In the Archean era, the first living organisms arose. They were heterotrophs and used organic compounds of the "primary" broth" as food. First anaerobic bacteria were the inhabitants of our planet. The most important stage in the evolution of life on Earth is associated with the emergence of photosynthesis, which leads to the division of the organic world into flora and fauna. The first photosynthetic organisms were prokaryotic (pre-nuclear) cyanobacteria and blue-green algae. The eukaryotic green algae that then appeared released free oxygen into the atmosphere from the ocean, which contributed to the emergence of bacteria capable of living in an oxygen environment. At the same time - on the border of the Archean Proterozoic era, two more major evolutionary events occurred - sexual process and multicellularity.

To better understand the meaning of the last two aromorphoses, let us dwell on them in more detail. Haploid organisms (microorganisms, blue-green) have one set of chromosomes. Each new mutation immediately manifests itself in their phenotype. If the mutation is beneficial, it is retained by selection; if it is harmful, it is eliminated by selection. Haploid organisms continuously adapt to the environment, but they do not develop fundamentally new features and properties. The sexual process sharply increases the possibility of adapting to environmental conditions, due to the creation of countless combinations in the chromosomes. Diploidy, which arose simultaneously with the formed nucleus, allows you to save mutations in a heterozygous state and use them as reserve of hereditary variability for further developments. In addition, in the heterozygous state, many mutations often increase the viability of individuals and, therefore, increase their chances in the struggle for existence.

The emergence of diploidy and genetic diversity of unicellular eukaryotes, on the one hand, led to the heterogeneity of the structure of cells and their association in colonies, on the other hand, the possibility of a “division of labor” between the cells of the colony, i.e. the formation of multicellular organisms. The separation of cell functions in the first colonial multicellular organisms led to the formation of primary tissues - ectoderm and endoderm, which later made it possible for the emergence of complex organs and organ systems. Improving the interaction between cells, first through contact, and then with the help of the nervous and endocrine systems, ensured the existence of multicellular

organism as a whole.

The paths of evolutionary transformations of the first multicellular organisms were different. Some moved to a sedentary lifestyle and turned into organisms of the type sponges. Others began to crawl with the help of cilia. From them came flatworms. Still others retained a floating lifestyle, acquired a mouth and gave rise to coelenterates.

3. The history of the Earth, from the time of the appearance of organic life on it and until the appearance of man on it, is divided into three large periods - eras that differ sharply from one another and bear the names: Paleozoic - ancient life, Mesozoic - middle, Neozoic - new life .

Of these, the largest in time is the Paleozoic, it is sometimes divided into two parts: the early Paleozoic and the late, since the astronomical, geological, climatic and floristic conditions of the late differ sharply from the early. The first includes: the Cambrian, Silurian and Devonian periods, the second - the Carboniferous and Permian.

Before the Paleozoic, there was the Archean era, but then there was no life yet. The first life on Earth is algae and plants in general. The first algae originated in water: this is how the emergence of the first organic life appears to modern science, and only later do mollusks that feed on algae appear.

Algae pass into ground grass, giant grasses pass into Paleozoic herbaceous trees.

In the Devonian period, lush vegetation appears on Earth, and life in the water in the form of its small representatives: protozoa, trilobites, etc. A warm climate is all over the globe, because there is still no modern sky with its sun, moon and stars; everything was covered with a dense, poorly permeable, powerful fog of water vapor, still in a colossal amount surrounding the earth, and only a part settled into the water basins of the oceans. The earth is rushing in the cold world space, but then it was dressed in a warm, impenetrable shell. Due to the greenhouse (greenhouse) effect, the entire early Paleozoic, including even the Carboniferous period, has warm-water flora and fauna throughout the earth: both in Svalbard and in the Antarctic - everywhere deposits of coal, which is a product of the tropical forest, everywhere there was warm-water marine fauna. Then the rays of the sun did not penetrate directly to the earth, but were refracted at a certain angle through the vapors and illuminated it then differently than now: the night was not so dark and not so long, and the day was not so bright. The days were shorter than today. There was neither winter nor summer, there are no astronomical and geophysical reasons for this yet. Coal deposits are composed of trees that do not have annual rings, their structure is tubular, like that of grass, and not ring-shaped. So there were no seasons. There were no climatic zones, also due to the greenhouse effect.

Modern paleontology has already sufficiently studied all types of living organisms of the Cambrian period: about a thousand different types of mollusks, but there is reason to believe that the first vegetation and even the first mollusks appeared at the end of the Archean era.

In the next, Silurian period, the number of mollusks increases to 10,000 varieties, and in the Devonian period, lungfish appear, that is, fish that do not have a backbone, but are covered with a shell, as a transitional form from mollusks to fish. They breathed with both gills and lungs. They make an attempt to become land dwellers, but they don't have to do it. The transition from the sea to the land will be performed by amphibians, from the class of vertebrates such as amphibious lizards.

The first representative of the lizards - the archosaurus - appears at the end of the Paleozoic, it develops at the beginning of the Mesozoic era, during the Triassic period.

Distinctive properties of the Paleozoic: light was not separated from darkness, the intermediate state between light and darkness, between day and night, is partially extended until the beginning of the Carboniferous. There were no lights visible in the sky. There were no seasons and climatic zones.

Proof: the absence of growth rings on the trees of the Paleozoic, except for the last, Permian period, when they first appear; the disappearance since that time of all herbaceous trees with a tubular trunk structure; distribution of tropical vegetation over the entire surface of the earth, including the poles; the same heat-loving fauna all over the earth; the formation of gigantic quantities of deposits of coal, as a result of the death of herbaceous forests, not adapted to the direct rays of the sun and naturally charred and died from ultraviolet and solar radiation, as grass is charred in a hot summer during a drought.

Since the Permian period, climatic zones and the distribution of late flora and fauna appear, adapting to climatic zones in different ways.

The next period in the life of the Earth corresponds to the entire Mesozoic era, that is, the periods: Triassic, Jurassic and Cretaceous. It was the heyday of the animal kingdom. The most diverse and bizarre forms of reptiles inhabited the Earth. They were both in the seas, and on land and in the air. It should be noted that the entire class of insects appeared at the end of the Paleozoic, and they were many times larger than their modern descendants.

