The modern concept of the origin of life on Earth is the result of a broad synthesis of natural sciences, many theories and hypotheses put forward by researchers of various specialties.

For the emergence of life on Earth, the primary atmosphere (of the planet) is important.

Earth's primary atmosphere contained methane, ammonia, water vapor, and hydrogen. By acting on a mixture of these gases with electric charges and ultraviolet radiation, scientists managed to obtain complex organic substances that make up living proteins. The elementary "building blocks" of living things are such chemical elements as carbon, oxygen, nitrogen and hydrogen.

In a living cell, by weight, it contains 70% oxygen, 17% carbon, 10% hydrogen, 3% nitrogen, followed by phosphorus, potassium, chlorine, calcium, sodium, magnesium, and iron.

So, the first step on the way to the emergence of life is the formation of organic substances from inorganic ones. It is associated with the presence of chemical "raw materials", the synthesis of which can occur under certain radiation, pressure, temperature and humidity.

The emergence of the simplest living organisms was preceded by a long chemical evolution. From a small number of compounds (as a result of natural selection), substances with properties suitable for life arose. Compounds that arose on the basis of carbon formed the "primary soup" of the hydrosphere. Substances containing nitrogen and carbon arose in the molten depths of the Earth and were brought to the surface during volcanic activity.

The second step in the emergence of compounds is associated with the emergence of biopolymers in the Earth's primary ocean: nucleic acids, proteins. If we assume that during this period all organic compounds were in the primary ocean of the Earth, then complex organic compounds could form on the surface of the ocean in the form of a thin film and in shallow water heated by the sun. The anaerobic environment facilitated the synthesis of polymers from inorganic compounds. Simple organic compounds began to combine into large biological molecules.

Enzymes were formed - protein substances - catalysts that contribute to the formation or disintegration of molecules. As a result of the activity of enzymes, the "primary elements" of life arose - nucleic acids, complex polymeric substances consisting of monomers.

Monomers in nucleic acids are arranged in such a way that they carry certain information, a code,

consisting in the fact that each amino acid included in the protein corresponds to a certain protein of 3 nucleotides (triplet). Proteins can be built on the basis of nucleic acids and exchange of matter and energy with the external environment can take place.

The symbiosis of nucleic acids formed "molecular genetic control systems".

At this stage, nucleic acid molecules acquired the properties of self-reproduction of their own kind, began to control the process of formation of protein substances.

The origins of all living things were revertase and matrix synthesis from DNA to RNA, the evolution of the r-RNA molecular system into DNA-nova. This is how the “genome of the biosphere” arose.

Heat and cold, lightning, ultraviolet reaction, atmospheric electric charges, gusts of wind and water jets - all this provided the beginning or attenuation of biochemical reactions, the nature of their course, gene "bursts".

By the end of the biochemical stage, such structural formations as membranes appeared, limiting the mixture of organic substances from the external environment.

Membranes have played a major role in the construction of all living cells. The bodies of all plants and animals are made up of cells.

Modern scientists have come to the conclusion that the first organisms on Earth were single-celled prokaryotes. In their structure, they resembled bacteria or blue-green algae that currently exist.

For the existence of the first "living molecules", prokaryotes, as for all living things, an influx of energy from the outside is necessary. Each cell is a small "energy station". ATP and other compounds containing phosphorus serve as a direct source of energy for cells. Cells receive energy from food, they are able not only to spend, but also to store energy.

Scientists suggest that many of the first lumps of living protoplasm arose on Earth. About 2 billion years ago, a nucleus appeared in living cells. Eukaryotes evolved from prokaryotes. There are 25-30 species of them on Earth. The simplest of them are amoeba. In eukaryotes, there is a decorated nucleus in the cell with a substance containing the code for protein synthesis.

By this time, there was a “choice” of a plant or animal lifestyle. The difference between these lifestyles is related to the mode of nutrition and the occurrence of photosynthesis, which consists in the creation of organic substances (for example, sugars from carbon dioxide and water using light energy).

Thanks to photosynthesis, plants produce organic matter, due to which an increase in the mass of plants occurs, and produce a large amount of organic matter.

With the advent of photosynthesis, oxygen began to enter the Earth's atmosphere, and a secondary Earth atmosphere with a high oxygen content was formed.

