only in younger deposits - about 1.5 billion years old. The scientist wrote about this discovery in his book. “The print quality of that edition was simply terrible. It was generally impossible to understand anything from the illustrations. The images were blurry gray spots,” says Alexander Markov, “so it’s not surprising that most readers, after flipping through this work, threw it aside, safely about forgetting him." The sensation, as often happens in science, lay for many years on a bookshelf.

The director of the Paleontological Institute of the Russian Academy of Sciences, Doctor of Geological and Mineralogical Sciences, Corresponding Member of the Russian Academy of Sciences Alexei Rozanov quite accidentally remembered Timofeev's work. He decided once again, using modern devices, to explore the collection of Karelian specimens. And very quickly he became convinced that he really had eukaryote-like organisms in front of him. Rozanov is sure that the discovery of his predecessor is an important discovery, which is a good reason to revise existing views on the time of the first appearance of eukaryotes. Very quickly, the hypothesis had supporters and opponents. But even those who share Rozanov's views speak with restraint on this issue: "In principle, the appearance of eukaryotes 3 billion years ago is possible. But it is difficult to prove," Alexander Markov believes. The average size prokaryotes range from 100 nanometers to 1 micron, eukaryotes - from 2-3 to 50 micrometers. In fact, the dimensional intervals overlap. Researchers often find specimens of both giant prokaryotes and tiny eukaryotes. Size is not 100% proof. It is really not easy to test the hypothesis. There are no more samples of eukaryotic organisms in the world extracted from Archean deposits. It is also not possible to compare ancient artifacts with their modern counterparts, because the descendants of the acritarchs did not survive to this day.

Revolution in science

Nevertheless, a big fuss arose in the scientific community around Rozanov's idea. Someone categorically does not accept Timofeev's find, because he is sure that 3 billion years ago there was no oxygen on Earth. Others are confused by the temperature factor. Researchers believe that if eukaryotic organisms appeared during the time of the Archaean, then, roughly speaking, they would have been cooked immediately. Aleksey Rozanov says the following: “Usually, parameters such as temperature, the amount of oxygen in the air, and water salinity are determined based on geological and geochemical data. I propose a different approach. First, assess the level of biological organization based on paleontological findings. Then, based on these data, determine , how much oxygen should have been contained in the Earth's atmosphere so that one or another form of life could feel normal.If eukaryotes appeared, then oxygen should already have been present in the atmosphere, in the region of several percent of the current level.If a worm appeared, the oxygen content should was already tens of percent.Thus, it is possible to draw up a graph reflecting the appearance of organisms different levels organization depending on the increase in oxygen and decrease in temperature. "Aleksey Rozanov is inclined to push back as much as possible the moment of the appearance of oxygen in the past and to reduce the temperature of the ancient Earth to the maximum.

If it is possible to prove that Timofeev found fossilized eukaryote-like microorganisms, this will mean that in the near future humanity will have to change the usual idea about the course of evolution. This fact will allow us to say that life on Earth appeared much earlier than expected. In addition, it turns out that it is necessary to revise the evolutionary chronology of life on Earth, which, it turns out, is almost 2 billion years older. But in this case, it remains unclear when, where, at what stage of development the break in the evolutionary chain occurred or why its course slowed down. In other words, it is completely unclear what happened on Earth for 2 billion years, where eukaryotes were hiding all this time: too much white spot is formed in the history of our planet. Another revision of the past is required, and this is a colossal work in its scope, which, perhaps, will never end.

OPINIONS

Life long

Vladimir Sergeev, Doctor of Geology and Mineralogy, Leading Researcher at the Geological Institute of the Russian Academy of Sciences:

In my opinion, one should be more careful with such conclusions. Timofeev's data are based on material with secondary changes. And this is the main problem. The cells of eukaryote-like organisms were chemically degraded and could be destroyed by bacteria. I consider it necessary to re-examine the Timofeev finds. As for the time of the appearance of eukaryotes, most experts believe that they appeared 1.8-2 billion years ago. There are some finds whose biomarkers indicate the emergence of these organisms 2.8 billion years ago. In principle, this problem is associated with the appearance of oxygen in the Earth's atmosphere. It is generally accepted that it formed 2.8 billion years ago. And Alexei Rozanov pushes this time back to 3.5 billion years. From my point of view, this is not true.

Alexander Belov, paleoanthropologist:

Everything that science finds today is only a fraction of the material that may still exist on the planet. Surviving forms are very rare. The fact is that special conditions are necessary for the preservation of organisms: a humid environment, lack of oxygen, and mineralization. Microorganisms that lived on land, in general, could not reach the researchers. It is by mineralized or petrified structures that scientists judge what kind of life was on the planet. The material that falls into the hands of scientists is a mixed fragments from different eras. Classical conclusions about the origin of life on Earth may not correspond to reality. In my opinion, it did not develop from simple to complex, but appeared at once.

Maya Prygunova, Itogi magazine No. 45 (595)