The first birds appear in the Jurassic period. They reproduced not only quantitatively, but also in various species. One species of birds gave birth to chicks with their own characteristics, which gave rise to a new species of birds, which in turn had chicks that were not quite similar to them. This is how the diverse world of living beings developed. At some moments there were absolutely amazing metamorphoses.

Paleontologists know many specimens of different stages in the development of birds and not a single intermediate species between them: these are pterodactyls, archeopteryxes and fully developed birds.

Pterodactyls are half birds, half reptiles. This is a lizard whose toes have developed strongly and there are films between them, like a bat. But the next generation, which retained the same long spine, on both sides of which feathers grew, differs sharply from its predecessors. The body and wings were covered with feathers, but the claws remained on the wings for clinging to branches.

The head of Archeopteryx is the muzzle of the beast, inherited from the pterodactyl, with sharp large teeth and soft lips. And only in the next generation the vertebral tail disappears and the head becomes the head of a bird with a beak.

The last era is coming - the Neozoic. It includes the Tertiary and Ice (Quaternary) periods. Man appears at the end of the Ice Age. It was during the Neozoic era that mammals appeared. This is almost the modern world of animals. The fauna of that time can be seen to some extent in Africa, which was not touched by the glacier.

The biggest question for many is the question of monkeys. Most scientists are inclined to believe that the ape can in no way be the predecessor of man; but some say there must be some common ancestor. But this common ancestor has not yet been found.

Geological table of the Earth

The question of when life appeared on Earth has always worried not only scientists, but all people. Answers to it

almost all religions. Although there is still no exact scientific answer to it, some facts allow us to make more or less well-founded hypotheses. In Greenland, researchers have found a rock sample

with tiny inclusions of carbon. The age of the sample is more than 3.8 billion years. The source of carbon, most likely, was some kind of organic matter - during such a time it completely lost its structure. Scientists believe that this clump of carbon may be the oldest trace of life on Earth.

What did the primitive earth look like?

Fast forward to 4 billion years ago. The atmosphere does not contain free oxygen, it is only in the composition of oxides. Almost no sounds, except for the whistle of the wind, the hiss of water erupting with lava and the impact of meteorites on the surface of the Earth. No plants, no animals, no bacteria. Maybe this is what the Earth looked like when life appeared on it? Although this problem has been of concern to many researchers for a long time, their opinions on this matter differ greatly. The conditions on the Earth of that time could be evidenced by rocks, but they have long been destroyed as a result of geological processes and movements of the earth's crust.

In this article, we will briefly talk about several hypotheses for the origin of life, reflecting modern scientific ideas. According to Stanley Miller, a well-known specialist in the field of the origin of life, one can speak about the origin of life and the beginning of its evolution from the moment when organic molecules self-organized into structures that could reproduce themselves. But this raises other questions: how did these molecules come about; why they could reproduce themselves and assemble into those structures that gave rise to living organisms; what are the conditions for this?

According to one hypothesis, life began in a piece of ice. Although many scientists believe that the presence of carbon dioxide in the atmosphere maintained greenhouse conditions, others believe that winter dominated the Earth. At low temperatures, all chemical compounds are more stable and therefore can accumulate in greater quantities than at high temperatures. Space-borne fragments of meteorites, emissions from hydrothermal vents, and chemical reactions occurring during electrical discharges in the atmosphere were sources of ammonia and organic compounds such as formaldehyde and cyanide. Getting into the water of the oceans, they froze along with it. In the ice layer, the molecules of organic substances closely approached each other and entered into interactions that led to the formation of glycine and other amino acids. The ocean was covered with ice, which protected the newly formed compounds from destruction by ultraviolet radiation. This ice world could melt, for example, when a huge meteorite fell on the planet (Fig. 1).

Charles Darwin and his contemporaries believed that life could have originated in a body of water. This point of view is still held by many scholars. In a closed and relatively small body of water, organic matter brought by the waters flowing into it could accumulate in the required quantities. Then these compounds were further concentrated on the inner surfaces of layered minerals, which could be catalysts for reactions. For example, two molecules of phosphaldehyde that met on the surface of a mineral reacted with each other to form a phosphorylated carbohydrate molecule, a possible precursor of ribonucleic acid (Fig. 2).

Or maybe life arose in areas of volcanic activity? Immediately after its formation, the Earth was a fire-breathing ball of magma. During volcanic eruptions and with gases released from molten magma, a variety of chemicals necessary for the synthesis of organic molecules were brought to the earth's surface. Thus, carbon monoxide molecules, once on the surface of the catalytic pyrite mineral, could react with compounds that had methyl groups and form acetic acid, from which other organic compounds were then synthesized (Fig. 3).

For the first time, the American scientist Stanley Miller managed to obtain organic molecules - amino acids - in laboratory conditions simulating those that were on the primitive Earth in 1952. Then these experiments became a sensation, and their author gained worldwide fame. He currently continues to do research in prebiotic (pre-life) chemistry at the University of California. The installation on which the first experiment was carried out was a system of flasks, in one of which it was possible to obtain a powerful electric discharge at a voltage of 100,000 V.

Miller filled this flask with natural gases - methane, hydrogen and ammonia, which were present in the atmosphere of the primitive Earth. The flask below contained a small amount of water, simulating the ocean. An electric discharge was close to lightning in its strength, and Miller expected that under its action chemical compounds were formed, which, having then got into the water, would react with each other and form more complex molecules.

The result exceeded all expectations. Turning off the installation in the evening and returning the next morning, Miller found that the water in the flask had acquired a yellowish color. What formed was a broth of amino acids, the building blocks of proteins. Thus this experiment showed how easily the primary ingredients of the living could be formed. All that was needed was a mixture of gases, a small ocean and a small lightning bolt.

Other scientists tend to believe that the ancient atmosphere of the Earth was different from the one that Miller modeled, and most likely consisted of carbon dioxide and nitrogen. Using this gas mixture and Miller's experimental setup, chemists tried to make organic compounds. However, their concentration in the water was as negligible as if a drop of food coloring had been dissolved in a swimming pool. Naturally, it is difficult to imagine how life could have arisen in such a dilute solution.

