How the first living things appeared. The origin of life on Earth: The main stages of the development of the biosphere

Life appeared on our planet about half a billion years after the origin of the Earth, that is, about 4 billion years ago: it was then that the first common ancestor of all living beings arose. It was a single cell, the genetic code of which included several hundred genes. This cell had everything necessary for life and further development: mechanisms responsible for protein synthesis, reproduction of hereditary information and the production of ribonucleic acid (RNA), which is also responsible for encoding genetic data.

Scientists understood that the first common ancestor of all living beings arose from the so-called primordial soup - amino acids that arose from the compounds of water with chemical elements that filled the reservoirs of the young Earth.

The possibility of forming amino acids from a mixture of chemical elements was proven as a result of the Miller-Urey experiment, which Gazeta.Ru reported on several years ago. During the experiment, Stanley Miller simulated the atmospheric conditions of the Earth about 4 billion years ago in test tubes, filling them with a mixture of gases - methane, ammonia, carbon and carbon monoxide - adding water and passing an electric current through the test tubes, which was supposed to produce the effect of lightning discharges.

As a result of the interaction of chemicals, Miller obtained five amino acids in test tubes - the basic building blocks of all proteins.

Half a century later, in 2008, researchers re-analyzed the contents of the test tubes, which Miller kept intact, and found that in fact the mixture of products contained not 5 amino acids at all, but 22, it was just that the author of the experiment could not identify them several decades ago.

After this, scientists were faced with the question of which of the three basic molecules contained in all living organisms (DNA, RNA or proteins) became the next step in the formation of life. The complexity of this issue lies in the fact that the process of formation of each of the three molecules depends on the other two and cannot be carried out in its absence.

Thus, scientists had to either recognize the possibility of the formation of two classes of molecules at once as a result of random successful combination amino acids, or agree that the structure of their complex relationships formed spontaneously, after the emergence of all three classes.

The problem was resolved in the 1980s, when Thomas Check and Sidney Altman discovered the ability of RNA to exist completely autonomously, acting as an accelerator chemical reactions and synthesizing new RNAs similar to themselves. This discovery led to the “RNA world hypothesis,” first proposed by microbiologist Carl Woese in 1968 and finally formulated by laureate biochemist Nobel Prize in chemistry by Walter Gilbert in 1986. The essence of this theory is that the basis of life is recognized as ribonucleic acid molecules, which in the process of self-reproduction could accumulate mutations. These mutations eventually led to the ability of ribonucleic acid to create proteins. Protein compounds are more efficient catalysts than RNA, which is why the mutations that created them became entrenched in the process. natural selection.

At the same time, “repositories” of genetic information—DNA—were formed. Ribot nucleic acids preserved as an intermediary between DNA and proteins, performing many different functions:

they store information about the sequence of amino acids in proteins, transfer amino acids to sites of synthesis of peptide bonds, and take part in regulating the degree of activity of certain genes.

On this moment Scientists do not have clear evidence that such RNA synthesis as a result of random amino acid combinations is possible, although there is certain confirmation of this theory: for example, in 1975, scientists Manfred Samper and Rudiger Luce demonstrated that under certain conditions RNA can spontaneously arise in a mixture containing only nucleotides and a replicase, and in 2009, researchers from the University of Manchester proved that uridine and cytidine - the constituent parts of ribonucleic acid - could be synthesized under the conditions of the early Earth. However, some researchers continue to criticize the “RNA world hypothesis” due to the extremely low probability of spontaneous emergence of ribonucleic acid with catalytic properties.

Scientists Richard Wolfenden and Charles Carter from the University of North Carolina proposed their version of the formation of life from the primordial " building material" They believe that amino acids, formed from a set of chemical elements that existed on Earth, became the basis for the formation not of ribonucleic acids, but of other, simpler substances - protein enzymes, which made the appearance of RNA possible. The researchers published the results of their work in the journal PNAS .