The appearance of oxygen and the intensive development of land plants is the greatest stage in the development of life on Earth. From that moment, a gradual modification and development of living forms began.

Life with all its manifestations has produced profound changes in the development of our planet. Improving in the process of evolution, living organisms spread more and more widely on the planet, taking a great part in the redistribution of energy and substances in the earth's crust, as well as in the air and water shells of the Earth.

The emergence and spread of vegetation led to a fundamental change in the composition of the atmosphere, initially containing very little free oxygen, and consisting mainly of carbon dioxide and, probably, methane and ammonia.

Plants assimilating carbon from carbon dioxide have created an atmosphere containing free oxygen and only traces of carbon dioxide. Free oxygen in the composition of the atmosphere served not only as an active chemical agent, but also as a source of ozone, which blocked the path of short ultraviolet rays to the Earth's surface (ozone screen).

At the same time, carbon, accumulated for centuries in the remains of plants, formed energy reserves in the earth's crust in the form of deposits of organic compounds (coal, peat).

The development of life in the oceans led to the creation of sedimentary rocks consisting of skeletons and other remains of marine organisms.

These deposits, their mechanical pressure, chemical and physical transformations have changed the surface of the earth's crust. All this testified to the presence of a biosphere on Earth, in which life phenomena unfolded and continue to this day.

Panspermia

Panspermia (from the Greek “mixture” and “seed”) is a very authoritative theory in our time about the appearance of life on Earth as a result of the transfer of “life germs” from other planets. This hypothesis was put forward by the German scientist G. Richter in 1865, who meant the transfer of spores of microorganisms either by meteorites or under the action of light pressure. Later, cosmic radiation was discovered, which acts on living organisms no less destructive than the decay of uranium. And the theory of panspermia "fell in dust" until the first flight to the Moon - when living microorganisms from the Earth were nevertheless found on the landed probe "Surveyor-3", which successfully survived a long flight in outer space.

In 2006, the presence of both water and the simplest organic compounds in the cometary substance was discovered. It's funny, but this means that a small meteorite with a luminous plume, which is approaching a much larger ball of the planet, is something like a cosmic analogue of the female and male germ cells, together giving rise to a new life.

Part of the followers of panspermia believes that the exchange of bacteria took place between the Earth and Mars at a time when the Red Planet was still flourishing and was partially covered by oceans. Moreover, meteorites did not necessarily serve this - perhaps intelligent visitors brought bacteria here (but this is a separate issue). But even if such events took place in history, we will have to figure out where life came from on another planet.

Electricity and primordial soup

The famous Miller-Urey experiment in 1953 proved that electrical sparks can generate the basis of life - amino acids and sucrose - in the presence of water, methane, ammonia and hydrogen in the atmosphere. This means that ordinary lightning could create the basic building blocks of life on ancient Earth, called the primordial soup. This term was introduced in 1924 by the Soviet biologist Oparin. According to his theory, this "soup" arose about 4 billion years ago in shallow water bodies of the planet under the influence of electrical discharges, cosmic radiation and high liquid temperature. At first, nucleotides, polypeptides, nitrogenous bases and amino acids predominated in its composition. Then, over millions of years, more complex molecules formed in the primordial soup until they formed the simplest single-celled organisms - bacteria.

clay life

According to religious sources, Adam was created from the dust of the earth, and in the Koran and among some peoples (for example, the Japanese), the gods molded people from clay. According to organic chemist Alexander Graham Kearns-Smith of the University of Glasgow in Scotland, this may not be a simple allegory: the first molecules of life could have formed on clay. Initially, primitive carbon compounds did not have DNA, which means that they could not reproduce their own kind - “reproduction” could only be stimulated by sources from the external environment.

Such a source could be clayey rock, which is not just a certain mass of earth - it is an organized, ordered sequence of molecules. The clay surface could not only concentrate and combine organic compounds, but at the microscopic level organize them into structures, acting like a genome. Over time, organic molecules "remembered" this sequence and learned to self-organize. Subsequently, they became more complicated: they had a prototype of DNA, RNA and other nucleic acids.