If indeed the contribution of terrestrial processes to the creation of reserves of primary organic matter was so insignificant, then where did it come from? Maybe from space? Asteroids, comets, meteorites, and even interplanetary dust particles could carry organic compounds, including amino acids. These extraterrestrial objects could provide enough organic compounds to enter the primary ocean or a small body of water for the origin of life.

The sequence and time interval of events, starting from the formation of primary organic matter and ending with the appearance of life as such, remains and will probably forever remain a mystery that worries many researchers, as well as the question of what. in fact, consider life.

Currently, there are several scientific definitions of life, but they are not all accurate. Some of them are so wide that inanimate objects such as fire or mineral crystals fall under them. Others are too narrow, and according to them, mules that do not produce offspring are not considered alive.

One of the most successful defines life as a self-sustaining chemical system capable of behaving in accordance with the laws of Darwinian evolution. This means that, firstly, a group of living individuals must produce descendants similar to themselves, who inherit the characteristics of their parents. Secondly, in the generations of descendants, the consequences of mutations should appear - genetic changes that are inherited by subsequent generations and cause population variability. And thirdly, it is necessary that a system of natural selection operate, as a result of which some individuals gain an advantage over others and survive in changed conditions, giving offspring.

What elements of the system were necessary for it to have the characteristics of a living organism? A large number of biochemists and molecular biologists believe that RNA molecules possessed the necessary properties. RNA - ribonucleic acids - are special molecules. Some of them can replicate, mutate, thus transmitting information, and, therefore, they could participate in natural selection. True, they are not able to catalyze the replication process themselves, although scientists hope that in the near future an RNA fragment with such a function will be found. Other RNA molecules are involved in “reading” genetic information and transferring it to ribosomes, where the synthesis of protein molecules takes place, in which RNA molecules of the third type take part.

Thus, the most primitive living system could be represented by RNA molecules that doubled, mutated and were subject to natural selection. In the course of evolution, on the basis of RNA, specialized DNA molecules arose - the keepers of genetic information - and no less specialized protein molecules, which assumed the functions of catalysts for the synthesis of all currently known biological molecules.

At some point in time, a “living system” of DNA, RNA and protein found shelter inside a sac formed by a lipid membrane, and this structure, more protected from external influences, served as the prototype for the very first cells that gave rise to the three main branches of life, which are represented in the modern world by bacteria. , archaea and eukaryotes. As for the date and sequence of the appearance of such primary cells, this remains a mystery. In addition, according to simple probabilistic estimates, there is not enough time for the evolutionary transition from organic molecules to the first organisms - the first simple organisms appeared too suddenly.

For many years, scientists believed that life could hardly have arisen and developed during the period when the Earth was constantly subjected to collisions with large comets and meteorites, and this period ended about 3.8 billion years ago. However, recently, in the oldest sedimentary rocks on Earth found in southwestern Greenland, traces of complex cellular structures were found that are at least 3.86 billion years old. This means that the first forms of life could have arisen millions of years before the bombardment of our planet by large cosmic bodies stopped. But then a completely different scenario is possible (Fig. 4).

Space objects that fell to Earth could play a central role in the emergence of life on our planet, since, according to some researchers, cells like bacteria could originate on another planet and then get to Earth along with asteroids. One of the pieces of evidence in favor of the extraterrestrial origin of life was found inside a potato-shaped meteorite named ALH84001. Initially, this meteorite was a piece of the Martian crust, which was then ejected into space as a result of an explosion when a huge asteroid collided with the surface of Mars, which occurred about 16 million years ago. And 13 thousand years ago, after a long journey within the solar system, this fragment of Martian rock in the form of a meteorite landed in Antarctica, where it was recently discovered. A detailed study of the meteorite inside it revealed rod-shaped structures resembling fossilized bacteria in shape, which gave rise to heated scientific debate about the possibility of life in the depths of the Martian crust. It will not be possible to resolve these disputes until 2005, when the US National Aeronautics and Space Administration will carry out an interplanetary mission to Mars to take samples of the Martian crust and deliver samples to Earth. And if scientists manage to prove that microorganisms once inhabited Mars, then it will be possible to speak with a greater degree of certainty about the extraterrestrial origin of life and the possibility of bringing life from space (Fig. 5).

Rice. 5. Our origin is from microbes.

What have we inherited from ancient life forms? The following comparison of unicellular organisms with human cells reveals many similarities.

1. Sexual reproduction Two specialized reproductive cells of algae - gametes - mating, form a cell that carries genetic material from both parents. This surprisingly resembles the fertilization of a human egg by a spermatozoon.

2. Eyelashes Thin cilia on the surface of a single-celled paramecium sway like tiny oars and provide it with movement in search of food. Similar cilia cover the human respiratory tract, secrete mucus and trap foreign particles.

3. Capturing other cells The amoeba absorbs food, surrounding it with pseudopodia, which is formed by the extension and elongation of part of the cell. In an animal or human body, amoeboid blood cells similarly extend the pseudopodium to engulf dangerous bacteria. This process is called phagocytosis.

4. Mitochondria The first eukaryotic cells arose when the amoeba captured the prokaryotic cells of aerobic bacteria, which turned into mitochondria. And although bacteria and mitochondria of a cell (pancreas) are not very similar, they have one function - to produce energy in the process of oxidizing food.

5. Flagella The long flagellum of the human sperm allows it to move at high speed. Bacteria and protozoan eukaryotes also have flagella with a similar internal structure. It consists of a pair of microtubules surrounded by nine others.

The evolution of life on Earth: from simple to complex

At present, and probably in the future, science will not be able to answer the question of what the very first organism that appeared on Earth looked like - the ancestor from which the three main branches of the tree of life originate. One of the branches is eukaryotes, whose cells have a formed nucleus containing genetic material, and specialized organelles: mitochondria that produce energy, vacuoles, etc. Eukaryotic organisms include algae, fungi, plants, animals and humans.