Richard Wolfenden analyzed physical properties 20 amino acids and came to the conclusion that amino acids could independently ensure the process of forming the structure of a complete protein. These proteins, in turn, were enzymes—molecules that speed up chemical reactions in the body. Charles Carter continued the work of his colleague, showing with the example of an enzyme called aminoacyl-tRNA synthetase the enormous importance that enzymes could play in the further development of the foundations of life: these

protein molecules are able to recognize transport ribonucleic acids, ensure their correspondence to sections of the genetic code, and thereby organize the correct transfer of genetic information to subsequent generations.

According to the authors of the study, they were able to find the very “missing link”, which was an intermediate stage between the formation of amino acids from primary chemical elements and the folding of complex ribonucleic acids from them. The process of formation of protein molecules is quite simple compared to the formation of RNA, and its feasibility was proven by Wolfenden by studying 20 amino acids.

The scientists’ findings also provide an answer to another question that has worried researchers for a long time, namely: when did the “division of labor” occur between proteins and nucleic acids, which include DNA and RNA. If Wolfenden and Carter’s theory is correct, then we can safely say: proteins and nucleic acids “divided” their main functions among themselves at the dawn of life, namely about 4 billion years ago.

Is life the result of evolution or creation? This dilemma has troubled the minds of more than one generation of scientists. Endless debates on this matter give rise to more and more interesting theories.

Order vs chaos

The second law of thermodynamics (entropy) states that all elements of the cosmos move from order to chaos. This is noted by NASA scientist Robert Destrow, who claims that “the universe stops like a clock.” Creationists rely on the law of entropy to prove the inconsistency of the point of view of evolutionists, which assumes the spontaneous development and complication of all elements of the surrounding world.

The 19th century theologian William Peley made the following analogy. We know that pocket watch did not arise by themselves, but were made by man: it follows that such a complex structure as the human body is also the result of creation.

Charles Darwin opposed this point of view with his theory of the power of natural selection, which, relying on hereditary variability in the process of long-term evolution, is capable of forming the most complex organic structures.

“But organic life could not arise from inanimate matter,” creationists pointed out. vulnerable spot Darwin's theories.

Only relatively recently did the research of chemists Stanley Miller and Harold Urey provide arguments in defense of the theory of evolution.

An experiment by American scientists confirmed the hypothesis that conditions existed on the primitive Earth that contributed to the emergence of biological molecules from inorganic substances. According to their findings, the molecules were formed in the atmosphere as a result of ordinary chemical reactions, and then, falling with rain into the ocean, led to the birth of the first cell.

How old is the Earth?

In 2010, American biochemist Douglas Theobald tried to prove that all life on Earth has a common ancestor. He mathematically analyzed the sequences of the most common proteins and found that the selected molecules are found in humans, flies, plants, and bacteria. The probability of a common ancestor, according to the scientist’s calculations, was 102,860.

According to the theory of evolution, the process of transition from the simplest organisms to the higher ones takes billions of years. But creationists claim that this is impossible, since the age of the Earth does not exceed several tens of thousands of years.

All species of animals and plants, in their opinion, appeared almost simultaneously and independently of each other - in the form in which we can observe them now.

Modern science, based on data from radioisotope analysis of terrestrial samples and meteorite matter, determines the age of the Earth at 4.54 billion years. However, as some experiments have shown, this dating method can have very serious errors.

In 1968, the American "Magazine geographical research» published radioisotope analysis of volcanic rocks formed in Hawaii as a result of a volcanic eruption that occurred in 1800. The age of the rocks was determined to range from 22 million to 2 billion years.

Radiocarbon analysis, which is used to date biological remains, also leaves many questions. This method allows the age limit of the samples to be set at 60,000 years with 10 half-lives of carbon-14. But how to explain the fact that carbon-14 is found in samples of “Jurassic wood”? “Only because the age of the Earth has been unreasonably advanced,” creationists insist.