Life from the oceans

The Underwater Hydrothermal Vent Theory suggests that life may have originated at the source of underwater volcanoes, which ejected hydrogen-rich molecules and a lot of heat through cracks in the ocean floor. These molecules combined on the surface of the rocks, which provided mineral catalysts for new chemical reactions.

Thus, bacteria were born that formed the world-famous geological curiosity - stromatolites (from "stromatos" - carpet and "lithos" - stone). In petrified form, these formations have survived to this day. And underwater sources of this type continue to play an important role in maintaining a variety of marine ecosystems in our time.

Cold is the catalyst for evolution

Whichever of the scientists was right, but simple single-celled bacteria still populated the planet - and in this form they have invariably existed for more than a billion years. Then there was an incredibly fast explosion by the standards of evolution - much more complex life forms began to develop, which first mastered the oceans, and then land, soils and, finally, air. Not so long ago, scientists were able to figure out what was the impetus for decisive changes. It turned out to be the most powerful ice age in the history of the Earth, which began about 3 billion years ago. The planet was completely covered in ice up to one kilometer thick - experts called this phenomenon "Snowball Earth" (like those that children play).

The living conditions for the simplest microorganisms have changed dramatically - but, on the other hand, hardy extremophile bacteria had to adapt under the thickness of ice! It was during this "incubatory" period that the primary division of bacteria took place according to the ways of survival: some of them learned to receive energy from sunlight, others drew strength by processing substances dissolved in water. This marked the beginning of the kingdoms of wildlife - the first will become plants and unicellular photosynthetic animals in the future, the second - multicellular animals and fungi.

But one day, hot volcanoes woke up again, and splashed into the atmosphere a huge amount of carbon dioxide, which caused a powerful greenhouse effect. The planet warmed up, the ice melted and released "grown-up" bacteria. The process of photosynthesis occurring in cyanobacteria (blue-green algae) gave a new reaction - and the atmosphere was saturated with oxygen in a short time. And the fragments of mineral rocks brought by the glacier that fell into the ocean gave new options for chemical reactions. This, as it is already becoming clear, allowed animals to evolve. Soon, instead of dividing bacteria into two new ones, they began to divide without leaving the "free swimming", and form the first multicellular structures. An example is the most ancient multicellular animals without nervous, blood and digestive systems - sea sponges.

According to this theory, life is quite likely under a thick layer of ice on one of Jupiter's moons - in the cold oceans of Europa, hidden from space probes. A team of researchers from NASA also found that there is geothermal activity under the ice of the satellite. Therefore, it is quite possible that Europe is repeating our own path, and as our sun begins to age and become brighter, evolution will also take over the eternal cold.

Life appeared on our planet about half a billion years after the emergence of the Earth, that is, about 4 billion years ago: it was then that the first common ancestor of all living beings was born. It was a single cell, the genetic code of which included several hundred genes. This cell had everything necessary for life and further development: the mechanisms responsible for the synthesis of proteins, the 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 things originated from the so-called primordial soup - amino acids that arose from the combination 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 proved as a result of the Miller-Urey experiment, about which Gazeta.Ru reported 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 there 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 received five amino acids in test tubes - the basic building blocks of all proteins.

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

After that, scientists faced 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 a random successful combination of amino acids, or agree that the structure of their complex relationships formed spontaneously, after the emergence of all three classes.

The problem was solved in the 1980s, when Thomas Check and Sydney Altman discovered the ability of RNA to exist completely autonomously, acting as an accelerator of chemical reactions and synthesizing new RNAs similar to itself. This discovery led to the "RNA World Hypothesis", first proposed by the microbiologist Carl Wese in 1968 and finally formulated by the biochemist and Nobel Laureate in Chemistry Walter Gilbert in 1986. The essence of this theory lies in the fact that ribonucleic acid molecules are recognized as the basis of life, 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, and that is why the mutations that created them have become fixed in the process of natural selection.

At the same time, “repositories” of genetic information, DNA, were also formed. Ribonucleic acids have survived 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 the sites of synthesis of peptide bonds, and take part in regulating the degree of activity of certain genes.