The second branch is bacteria - prokaryotic (pre-nuclear) unicellular organisms that do not have a pronounced nucleus and organelles. And finally, the third branch is unicellular organisms called archaea, or archaebacteria, whose cells have the same structure as those of prokaryotes, but a completely different chemical structure of lipids.

Many archaebacteria are able to survive in extremely unfavorable environmental conditions. Some of them are thermophiles and live only in hot springs with a temperature of 90 ° C and even higher, where other organisms would simply die. Feeling great in such conditions, these single-celled organisms consume iron and sulfur-containing substances, as well as a number of chemical compounds that are toxic to other life forms. According to scientists, the found thermophilic archaebacteria are extremely primitive organisms and, in evolutionary terms, are close relatives of the most ancient forms of life on Earth.

Interestingly, modern representatives of all three branches of life, most similar to their ancestors, still live in places with high temperatures. Based on this, some scientists tend to believe that, most likely, life arose about 4 billion years ago at the bottom of the ocean near hot springs, spewing streams rich in metals and high-energy substances. Interacting with each other and with the water of the then sterile ocean, entering into a wide variety of chemical reactions, these compounds gave rise to fundamentally new molecules. So, for tens of millions of years in this "chemical kitchen" the biggest dish was prepared - life. And about 4.5 billion years ago, single-celled organisms appeared on Earth, the lonely existence of which continued throughout the Precambrian period.

The burst of evolution that gave rise to multicellular organisms occurred much later, a little over half a billion years ago. Although the size of microorganisms is so small that billions can fit in one drop of water, the scale of their work is enormous.

It is believed that initially there was no free oxygen in the earth's atmosphere and the World Ocean, and only anaerobic microorganisms lived and developed under these conditions. A special step in the evolution of living things was the emergence of photosynthetic bacteria, which, using the energy of light, converted carbon dioxide into carbohydrate compounds that serve as food for other microorganisms. If the first photosynthetics emitted methane or hydrogen sulfide, then the mutants that once appeared began to produce oxygen in the process of photosynthesis. With the accumulation of oxygen in the atmosphere and waters, anaerobic bacteria, for which it is destructive, occupied anoxic niches.

In ancient fossils found in Australia, which are 3.46 billion years old, structures have been discovered that are believed to be the remains of cyanobacteria - the first photosynthetic microorganisms. The former dominance of anaerobic microorganisms and cyanobacteria is evidenced by stromatolites found in shallow coastal waters of unpolluted salt water bodies. In shape, they resemble large boulders and represent an interesting community of microorganisms living in limestone or dolomite rocks formed as a result of their vital activity. To a depth of several centimeters from the surface, stromatolites are saturated with microorganisms: photosynthetic cyanobacteria that produce oxygen live in the uppermost layer; bacteria are found deeper, which are to a certain extent tolerant of oxygen and do not need light; the bottom layer contains bacteria that can only live in the absence of oxygen. Located in different layers, these microorganisms form a system united by complex relationships between them, including food ones. Behind the microbial film, a rock is found, which is formed as a result of the interaction of the remains of dead microorganisms with calcium carbonate dissolved in water. Scientists believe that when there were no continents on the primitive Earth and only archipelagos of volcanoes rose above the surface of the ocean, shallow water abounded in stromatolites.

As a result of the vital activity of photosynthetic cyanobacteria, oxygen appeared in the ocean, and about 1 billion years after that, it began to accumulate in the atmosphere. First, the formed oxygen interacted with iron dissolved in water, which led to the appearance of iron oxides, which gradually settled to the bottom. So over the course of millions of years, with the participation of microorganisms, huge deposits of iron ore arose, from which steel is smelted today.

Then, when the main amount of iron in the oceans was oxidized and could no longer bind oxygen, it escaped into the atmosphere in gaseous form.

After photosynthetic cyanobacteria created a certain supply of energy-rich organic matter from carbon dioxide and enriched the earth's atmosphere with oxygen, new bacteria arose - aerobes, which can exist only in the presence of oxygen. They need oxygen for the oxidation (burning) of organic compounds, and a significant part of the energy received in this case is converted into a biologically available form - adenosine triphosphate (ATP). This process is energetically very favorable: anaerobic bacteria, when decomposing one glucose molecule, receive only 2 ATP molecules, and aerobic bacteria that use oxygen, 36 ATP molecules.

With the advent of oxygen sufficient for an aerobic lifestyle, eukaryotic cells also debuted, which, unlike bacteria, have a nucleus and organelles such as mitochondria, lysosomes, and in algae and higher plants, chloroplasts, where photosynthetic reactions take place. Regarding the emergence and development of eukaryotes, there is an interesting and well-founded hypothesis, expressed almost 30 years ago by the American researcher L. Margulis. According to this hypothesis, the mitochondria that function as energy factories in the eukaryotic cell are aerobic bacteria, and the chloroplasts of plant cells in which photosynthesis occurs are cyanobacteria, probably absorbed by primitive amoebae about 2 billion years ago. As a result of mutually beneficial interactions, the absorbed bacteria became internal symbionts and formed a stable system, the eukaryotic cell, with the cell that absorbed them.

Studies of fossil remains of organisms in rocks of different geological ages have shown that for hundreds of millions of years after the emergence of eukaryotic life forms, they were represented by microscopic spherical unicellular organisms, such as yeast, and their evolutionary development proceeded at a very slow pace. But a little over 1 billion years ago, many new types of eukaryotes arose, which marked a sharp leap in the evolution of life.

First of all, this was due to the appearance of sexual reproduction. And if bacteria and unicellular eukaryotes reproduced, producing genetically identical copies of themselves and not needing a sexual partner, then sexual reproduction in more highly organized eukaryotic organisms occurs as follows. Two haploid, having a single set of chromosomes, the germ cells of the parents, merging, form a zygote that has a double set of chromosomes with the genes of both partners, which creates opportunities for new gene combinations. The emergence of sexual reproduction led to the emergence of new organisms, which entered the arena of evolution.

For three quarters of the entire existence of life on Earth, it was represented exclusively by microorganisms, until a qualitative leap in evolution took place, which led to the emergence of highly organized organisms, including humans. Let us trace the main milestones in the history of life on Earth in a descending line.