Paleontologist Harold Coffin notes that the formation of sedimentary rocks occurred unevenly and is difficult to recognize true age of our planet. For example, fossils of fossil trees near Joggins (Canada), vertically penetrating the ground layer for 3 meters or more, indicate that plants were buried under a very short period of time as a result of catastrophic events.

Rapid evolution

If we assume that the Earth is not so ancient, is it possible for evolution to fit into a more compressed time frame? In 1988, a team of American biologists led by Richard Lenski decided to conduct a long-term experiment simulating the evolutionary process in the laboratory using the example of the bacterium Escherichia coli.

12 colonies of bacteria were placed in an identical environment, where only glucose was present as a food source, as well as citrate, which, in the presence of oxygen, could not be absorbed by the bacteria.

Scientists observed E. coli for 20 years, during which time more than 44 thousand generations of bacteria changed. In addition to changes in the size of bacteria typical of all colonies, scientists discovered interesting feature, inherent in only one colony: in it, bacteria somewhere between the 31st and 32nd thousand generations showed the ability to assimilate citrate.

In 1971, Italian scientists brought 5 wall lizards to the island of Pod Markaru, located in the Adriatic Sea. Unlike their previous habitat, there were few insects on the island, which the lizards fed on, but a lot of grass. Scientists checked the results of their experiment only in 2004. What did they see?

The lizards have adapted to an unusual environment: their population has reached 5,000 individuals, but most importantly, the reptiles have changed appearance and structure internal organs. In particular, the head and bite force increased to cope with large leaves, and a new section in the digestive tract appeared - a fermentation chamber, which allowed the lizards' intestines to digest tough cellulose. So, in just 33 years, wall lizards turned from predators into herbivores!

Weak link

If science is able to confirm intraspecific changes experimentally, then the possibility of the emergence of a new species during evolution remains exclusively in theory. Proponents of creationism not only point out to evolutionists the absence of intermediate forms of living organisms, but also try to scientifically confirm the inconsistency of the evolutionary theory of the origin of species.

Spanish geneticist Svante Pääbo managed to extract DNA from a fragment of a Neanderthal vertebra, believed to have lived about 50,000 years ago. Comparative analysis The DNA of modern humans and Neanderthals showed that the latter is not our ancestor.

US geneticist Alan Wilson using the method mitochondrial DNA could presumably say when “Eve” appeared on Earth. His studies gave an age of 150-200 thousand years. Japanese scientist Satoshi Horai provides similar data. In his opinion, modern man appeared in Africa about 200 thousand years ago, and from there moved to Eurasia, where it quickly replaced the Neanderthal.

Based on data from the fossil record, biologist Jonathan Wells notes: “It is quite clear that, at the level of kingdoms, phyla and classes, the origin of common ancestors through modification cannot be considered an immutable fact.”

Russian mathematician and philosopher Julius Schroeder notes that we do not know how to measure, on a scale known to us, the duration of the six days during which God created the world, because time itself was created during these same days. “The order of creation is fully consistent with the ideas of modern cosmology,” the scientist notes.

Doctor biological sciences Yuri Simakov even considers humans to be a product of genetic engineering. He suggests that the experiment was carried out at the junction of two species - Neanderthal and Homo sapiens. According to the biologist, there is “a complex and deliberate intervention of an intelligence that must be orders of magnitude superior to ours.”

The staff of the Hall of Evolution, which is located at the St. Louis Zoo, decided to reconcile the two theories in a humorous manner. At the entrance they hung a notice that read: “This is not at all saying that the living world could not have been created immediately - it just looks as if it appeared as a result of a long evolution.”

It has long history. It all started approximately 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 can be heard on the surface of the planet except erupting volcanoes and the noise of the wind. Scientists believe that this is what our planet looked like when life began to appear on it. It is very difficult to confirm or refute this. Rocks that could give more information to people were destroyed a long time ago, thanks to geological processes planets. So, the main stages of the evolution of life on Earth.

Evolution of life on Earth. Unicellular organisms.