At the moment, scientists do not have unambiguous evidence that such RNA synthesis as a result of random combinations of amino acids is possible, although there is some evidence for this theory: for example, in 1975, scientists Manfred Samper and Rudiger Lewis demonstrated that, under certain conditions, RNA can spontaneously arise in a mixture containing only nucleotides and replicase, and in 2009, researchers from the University of Manchester proved that uridine and cytidine, the constituents 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 generation of ribonucleic acid with catalytic properties.

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

Richard Wolfenden analyzed the physical properties of 20 amino acids and came to the conclusion that amino acids could independently provide 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 his colleague's work by showing, using 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 capable of recognizing transport ribonucleic acids, ensuring their correspondence to sections of the genetic code, and thereby organizing the correct transmission of genetic information to subsequent generations.

According to the authors of the study, they managed to find the very “missing link”, which was an intermediate step 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 realism was proved by Wolfenden using the example of studying 20 amino acids.

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

Lifeless mountains, rocks and water, a huge moon in the sky and a constant bombardment of meteorites - the most likely landscape of the Earth 4 billion years ago

Did life originate from inorganic matter in space, or did it originate on Earth? This dilemma necessarily confronts the researcher interested in the problem of the origin of life. So far, no one has been able to prove the correctness of any of the two hypotheses that currently exist, just as, however, it has not been possible to come up with a third way of solution.

The first hypothesis about the origin of life on Earth is old, it has solid figures of European science in its assets: G. Helmholtz, L. Pasteur, S. Arrhenius, V. Vernadsky, F. Crick. The complexity of living matter, the low probability of its spontaneous generation on the planet, as well as the failure of experimenters to synthesize living matter from non-living matter, lead scientists to the camp of adherents of this approach. There are numerous variations on exactly how life got to Earth, the most famous of which is the panspermia theory. According to her, life is widespread in interstellar space, but since there are no conditions for development, living matter turns into sperm, or spores, and thus moves through space. Billions of years ago, comets brought sperm to Earth, where an environment favorable for their disclosure was formed.

According to scientists, the synthesis of a living cell is not far off, it is a matter of technology and a matter of time. But will a test-tube-born cell be the answer to the question of how life on Earth began? Hardly. The synthetic cell will only prove that abiogenesis is somehow possible. But 4 billion years ago on Earth, things could have happened differently. For example, yes. The surface of the Earth cooled 4.5 billion years ago. The atmosphere was thin, and comets were actively bombarding the Earth, delivering organics in abundance. Extraterrestrial matter settled in shallow warm reservoirs heated by volcanoes: lava poured out at the bottom, islands grew, hot springs - fumaroles - beat. The continents at that time were not as strong and large as they are now, they easily moved along the earth's crust, connected and disintegrated.

The moon was closer, the earth was spinning faster, the days were shorter, the tides were higher, and the storms were more severe. Above it all stretched steel-colored skies, darkened by dust storms, clouds of volcanic ash, and shards of rock blasted out by meteorite impacts. An atmosphere rich in nitrogen, carbon dioxide and water vapor gradually developed. The abundance of greenhouse gases has caused global warming. Under such extreme conditions, the synthesis of living matter took place. Was it a miracle, an accident that happened despite the evolution of the universe, or is this the only way life can appear? Already in the early stages, one of the main features of living matter manifested itself - adaptability to environmental conditions. The early atmosphere contained little free oxygen, ozone was deficient, and the earth was bathed in ultraviolet rays that are deadly to life. So the planet would have remained uninhabited if the cells had not invented a mechanism for protecting against ultraviolet radiation. This scenario for the emergence of life as a whole does not differ from that proposed by Darwin. New details were added - they learned something by studying the most ancient rocks and experimenting, they guessed something. While the most reasonable, this scenario is also the most controversial. Scientists are fighting on each point, offering numerous alternatives. Doubts arise from the very beginning: where did the primary organic matter come from, did it synthesize on Earth or did it fall from the sky?

revolutionary idea

The scientific foundations of abiogenesis, or the origin of living things from non-living things, were laid by the Russian biochemist A.I. Oparin. In 1924, as a 30-year-old scientist, Oparin published the article "The Origin of Life", which, according to his colleagues, "contained the seeds of an intellectual revolution." The publication of Oparin's book in English in 1938 became a sensation and attracted significant Western intellectual resources to the problem of life. In 1953, S. Miller, a graduate student at the University of Chicago, conducted a successful experiment on abiogenic synthesis. He created the conditions of the early Earth in a laboratory test tube and obtained a set of amino acids as a result of a chemical reaction. Thus, Oparin's theory began to receive experimental confirmation.