1.2 billion years ago there was an explosion of evolution, due to the appearance of sexual reproduction and marked by the emergence of highly organized forms of life - plants and animals.

The formation of new variations in the mixed genotype that occurs during sexual reproduction manifested itself in the form of a biodiversity of new life forms.

Complexly organized eukaryotic cells appeared 2 billion years ago, when unicellular organisms complicated their structure by absorbing other prokaryotic cells. Some of them - aerobic bacteria - turned into mitochondria - energy stations of oxygen respiration. Others - photosynthetic bacteria - began to carry out photosynthesis inside the host cell and became chloroplasts in the cells of algae and plants. Eukaryotic cells with these organelles and a well-defined nucleus, including genetic material, make up all modern complex forms of life - from molds to humans.

3.9 billion years ago, unicellular organisms appeared, which probably looked like modern bacteria, and archaebacteria. Both ancient and modern prokaryotic cells are relatively simple in structure: they do not have a well-formed nucleus and specialized organelles, their jelly-like cytoplasm contains DNA macromolecules - carriers of genetic information, and ribosomes on which protein synthesis occurs, and energy is produced on the cytoplasmic membrane surrounding cell.

4 billion years ago, RNA mysteriously arose. It is possible that it was formed from simpler organic molecules that appeared on the primitive earth. It is believed that ancient RNA molecules had the functions of carriers of genetic information and catalytic proteins, they were capable of replication (self-doubling), mutated and were subjected to natural selection. In modern cells, RNA does not have or does not exhibit these properties, but plays a very important role as an intermediary in the transfer of genetic information from DNA to ribosomes, in which protein synthesis occurs.

A.L. Prokhorov
Based on an article by Richard Monasterski
in National Geographic Magazine 1998 #3

The very first organisms

breeds archaea And early Proterozoic have come down to us in a greatly altered state. High pressures and temperatures have transformed the original appearance of the rock, destroying all traces of ancient life. Therefore, the study of the ancient animal and plant world is associated with enormous difficulties. However, over the past century, with the help of instruments, something has been clarified in the appearance the earliest organisms on earth.

Using an electron microscope, chemical and isotopic analyzes of the slates of the Onverwacht Suite (Rhodesia), which are more than 3.2 billion years old, scientists at the University of Arizona (USA) found thousands of tiny formations of spherical, filamentous and shell-shaped forms in them. The particle sizes did not exceed 0.01 mm. The studies were carried out in a specially equipped laboratory, which excluded the possibility of contamination of samples by foreign organisms. Scientists believe that the formations found are fossilized remains of unicellular algae. However, other researchers are critical of their findings, believing that these formations may have a non-biological origin.

Similar remains of algae and bacteria in rocks with an absolute age of 2.7-3.1 billion years have been found in siliceous and ferruginous shales of North America, Central Africa and Australia. These findings suggest that to the beginning of the Archean era chemical evolution ended and biological evolution began.

Based on the findings, it can be assumed that already in the oceans Archean and Early Proterozoic ages dominated by the simplest unicellular organisms: bacteria, algae, fungi, protozoa. In the Archean, the first organisms adapt to various forms of nutrition. Some organisms assimilated nutrients from water, carbon dioxide and inorganic salts (autotrophic) in the process of photosynthesis; others lived either at the expense of autotrophs (heterotrophs) or fed on decaying organic residues (saprophages). The organic world was divided into the kingdom of plants and the kingdom of animals.

In the early Proterozoic, apparently, the first multicellular organisms appeared. These are the most primitive forms without clearly differentiated tissues. These include, in particular, a representative of the sponge type - aquatic organisms leading a benthic attached lifestyle. The shape of the sponges is varied, it can resemble a cylinder, a goblet, a glass, a ball. In the soft tissue of an animal there is an organic or mineral skeleton consisting of spicules. Representatives of sponges still inhabit the seas and oceans of our planet, but the first primitive sponges died out long ago and have come down to us only in a fossil state.

Somewhat later, representatives of the intestinal type appear. They already have differentiation of tissues and organs. Representatives of the intestinal cavities, as well as sponges, have survived to this day and are widely settled in the seas, oceans and even in fresh water, among them are well-known to us corals, jellyfish, hydras.

from plants in the Archean and early Proterozoic are actively developing blue green algae. The remains of these algae in the form of spherical, mushroom-shaped and columnar calcareous bodies, characterized by thin concentric layering, are often found in Proterozoic rocks. It is believed that the first representatives of organic life on Earth were precisely blue green algae . Experiments carried out at Moscow State University in the last century showed that they can exist in conditions that are "contraindicated" for other plants and animals. In a hermetically sealed glass bowl, these algae lived for more than 16 years! All other inhabitants of such glass balls quickly died, some bacteria “held” for 12 years, only blue-green ones survived. This proves that they can develop even in an oxygen-free environment.

The amazing adaptability of these algae is evident from the fact that they are now found in the icy Arctic, in hot geysers, at the bottom of the Dead Sea, in oil sources, in the mountains at an altitude of more than 5000 meters. These are the only living organisms that survived the explosions of atomic and hydrogen bombs. They are found even inside nuclear reactors. Such amazing resilience has led some scientists to speculate about an unearthly origin. blue-green algae. Be that as it may, these are the first organisms that appeared not only in the ancient oceans, but also on land.

A study by American professor E. Barghorn showed that blue green algae were the first to borrow gaseous oxygen from water. In the oceans near their colonies, a kind of “watery” atmosphere saturated with oxygen was created. This oxygen was breathed by the first marine organisms (coelenterates, sponges). Gradually, oxygen began to be released into the atmosphere, filling it. Thanks to vitality blue-green algae on our planet began to form oxygen atmosphere.

The history of the development of life is studied according to the data geology And paleontology, since many fossil remains produced by living organisms have been preserved in the structure of the earth's crust. In place of the former seas, sedimentary rocks were formed containing huge layers of chalk, sandstones and other minerals, representing bottom sediments of calcareous shells and silicon skeletons of ancient organisms. There are also reliable methods for determining the age of terrestrial rocks containing organic matter. The radioisotope method is usually used, based on measuring the content of radioactive isotopes in the composition of uranium, carbon, etc., which regularly changes with time.