Life began with the appearance of the simplest forms of life - single-celled organisms. The first unicellular organisms were prokaryotes. These organisms were the first to appear after the Earth became suitable for 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 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 were able to use nitrogen from the Earth’s atmosphere. Such creatures were blue-green algae. The evolution of single-celled organisms occurred after terrible events in the life of the planet and all stages of evolution were protected under magnetic field land.

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

Evolution of life on Earth. Multicellular organisms.

After the emergence of single-celled 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.

First stage of life - Colonial unicellular stage. The transition from unicellular organisms to multicellular ones, the structure of organisms and the genetic apparatus becomes more complex. 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 organisms of one colony. At this stage, specialization of body functions occurred at the tissue, organ and systemic 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 was a coordination center.

Third stage of life - Centrally differentiated stage. During this stage, the morphophysiological structure of organisms becomes more complex. Improvement of this structure occurs through increased tissue specialization. The nutritional, excretory, generative and other systems of multicellular organisms become more complex. U nervous systems a well-defined nerve center appears. Reproduction methods are improving - from external to internal fertilization.

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 land. Then other groups of terrestrial plants arose: ferns, mosses and others. These plants reproduced by spores, but preferred aquatic environment a habitat.

Plants have reached great diversity in Carboniferous period. Plants developed and could reach a height of up to 30 meters. During this period, the first gymnosperms appeared. The most widespread species were lycophytes and cordaites. Cordaites resembled coniferous plants in their trunk shape and had long leaves. After this period, the surface of the Earth was diversified with various plants that reached 30 meters in height. Later a large number of Over time, our planet became similar to the one we know now. Now there is a huge variety of animals and plants on the planet, and man has appeared. Man, as a rational being, after he got “on his feet”, devoted his life to studying. Riddles began to interest people, as well as the most important thing - where did man 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.

Hello dear readers of the blog site! In today's article I would like to talk about one of the theories of the origin of life. This is the theory of evolution that Darwin talked about so much. Here you can read about DNA, ancient fossils, some laboratory experiments, etc.

As a result of chemical reactions, approximately 3,800 million years ago, the first complex compound was formed that was capable of self-reproduction.

Still remains a mystery origin of life on earth . Scientists are of the opinion that all forms of life have been in a process of constant and continuous development since Charles Darwin first described the process and.

With each subsequent generation weak sides are eliminated, and the strong ones are honed and new opportunities are identified. One species of ancestors could give rise to several forms of life, after which it either found its own niche in the ecosystem or died out.

Their own niche in the ecosystem allowed them to survive and maintain their original form, and at this time the descendants of these species fit perfectly into other niches.

As a result, a complex system of kinship lines was formed, which today connects all organisms living on Earth with their already extinct ancestors. Today, ancient remains of many of the extinct species are preserved as fossils.

Fossils can be found in sedimentary rocks. The age of these fossils is determined using advanced radioisotope dating techniques.

This allowed scientists to recreate an approximate picture of life on Earth of any - approximate, because only small share remains from the entire diversity of animal and plant life that has ever existed.

Yet one thing is clear from the fossils found: there is a system between extinct and existing organisms family ties, which resembles a tree, and on this tree more and more new branches appear over time.

Many of these branches wither and die (like dinosaurs), while other branches grow and flourish. If we trace any of these branches to the very foundation, then ultimately we will come to a single trunk - the progenitor of all organisms that have ever lived, that is, the source of the origin of life.

Footprints in the rock.

Unfortunately, this is not easy to do. Approximately 4,500 million years, according to modern estimates, is the age of the Earth. The oldest fossils are believed to be no more than 590 million years old, which corresponds to the beginning of the Cambrian geological period (Cambrian).

Fossils found in Cambrian rocks include the remains of various forms life. For example, such as: descended from their primitive ancestors, mollusks and worms.

In other words, they were somewhere in the middle of the evolutionary tree. Their origin in the so-called Precambrian era remains unclear, due to the fact that there are no organic remains left in the rocks of this period.

It's easy to explain the reason for this. Soft-bodied organisms do not leave fossils because, usually, after death, they turn into hard rock, have time to completely decompose.