Oparin and the priest

According to the memoirs of colleagues, Academician A.I. Oparin was a convinced materialist and atheist. This is confirmed by his theory of abiogenesis, which, it would seem, leaves no hope for a supernatural explanation of the mysteries of life. Nevertheless, the views and personality of the scientist attracted people of completely opposite worldviews to him. Being engaged in scientific and educational work, participating in the pacifist movement, he traveled abroad a lot. Once, sometime in the 1950s, Oparin lectured in Italy on the problem of the origin of life. After the report, he was told that none other than the president of the Pontifical Academy of Sciences from the Vatican wanted to meet him. Alexander Ivanovich, being a Soviet man and knowing full well the biased attitude of the foreign intelligentsia towards the USSR, did not expect anything good from the representative of the Catholic Church, probably some kind of provocation. Nevertheless, the acquaintance took place. The Reverend Signor shook hands with Oparin, thanked him for the lecture, and exclaimed: “Professor, I am delighted with how beautifully you have revealed the providence of God!”

Probability of life

The theory of abiogenesis suggests that life originated at a certain stage in the development of matter. Since the formation of the Universe and the first particles, matter has embarked on a path of constant change. First, atoms and molecules arose, then stars and dust appeared, planets appeared from it, and life was born on the planets. The living arises from the inanimate, obeying some higher law, the essence of which is still unknown to us. Life could not have arisen on Earth, where there were suitable conditions. Of course, it is impossible to refute this metaphysical generalization, but the seeds of doubt have sprouted. The fact is that the conditions necessary for the synthesis of life are very numerous, often contradicting the facts and each other. For example, there is no evidence that the early Earth had a reducing atmosphere. It is unclear how the genetic code originated. Surprises with its complexity the structure of a living cell and its functions. What is the probability of the origin of life? Here are some examples.

The calculation that Fred Hoyle gives in his book "Evolution from Space" is impressive. The probability of randomly generating 2,000 cell enzymes of 200 amino acids each is 10 - 4,000 - an absurdly small number, even if the entire cosmos were organic soup.

The probability of synthesizing one protein consisting of 300 amino acids is one chance in 2×10 390 . Again, very little. If we reduce the number of amino acids in a protein to 20, then the number of possible combinations for the synthesis of such a protein will be 1018, which is only an order of magnitude greater than the number of seconds in 4.5 billion years. It is easy to see that evolution simply did not have time to sort through all the options and choose the best one. If we take into account that the amino acids in proteins are connected in certain sequences, and not randomly, then the probability of synthesizing a protein molecule will be the same as if a monkey randomly printed one of Shakespeare's tragedies, that is, almost zero.

Scientists calculated that the DNA molecule involved in the simplest protein coding cycle should have consisted of 600 nucleotides in a certain sequence. The probability of random synthesis of such DNA is 10 -400, in other words, this will require 10,400 attempts.

Not all scientists agree with such probability calculations. They point out that it is incorrect to calculate the chances of protein synthesis by random selection of combinations, since molecules have preferences, and some chemical bonds are always more likely than others. According to the Australian biochemist Ian Musgrave, it is generally meaningless to calculate the probability of abiogenesis. First, the formation of polymers from monomers is not accidental, but obeys the laws of physics and chemistry. Secondly, it is wrong to calculate the formation of modern protein, DNA or RNA molecules because they were not part of the first living systems. Perhaps in the structure of the organisms that exist today, nothing remains of past times. It is now believed that the first organisms were very simple systems of short molecules, consisting of only 30-40 monomers. Life began with very simple organisms, gradually complicating the design. Nature did not even try to build a Boeing 747 right away. Thirdly, do not be afraid of low probability. One chance in a million million? And so what, because it can fall out on the first try.