Let us immediately note that the development of life forms on Earth went in parallel with the geological restructuring of the structure and topography of the earth's crust, with changes in the boundaries of the continents and the oceans, the composition of the atmosphere, the temperature of the earth's surface, and other geological factors. These changes determined to a decisive extent the direction and dynamics of biological evolution.

The first traces of life on Earth date back to about 3.6–3.8 billion years old. Thus, life arose shortly after the formation of the earth's crust. In accordance with the most significant events of geobiological evolution in the history of the Earth, large time intervals are distinguished - eras, within them - periods, within periods - epochs, etc. For greater clarity, let's depict the life calendar as a conditional annual cycle, in which one month corresponds to 300 million years of real time (Fig. 6.2). Then the entire period of development of life on Earth will just be one conditional year of our calendar - from “January 1” (3600 million years ago), when the first protocells were formed, to “December 31” (zero years), when we live . As you can see, it is customary to count geological time in the reverse order.

(1) Archaea

Archean era(the era of ancient life) - from 3600 to 2600 million years ago, the length of 1 billion years - about a quarter of the entire history of life (on our conventional calendar, this is "January", "February", "March" and several days of "April").

Primitive life existed in the waters of the oceans in the form of primitive protocells. There was still no oxygen in the Earth's atmosphere, but there were free organic substances in the water, so the first bacterium-like organisms fed heterotrophically: they absorbed ready-made organic matter and received energy through fermentation. Autotrophic chemosynthetic bacteria or their new forms, archaea, could live in hot springs rich in hydrogen sulfide and other gases at temperatures up to 120°C. As the primary reserves of organic matter were depleted, autotrophic photosynthetic cells arose. In coastal areas, bacteria were released onto land, and soil began to form.

With the appearance of free oxygen in the water and atmosphere (from photosynthetic bacteria) and the accumulation of carbon dioxide, opportunities are created for the development of more productive bacteria, followed by the first eukaryotic cells with a real nucleus and organelles. Various protists (single-celled protozoa) subsequently developed from them, and then plants, fungi, and animals.

Thus, in the Archean era, pro- and eukaryotic cells with different types of nutrition and energy supply arose in the oceans. Prerequisites for the transition to multicellular organisms.

(2) Proterozoic

Proterozoic era(early life era), from 2600 to 570 million years ago, is the longest era, covering about 2 billion years, that is, more than half of the entire history of life.

Rice. 6.2. Eras and periods of development of life on Earth

Intensive processes of mountain building have changed the ratio of ocean and land. There is an assumption that at the beginning of the Proterozoic, the Earth underwent the first glaciation, caused by a change in the composition of the atmosphere and its transparency for solar heat. Many pioneer groups of organisms, having done their job, died out, and new ones came to replace them. But in general, biological transformations took place very slowly and gradually.

The first half of the Proterozoic was in full bloom and the dominance of prokaryotes - bacteria and archaea. At this time, the iron bacteria of the oceans, settling generation after generation to the bottom, form huge deposits of sedimentary iron ores. The largest of them are known near Kursk and Krivoy Rog. Eukaryotes were represented mainly by algae. Multicellular organisms were few and very primitive.

About 1000 million years ago, as a result of the photosynthetic activity of algae, the rate of oxygen accumulation increases rapidly. This is also facilitated by the completion of the oxidation of the iron in the earth's crust, which has so far absorbed the bulk of oxygen. As a result, the rapid development of protozoa and multicellular animals begins. The last quarter of the Proterozoic is known as the "age of jellyfish", since these and similar intestinal animals constituted the dominant and most progressive form of life at that time.

About 700 million years ago, our planet and its inhabitants are experiencing a second ice age, after which the progressive development of life becomes more dynamic. In the so-called Vendian period, several new groups of multicellular animals are laid down, but life is still concentrated in the seas.

At the end of the Proterozoic, triatomic oxygen O 3 accumulated in the atmosphere. This is ozone, which absorbs the ultraviolet rays of sunlight. The ozone shield reduced the level of mutagenicity of solar radiation. Further neoplasms were numerous and varied, but they were less radical in nature - within the already formed biological kingdoms (bacteria, archaea, protists, plants, fungi, animals) and the main types.

So, during the Proterozoic era, the dominance of prokaryotes was replaced by the dominance of eukaryotes, there was a radical transition from unicellularity to multicellularity, and the main types of the animal kingdom were formed. But these complex forms of life existed exclusively in the seas.

The earth's land at that time represented one large continent; geologists gave it the name Paleopangea. In the future, the global plate tectonics of the earth's crust and the corresponding drift of the continents will play a large role in the evolution of terrestrial life forms. In the meantime, in the Proterozoic, the rocky surface of the coastal areas was slowly covered with soil, bacteria, lower algae, and the simplest unicellular animals settled in the damp lowlands, which still perfectly existed in their ecological niches. The land was still waiting for its conquerors. And on our historical calendar it was already the beginning of “November”. Before the “New Year”, before our days, there were less than “two months”, only 570 million years.

(3) Paleozoic

Palaeozoic(era of ancient life) - from 570 to 230 million years ago, the total length is 340 million years.

The next period of intensive mountain building led to a change in the relief of the earth's surface. Paleopangea was divided into the giant continent of the Southern Hemisphere Gondwana and several small continents of the Northern Hemisphere. Former land areas were under water. Some groups became extinct, but others adapted and developed new habitats.

The general course of evolution, starting from the Paleozoic, is shown in Fig. 6.3. Please note that most of the directions of evolution of organisms that originated at the end of the Proterozoic continue to coexist with newly emerging young groups, although many reduce their volume. Nature parted with those who do not meet changing conditions, but preserves successful options as much as possible, selects and develops of them are the most adapted and, in addition, creates new forms, among them are chordates. Higher plants appear - land conquerors. Their body is divided into a root and a stem, which allows them to be well fixed on the soil and extract moisture and minerals from it.