It is likely that most organisms that lived during the Precambrian period were too fragile to leave clear deposits. This period accounts for 80% of the entire history of the Earth.

But this does not mean that they left no traces at all. Two researchers, in the early 1950s, began a thorough study of the rock formation on the shores of Upper in .

This layer of rock, known as chert, was 2,000 million years old. At first glance, there was nothing organic in them, but scientists, despite this, decided to examine small samples of rings using a microscope.

Amazing discovery.

They found unmistakable signs ancient life. These were the remains of tiny organisms that resemble microscopic single-celled bacteria and algae that still live today.

These fragile organisms were somehow miraculously imbued with glassy silica, which hardened into siliceous shale in which these organisms were preserved, like flies in amber. These curious white rings in the rock turned out to be the eroded remains of colonies of these organisms.

This find, which contained the organic remains of the specimens, was a revelation. Scientists around the world have resumed their study of breeds. They were in for a surprising reward after studying rocks they had previously thought were devoid of fossils.

The oldest form of life to date, about 3,500 million years old, was discovered in western Australia. But the study of the oldest rocks known to us, the Amitsoka gneiss in southwest Greenland, 3,800 million years old, did not produce the expected results.

No miracles.

Biologists do not find anything surprising in the fact that the found primitive remains resemble modern ones. Such single-celled organisms have always been considered the simplest forms of life, and it is natural that they are its most primitive forms.

The mode of existence of single-celled life forms is easy to figure out due to their simplicity. Biologists, instead of studying the mechanism of functioning of muscles and organs, study how the initial chemical substances turn into the “building blocks” of life - sugar, fats and proteins.

Simple cell.

These studies are especially important to uncover the mystery of the origin of life. Because the next transformation, which marked the beginning of the whole process - from inorganic living substances to living matter, had to take place.

The bacterium itself is a feeding protozoan cell; it is a fluid-filled, gelatinous membrane that processes simple chemicals, which consist of nitrogen, carbon, oxygen and hydrogen, into complex organic compounds: carbohydrates that give her energy (sugar) and the necessary proteins for her growth.

Structure of DNA.

Deoxyribonucleic acid (DNA) is an organic substance that ultimately controls these processes. DNA, in addition, has another important property: it can reproduce itself.

Each DNA molecule resembles spiral staircase, in which chains of atoms form lateral sides with jumpers (“steps”) located at different intervals.

If necessary, the entire molecule can be bifurcated, with the bridges being separated in the middle. After the spiral bifurcates, the shortened “steps” attract other substances, which, when added, form the missing halves of the “ladder” - thus, from one spiral, two are obtained.

This simple technique is the essence of life. Thanks to it, a single-celled organism grows and reproduces itself, splits down the middle and at the same time copies its internal chemical process.

Reproducing cells, in more complex forms of life, work together to form multicellular structures, each structure being only part of an extremely complex process. The genetic code controls the entire process. This code is embedded in the DNA molecule and differs different types and individuals.

Functions of DNA.

Mechanisms are all life processes (drinking, eating, removing waste products from the body) that serve to ensure the activity of DNA.

DNA is a very complex molecule, the more complex a life form, the more complex its DNA. The structure of the simplest DNA consists of thousands of atoms; these atoms are grouped into nucleotides - these are compounds of phosphates, sugars and nitrogenous bases.

Each nucleotide itself is also quite a complex structure. This also applies to other organic molecules, such as carbohydrates and proteins. From chains of amino acids (of which there are only 20 various types), located in a certain sequence, consist of proteins.

A simple chain can consist of 100 links, and a complex chain can consist of several thousand links. The genetic code of a given organism determines the entire structure.

The simplest bacterial cell contains DNA, carbohydrates and proteins, without which it cannot function. Of the life forms known today, these cells are the most primitive form.

From this we can conclude that they originated from non-living structures that synthesized these essential elements life before they found organic use.

"Primordial Broth".