What is life

Philosophers are not alone in the search for a definition of life. Such a definition is necessary for biochemists to understand: what happened in a test tube - living or non-living? Paleontologists who study the oldest rocks in search of the beginning of life. Exobiologists looking for organisms of extraterrestrial origin. It is not easy to define life. In the words of the Great Soviet Encyclopedia, "a strictly scientific distinction between living and non-living objects encounters certain difficulties." Indeed, what is characteristic only for a living organism? Maybe a set of external signs? Something white, soft, moving, making sounds. Plants, microbes and many other organisms do not fall into this primitive definition, because they are silent and do not move. You can consider life from a chemical point of view as matter consisting of complex organic compounds: amino acids, proteins, fats. But then a simple mechanical mixture of these compounds should be considered alive, which is not true. A better definition, on which there is broad scientific consensus, relates to the unique functions of living systems.

The ability to reproduce, when an exact copy of hereditary information is transmitted to descendants, is inherent in all earthly life, and even its smallest particle - a cell. That is why the cell is taken as the unit of measurement of life. The components of the cells: proteins, amino acids, enzymes - taken separately, will not be alive. This leads to the important conclusion that successful experiments on the synthesis of these substances cannot be considered an answer to the question of the origin of life. There will be a revolution in this field only when it becomes clear how the whole cell came into being. Without a doubt, the discoverers of the mystery will be awarded the Nobel Prize. In addition to the function of reproduction, there are a number of necessary, but insufficient properties of the system in order to be called alive. A living organism can adapt to environmental changes at the genetic level. This is very important for survival. Thanks to variability, life survived on the early Earth, during catastrophes and during severe ice ages.

An important property of a living system is catalytic activity, that is, the ability to carry out only certain reactions. Metabolism is based on this property - the choice of the necessary substances from the environment, their processing and obtaining the energy necessary for further life. The metabolic scheme, which is nothing more than a survival algorithm, is hardwired into the genetic code of the cell and is transmitted to descendants through the mechanism of heredity. Chemists know many systems with catalytic activity, which, however, cannot reproduce, and therefore cannot be considered alive.

Decisive experiment

There is no hope that one day the cell turned out by itself from the atoms of chemical elements. This is an incredible option. A simple bacterial cell contains hundreds of genes, thousands of proteins and different molecules. Fred Hoyle joked that the synthesis of a cell is as incredible as the assembly of a Boeing in a hurricane that swept over a junkyard. And yet, the Boeing exists, which means that it was somehow “assembled”, or rather “self-assembled”. According to current ideas, the “self-assembly” of the Boeing began 4.5 billion years ago, the process proceeded gradually and was extended in time for a billion years. At least 3.5 billion years ago, a living cell already existed on Earth.

For the synthesis of living things from non-living things, at the initial stage, simple organic and inorganic compounds must be present in the atmosphere and water bodies of the planet: C, C 2 , C 3 , CH, CN, CO, CS, HCN, CH 3 CH, NH, O, OH, H 2 O, S. Stanley Miller, in his famous experiments on abiogenic synthesis, mixed hydrogen, methane, ammonia and water vapor, then passed the heated mixture through electrical discharges and cooled it. A week later, a brown liquid formed in the flask containing seven amino acids, including glycine, alanine and aspartic acid, which are part of cellular proteins. Miller's experiment showed how prebiological organics could be formed - substances that are involved in the synthesis of more complex cell components. Since then, biologists consider this issue resolved, despite the serious problem. The fact is that the abiogenic synthesis of amino acids occurs only under reducing conditions, which is why Oparin believed the atmosphere of the early Earth to be methane-ammonia. But geologists do not agree with this conclusion.

Early atmosphere problem

Methane and ammonia have nowhere to come from in large quantities on Earth, experts say. In addition, these compounds are very unstable and are destroyed by sunlight, a methane-ammonia atmosphere could not exist even if these gases were released from the bowels of the planet. According to geologists, the Earth's atmosphere 4.5 billion years ago was dominated by carbon dioxide and nitrogen, which chemically creates a neutral environment. This is evidenced by the composition of the oldest rocks, which at that time were smelted from the mantle. The oldest rocks on the planet, 3.9 billion years old, were found in Greenland. These are the so-called gray gneisses - highly altered igneous rocks of medium composition. The change in these rocks went on for millions of years under the influence of carbon dioxide fluids of the mantle, which simultaneously saturated the atmosphere. Under such conditions, abiogenic synthesis is impossible.