Rice. 6.3. Evolutionary development of the living world from the end of the Proterozoic to our time

The area of ​​the seas either increases or decreases. At the end of the Ordovician, as a result of lowering the level of the world ocean and a general cooling, there was a rapid and massive extinction of many groups of organisms, both in the seas and on land. In the Silurian, the continents of the Northern Hemisphere merge into the supercontinent Laurasia, which is shared with the southern continent Gondwana. The climate becomes drier, milder and warmer. Armored “fish” appear in the seas, the first jointed animals come to land. With the new uplift of the land and the reduction of the seas in the Devonian, the climate becomes more contrasting. Mosses, ferns, mushrooms appear on the ground, the first forests are formed, consisting of giant ferns, horsetails and club mosses. Among animals, the first amphibians, or amphibians, appear. In the Carboniferous, marshy forests of huge (up to 40 m) tree-like ferns are widespread. It was these forests that left us deposits of coal (“coal forests”). At the end of the Carboniferous, the land rises and cools, the first reptiles appear, finally freed from water dependence. In the Permian period, another uplift of land led to the unification of Gondwana with Laurasia. The single mainland of Pangea was formed again. As a result of the next cooling, the polar regions of the Earth are subjected to glaciation. Tree-like horsetails, club mosses, ferns, and many ancient groups of invertebrates and vertebrates are dying out. In total, up to 95% of marine species and about 70% of terrestrial species died out by the end of the Permian period. But reptiles (reptiles) and new insects are rapidly progressing: their eggs are protected from drying out by dense shells, the skin is covered with scales or chitin.

The general result of the Paleozoic - the settlement of land by plants, fungi and animals. At the same time, both those and others, and the third, in the process of their evolution, become more complex anatomically, acquire new structural and functional adaptations for reproduction, respiration, and nutrition, which contribute to the development of a new habitat.

It ends with the Paleozoic, when on our calendar “December 7th”. Nature is “in a hurry”, the pace of evolution in groups is high, the timing of transformations is being compressed, but the first reptiles are only entering the scene, and the time of birds and mammals is still far ahead.

(4) Mesozoic

Mesozoic era(era of middle life) - from 230 to 67 million years ago, the total length is 163 million years.

The uplift of the land, which began in the previous period, continues. Initially, there is a single mainland Pangea. Its total area is much larger than the current land area. The central part of the continent is covered with deserts and mountains; the Urals, Altai and other mountain ranges have already been formed. The climate is becoming more and more arid. Only river valleys and coastal lowlands are inhabited by monotonous vegetation of primitive ferns, cycads and gymnosperms.

In the Triassic, Pangea gradually breaks up into northern and southern continents. Among the animals on land, herbivorous and predatory reptiles, including dinosaurs, begin their “triumphal procession”. Among them there are already modern species: turtles and crocodiles. Amphibians and various cephalopods still live in the seas, and bony fish of a completely modern look appear. This abundance of food attracts predatory reptiles to the sea, their specialized branch - ichthyosaurs - is separated. From some early reptiles, small groups separated themselves, giving rise to birds and mammals. They already have an important feature - warm-bloodedness, which will give great advantages in the further struggle for existence. But their time is still ahead, but for now dinosaurs continue to master the earthly spaces.

In the Jurassic period, the first flowering plants appeared, and giant reptiles dominate among animals, having mastered all habitats. In warm seas, in addition to marine reptiles, bony fish and a variety of cephalopods, similar to modern squids and octopuses, thrive. The split and drift of the continents continues with a general direction towards their present state. This creates conditions for isolation and relatively independent development of fauna and flora on different continents and island systems.

In the Cretaceous period, in addition to egg-laying and marsupial mammals, placental mammals appeared, which for a long time bear cubs in the mother's womb in contact with blood through the placenta. Insects begin to use flowers as a source of food, while simultaneously contributing to their pollination. Such cooperation has brought benefits to both insects and flowering plants. The end of the Cretaceous period was marked by a decrease in the level of the ocean, a new general cooling and the mass extinction of many groups of animals, including dinosaurs. It is believed that 10–15% of the former species diversity remained on land.

There are different versions of these dramatic events at the end of the Mesozoic. The most popular scenario is a global catastrophe caused by a giant meteorite or asteroid falling to the Earth and leading to the rapid destruction of the biospheric balance (shock wave, atmospheric dusting, powerful tsunami waves, etc.). However, everything could be much more prosaic. The gradual restructuring of the continents and climate change could lead to the destruction of the existing food chains built on a limited range of producers. First, some invertebrates, including large cephalopods, died out in the colder seas. Naturally, this led to the extinction of sea lizards, for which cephalopods were the main food. On land, there was a reduction in the growth zone and biomass of soft succulent vegetation, which led to the extinction of giant herbivorous dinosaurs, followed by predatory dinosaurs. The food supply for large insects was also reduced, and flying lizards began to disappear behind them. As a result, within a few million years, the main groups of dinosaurs became extinct. It must be borne in mind that the reptiles were cold-blooded animals and were not adapted to exist in a new, much more severe climate. Under these conditions, small reptiles survived and further developed - lizards, snakes; and relatively large ones, such as crocodiles, turtles, tuatara, survived only in the tropics, where the necessary food supply and mild climate remained.

Thus, the Mesozoic era is rightfully called the era of reptiles. For 160 million years, they survived their heyday, the widest divergence in all habitats and died out in the fight against the inevitable elements. Against the backdrop of these events, warm-blooded organisms – mammals and birds, who have moved to the development of liberated ecological niches, received huge advantages. But it was already a new era. Until the “New Year” there were “7 days”.

(5) Cenozoic

Cenozoic era(era of new life) - from 67 million years ago to the present. This is the era of flowering plants, insects, birds and mammals. In this era, a man appeared.