No one knows what our world was like 3,800 million years ago. Scientists Haldane and Oparin, in the 20s, put forward a theory according to which, in those distant times, the Earth was almost completely devoid of oxygen, and consisted of hydrogen, ammonia, water, methane, carbon monoxide and a number of other substances.

They assumed that hot water covered most of the Earth's surface, and the boiling of this water was maintained by magma, the molten rock that lies beneath thin oceanic rock.

According to their hypothesis, such a mixture hot water and gases could lead to the formation of the so-called “primary broth”, which is rich in the chemical elements necessary for the synthesis of life.

The reaction was initiated, could be volcanic activity, electrical discharge lightning or intense ultraviolet radiation that passes through a thin layer of the atmosphere. American scientist Stanley Miller tested this theory experimentally in 1953.

Stanley Miller created a model of the primordial world, which consisted of two flasks and glass tubes. One of these flasks contained a solution whose composition, theoretically, corresponded sea ​​water. It filled the space above the liquid with a mixture of gases.

This mixture of gases also, theoretically, corresponded to the proposed atmosphere. This flask was connected by a tube to another flask, which had two electrodes to produce a spark - a miniature model of lightning.

Another tube extended from the spark chamber; this tube led to the first flask through a U-shaped manifold condenser.

When Miller heated the mixture in the lower flask, it boiled and turned into gas, then entered the chamber with a spark, and then condensed and flowed back into the lower flask. This process was carried out continuously for a week, and then the liquid was pumped out for analysis.

The results were positive. The resulting mixture contained three amino acids - compounds from which proteins are formed. This idea was picked up by many researchers. They conducted experiments similar to these, and as a result, they obtained even more amino acids, and even simple nucleotides - the building blocks of DNA.

Amazing results.

The results of these experiments are considered convincing and they give reason to believe that all the protein (and not only it) could have been synthesized over several billion years. DNA, presumably, could also be created along with its thousands of strictly arranged atoms.

Once it arose, it could reproduce itself, create its own proteins and other complex organic matter and develop into a functional self-replicating life form, such as a bacterial cell.

Something possible could have happened, but the mathematical probability of creating such a complex substance as DNA or protein is infinitesimal in the “primordial soup”, as a result of a random combination of chemical elements.

The example of a monkey with a typewriter can illustrate this probability. For example, if you give a monkey enough paper and allow it to type at random for several years, it will be able to reproduce some words, but the likelihood of it creating a literary masterpiece is almost zero. In this example, an amino acid can be compared to a word, but the masterpiece is undoubtedly DNA.

Today, this theory is recognized by many scientists who continue to search for mechanisms that facilitate the combination of amino acids into proteins without control by DNA.

If such a mechanism is found, humanity will take an important step towards understanding the mystery of DNA formation and, therefore, to elucidating the origin of life on Earth.

This is an article about the evolutionary theory of the origin of life, which, of course, has not yet been fully completed, and which can be argued a lot, but we will not do this 😉

The Earth was probably formed 4.5-5 billion years ago from a giant cloud of cosmic dust. the particles of which were compressed into a hot ball. Water vapor was released from it into the atmosphere, and water fell from the atmosphere onto the slowly cooling Earth for millions of years in the form of rain. In the recesses earth's surface the prehistoric ocean was formed. In it, approximately 3.8 billion years ago, the original life arose.

There are several theories about the origin of life on Earth. For example, one of the long-standing hypotheses says that it was brought to Earth from space, but there is no conclusive evidence of this. In addition, the life that we know is surprisingly adapted to exist precisely in terrestrial conditions, so if it arose outside the Earth, it would have been on an terrestrial-type planet. Most modern scientists believe that life originated on Earth, in its seas. But how did the planet itself come about and how did the seas appear on it?