At the beginning of the Cenozoic, the location of the continents is already close to modern, but there are wide bridges between Asia and North America, the latter is connected through Greenland with Europe, and Europe is separated from Asia by a strait. South America was isolated for several tens of millions of years. India is also isolated, although it is gradually moving north towards the Asian continent. Australia, which at the beginning of the Cenozoic was associated with Antarctica and South America, about 55 million years ago completely separated and gradually moved north. On isolated continents, special directions and rates of evolution of flora and fauna are created. For example, in Australia, the absence of predators allowed the preservation of ancient marsupials and egg-laying mammals, long extinct on other continents. Geological rearrangements contributed to the emergence of ever greater biodiversity, as they created great variations in the living conditions of plants and animals.

About 50 million years ago, in the territory of North America and Europe, a detachment of primates appeared in the class of mammals, which subsequently gave rise to monkeys and humans. The first people appeared about 3 million years ago (7 hours before the New Year), apparently, in the eastern Mediterranean. At the same time, the climate became more and more cool, the next (fourth, counting from the early Proterozoic) ice age set in. In the northern hemisphere, four periodic glaciations have occurred over the past million years (as phases of an ice age, alternating with temporary warming). During this time, mammoths, many large animals, and ungulates died out. An important role in this was played by people who were actively engaged in hunting and farming. The human of the modern species was formed only about 100 thousand years ago (after “23 hours 45 minutes on December 31” of our conditional year of life; we exist this year for only its last quarter of an hour!).

In conclusion, we emphasize again that driving forces biological evolution must be seen in two interconnected planes - geological and proper biological. Each successive large-scale restructuring of the earth's surface entailed inevitable transformations in the living world. Each new cold snap led to the mass extinction of ill-adapted species. The drift of the continents determined the difference in the rates and directions of evolution in large isolates. On the other hand, the progressive development and reproduction of bacteria, plants, fungi, and animals also affected geological evolution itself. As a result of the destruction of the mineral basis of the Earth and its enrichment with metabolic products of microorganisms, the soil arose and was constantly rebuilt. The accumulation of oxygen at the end of the Proterozoic led to the formation of an ozone screen. Many waste products remained forever in the bowels of the earth, transforming them irreversibly. These are organogenic iron ores, and deposits of sulfur, chalk, coal, and much more. The living, generated from inanimate matter, evolves together with it, in a single biogeochemical flow of matter and energy. As for the inner essence and direct factors of biological evolution, we will consider them in a special section (see 6.5).

Most modern scientists believe that the Earth was formed a little earlier than 4.5 billion years ago. Life on it arose relatively quickly. The earliest remains of extinct microorganisms have been found in silica deposits dating back to 3.8 billion years (see Life and Its Origins).

The first inhabitants of the Earth were prokaryotes - organisms without a formed nucleus, similar to modern bacteria. They were anaerobes, that is, they did not use free oxygen for respiration, which was not yet in the atmosphere. The source of food for them was organic compounds that arose even on the lifeless Earth as a result of the action of ultraviolet solar radiation, lightning discharges and the heat of volcanic eruptions. Another source of energy for them was reduced inorganic substances (sulfur, hydrogen sulfide, iron, etc.). Photosynthesis also arose relatively early. Bacteria were also the first photosynthetics, but they used not water, but hydrogen sulfide or organic substances as a source of hydrogen ions (protons). Life then was represented by a thin bacterial film at the bottom of reservoirs and in wet places on land. This era of the development of life is called Archean, the most ancient (from the Greek word ἀρχαῖος - ancient).

An important evolutionary event occurred at the end of the Archean. About 3.2 billion years ago, one of the groups of prokaryotes - cyanobacteria developed a modern, oxygenic mechanism of photosynthesis with the splitting of water under the action of light. The resulting hydrogen combined with carbon dioxide, and carbohydrates were obtained, and free oxygen entered the atmosphere. The Earth's atmosphere gradually became oxygenic, oxidizing. (It is possible that a significant part of the oxygen could have been released from the rocks when the metal core of the Earth was formed.)

All this had important consequences for life. Oxygen in the upper atmosphere under the influence of ultraviolet rays turned into ozone. The ozone shield reliably protected the Earth's surface from the harsh solar radiation. The emergence of oxygen respiration, which is energetically more favorable than fermentation, glycolysis, and, consequently, the emergence of larger and more complex eukaryotic cells, became possible. First unicellular, and then multicellular organisms arose. Oxygen also played a negative role - all mechanisms of atmospheric nitrogen binding are suppressed by it. Therefore, atmospheric nitrogen is still bound by bacteria - anaerobes and cyanobacteria. The life of all other organisms on Earth, which arose later, already in an oxygen atmosphere, practically depends on them.

Cyanobacteria, along with bacteria, were widespread on the surface of the Earth at the end of the Archean and the subsequent era - the Proterozoic, the era of primary life (from the Greek words πρότερος - earlier and ζωή - life). The deposits formed by them are known - stromatolites (“carpet stones”). These ancient photosynthetics used soluble calcium bicarbonate as a source of carbon dioxide. At the same time, insoluble carbonate settled on the colony with a calcareous crust. Stromatolites in many places form whole mountains, but the remains of microorganisms are preserved only in some of them.

Somewhat later, cyanobacteria, the ancestors of chloroplasts, became symbionts of some of the first eukaryotes. The remains of the first undoubted eukaryotes - protozoa and colonial algae - were found in the deposits of the Proterozoic era. They look like Volvox.

In the next, Devonian period (from the name of the county in Great Britain), which lasted about 60 million years, various ferns replaced psilophytes, and fish, in which the anterior pair of gill arches turned into jaws, were jawless. In the Devonian, the main groups of fish already appeared - cartilaginous, ray-finned and lobe-finned. Some of the latter came to land at the end of the Devonian, giving rise to a large group of amphibians.

Cenozoic begins with the Tertiary period. The early Tertiary, or Paleogene, period includes the epochs: Paleocene, Eocene and Oligocene, which lasted 40 million years. At this time, all living orders of mammals and birds arose. New life reached its peak at the beginning of the Neogene period, during the Miocene epoch, which began 25 million years ago. At the same time, the first great apes appeared. A severe cooling at the end of the next epoch, the Pliocene, led to the extinction of heat-loving flora and fauna in large areas of Eurasia and North America. About 2 million years ago, the last period of the Earth's history begins - the Quaternary. This is the period of the formation of man, so it is often called an anthropogen.