There is one widely accepted theory about this. According to it, the Earth was formed from clouds of cosmic dust containing all known in nature chemical elements, which were compressed into a ball. Hot water vapor escaped from the surface of this red-hot ball, enveloping it in a continuous cloud cover. The water vapor in the clouds slowly cooled and turned into water, which fell in the form of abundant continuous rains on the still hot, burning Earth. On its surface it again turned into water vapor and returned to the atmosphere. Over millions of years, the Earth gradually lost so much heat that its liquid surface began to harden as it cooled. This is how the earth's crust was formed.

Millions of years passed, and the temperature of the Earth's surface dropped even more. Stormwater stopped evaporating and began to flow into huge puddles. Thus began the influence of water on the earth's surface. And then, due to the drop in temperature, a real flood occurred. Water that previously evaporated into the atmosphere and turned into it component, continuously fell to the Earth, with thunder and lightning, powerful showers fell from the clouds. Little by little, water accumulated in the deepest depressions of the earth's surface, which no longer had time to completely evaporate. There was so much of it that gradually a prehistoric Ocean formed on the planet. Lightning streaked the sky. But no one saw this. There was no life on Earth yet. The continuous rain began to erode the mountains. Water flowed from them in noisy streams and stormy rivers. Over millions of years, water flows have deeply eroded the earth's surface and valleys have appeared in some places. The water content in the atmosphere decreased, and more and more accumulated on the surface of the planet. The continuous cloud cover became thinner, until one fine day the first ray of the sun touched the Earth. The continuous rain has stopped. Most of the land was covered by the prehistoric Ocean. From its upper layers, the water washed away a huge amount of soluble minerals and salts, which fell into the sea. The water from it continuously evaporated, forming clouds, and the salts settled, and over time there was a gradual salinization of sea water. Apparently, under some conditions that existed in ancient times, substances were formed from which special crystalline forms arose. They grew, like all crystals, and gave rise to new crystals, which added more and more substances to themselves. sunlight and perhaps very strong electrical discharges served as a source of energy in this process. Perhaps the first inhabitants of the Earth - prokaryotes, organisms without a formed nucleus, similar to modern bacteria - arose from such elements. They were anaerobes, that is, they did not use free oxygen for breathing, which did not yet exist in the atmosphere. The source of food for them was organic compounds that arose on the still lifeless Earth as a result of exposure to ultraviolet radiation from the Sun, lightning discharges and heat generated during volcanic eruptions. Life then existed in a thin bacterial film at the bottom of reservoirs and in damp places. This era of the development of life is called Archean. From bacteria, and perhaps in a completely independent way, tiny single-celled organisms arose - the most ancient protozoa.

They still form the basis of life in the seas and freshwater bodies. They are so small that they can only be seen with a microscope. There are thousands and thousands of them in a drop of water from a small pond. The development of all animal life began with these simplest single-celled organisms. At the end of the Proterozoic, the next era after the Archean, 1000 - 600 million years ago, a fairly rich fauna already existed: jellyfish, polyps, flatworms, molluscs and echinoderms.

The picture shows primitive creatures that lived approximately 600 - 570 million years ago in the Cambrian geological period, the first period Paleozoic era. We first learned about them through fossils discovered by geologists studying the Cambrian Mountains in Great Britain. This is where the name of the geological period of history comes from.

There are no traces of the simpler animals and plants that inhabited the sea at the end of the Proterozoic. One can only assume that these were organisms consisting only of soft tissues, which quickly completely decomposed after death. There were no real fish in the Cambrian yet, but coelenterates, sponges, the now extinct archaeocyaths, flat and polychaete worms, snails, cuttlefish, crayfish and trilobites. The latter looked like crayfish up to 10 cm long. For that time they were real giants, larger than all other creatures. (There was no life on land at that time.) At the end of the Cambrian, the first chordates, similar to modern lancelets, apparently already appeared. Over the next millions of years, animals gradually changed, and in the next geological period - the Silurian, which began 500 - 400 million years ago, in addition to numerous trilobites on seabed new inhabitants appeared - sea scorpions.

In the water column of the Silurian Sea, single-celled organisms and jellyfish drifted passively. And crustaceans and trilobites, worms and animals protected by shells crawled along the seabed, for example bivalves and snails. Only a very few of them could swim. Even the first vertebrates, which already resembled fish in appearance, lived on the seabed. In Silurian in the seas and fresh waters Strange “fish” also appeared - without jaws and paired fins. Their relatives, hagfishes and lampreys, have survived to this day. During the Silurian period, the first true fish already appeared. These shark-like swimmers had a streamlined, shell-covered body, fins, and a mouth with a movable beak-like jaw lined with sharp teeth. Approximately 450 million years ago, in the Silurian, the first vertebrates appeared - fish. The body of one of the oldest - cephalaspis - was covered with armored scales, and the head with a bone shell. Apparently, Cephalaspis was a poor swimmer. Over millions of years in the same geological period, two large classes of fish developed - cartilaginous and bony (lungfish, lobe-finned and ray-finned). And cartilaginous, that is, having a cartilaginous skeleton, include sharks and rays. In contrast, the skeleton bony fish consists partly or entirely of bone tissue. Bony fish include almost all the commercial fish we are familiar with: herring, flounder, cod and mackerel, carp, pike and many others. In total, there are 20 thousand species of fish on Earth today, and they inhabit not only the seas, but also other bodies of water.

400 million years ago the Silurian gave way to the Devonian geological period, which lasted about 60 million years. Then the first plants appeared on land - lichens, which overgrown the moistened banks of reservoirs. During the Devonian, other forms evolved from them, including the first higher plants- ferns and horsetails. In addition, if previously all animals breathed only oxygen dissolved in water, now some of them have learned to extract it from the air. These first land animals - millipedes, scorpions and wingless primitive insects - probably lived near water. The ancestor of all land vertebrates was a lobe-finned fish with paw-like pectoral and ventral fins. Gradually, lobe-finned fish developed true upper and lower limbs, and over time, amphibians (amphibians) and reptiles (reptiles) appeared.

How do we know what ancient animals looked like?

All the changes that the Earth has undergone since the formation of its crust are studied by historical geology. Scientists determine the age of geological layers by fossils - the remains of ancient animals and plants, since each era had its own characteristic representatives of flora and fauna. Paleontology is the study of fossils. Paleontologists study the fossil remains of ancient organisms and restore the appearance of extinct animals. When living organisms died in the prehistoric Ocean, they sank to the bottom, where they were covered with silt or sand brought by rivers. Over millions of years, the silty soils, along with the remains buried under them, compacted, turning into stone. The soft tissues of the animals were completely decomposed, but the imprint remained. Hard mollusk shells or crustacean shells were often preserved intact. During historical development The earth's seabed is repeatedly affected powerful forces and the molten bowels of the planet were pushed out onto greater height and became part of the land. Researchers find remains and imprints of ancient animals embedded in rock and use them to study geological processes. Layers rocks for scientists - like the pages of a book with many drawings, and you just need to correctly decipher the “text” in order to understand how life on the planet developed. Layers of sand and silt containing fossils were deposited on top of each other over millions of years. This is how they were compressed: the more ancient layers are lower, the later layers are higher. By accumulating information about which layers are dominated by certain types of fossils, scientists have learned to determine what geological time they belong to. After this, it is quite easy to determine the age of the geological rock in which they were found from the found fossils.

The Grand Canyon of the Colorado River in the US state of Arizona is one of the few places where a huge, easy-to-read stone record of life on the planet has been preserved. Here the river cut through a layer of sedimentary rocks - limestone, sandstone and shale - to a depth of 1800 m. The river formed a canyon, that is, a deep valley with very steep slopes and a narrow bottom, eroding the bottom of the ancient sea. It rose very slowly and evenly. Mountain building, which is always accompanied by giant shifts and faults of rocks, did not occur here. Therefore, the sequence of occurrence of geological rocks has hardly changed. By studying the fossils of the layers of a steep slope, you can trace all the changes that occurred in the animal world of the ancient sea over hundreds of millions of years.