Academician E.M. Galimov, director of the Institute of Geochemistry and Analytical Chemistry. IN AND. Vernadsky RAS. He calculated that the earth's crust arose very early, in the first 50-100 million years after the formation of the planet, and was predominantly metallic. In such a case, the mantle must indeed have released methane and ammonia in sufficient quantities to create reducing conditions. American scientists K. Sagan and K. Chaiba proposed a mechanism for self-protection of the methane atmosphere from destruction. According to their scheme, the decomposition of methane under the action of ultraviolet radiation could lead to the creation of an aerosol from organic particles in the upper atmosphere. These particles absorbed solar radiation and protected the planet's reducing environment. True, this mechanism was developed for Mars, but it is applicable to the early Earth.

Suitable conditions for the formation of prebiological organics did not last long on Earth. Over the next 200-300 million years, the mantle began to oxidize, which led to the release of carbon dioxide from it and a change in the composition of the atmosphere. But by that time, the environment for the origin of life had already been prepared.

At the bottom of the sea

Primordial life could have originated around volcanoes. Imagine numerous faults and cracks on the still fragile bottom of the oceans, oozing magma and seething gases. In such zones, saturated with hydrogen sulfide vapor, deposits of metal sulfides are formed: iron, zinc, copper. What if the synthesis of primary organics took place directly on the surface of iron-sulfur minerals using the reaction of carbon dioxide and hydrogen? Fortunately, there is a lot of both around: carbon dioxide and monoxide are released from magma, and hydrogen is released from water during its chemical interaction with hot magma. There is also an influx of energy necessary for synthesis.

This hypothesis is consistent with geological data and is based on the assumption that early organisms lived in extreme conditions, like modern chemosynthetic bacteria. In the 60s of the XX century, researchers discovered underwater volcanoes at the bottom of the Pacific Ocean - black smokers. There, in clubs of poisonous gases, without access to sunlight and oxygen, at a temperature of + 120 ° there are colonies of microorganisms. Similar conditions to black smokers were already on Earth 2.5 billion years ago, as evidenced by the layers of stromatolites - traces of the vital activity of blue-green algae. Forms similar to these microbes are among the remains of the most ancient organisms 3.5 billion years old.

To confirm the volcanic hypothesis, an experiment is needed that would show that abiogenic synthesis is possible under the given conditions. Work in this direction is carried out by groups of biochemists from the USA, Germany, England and Russia, but so far without success. Encouraging results were obtained in 2003 by a young researcher Mikhail Vladimirov from the laboratory of evolutionary biochemistry of the Institute of Biochemistry. A.N. Bach RAS. He created an artificial black smoker in the laboratory: a disk of pyrite (FeS 2) was placed in an autoclave filled with saline, which served as the cathode; carbon dioxide and electric current passed through the system. A day later, formic acid appeared in the autoclave - the simplest organic matter, which is involved in the metabolism of living cells and serves as a material for the abiogenic synthesis of more complex biological substances.

Cyanobacteria capable of fixing atmospheric nitrogen

Habitable Planet Hunters

Both theories of the origin of life, panspermia and abiogenesis, assume that life is not a unique phenomenon in the Universe, it must be on other planets. But how to find it? For a long time there was the only method of searching for life, which has not yet given positive results - by radio signals from aliens. At the end of the 20th century, a new idea arose - using telescopes to look for planets outside the solar system. The hunt for exoplanets has begun. In 1995, the first specimen was caught: a planet with a mass of half the Jupiter, rapidly orbiting the 51st star in the constellation Pegasus. As a result of almost 10 years of searching, 118 planetary systems containing 141 planets were discovered. None of these systems is similar to the Solar, none of the planets - to the Earth. The found exoplanets are close in mass to Jupiter, that is, they are much larger than the Earth. Distant giants are uninhabitable due to the peculiarities of their orbits. Some of them rotate very close to their star, which means that their surfaces are hot and there is no liquid water in which life develops. The rest of the planets - their minority - move in an elongated elliptical orbit, which dramatically affects the climate: the change of seasons there must be very sharp, and this is detrimental to organisms.

Both theories of the origin of life, panspermia and abiogenesis, assume that life is not a unique phenomenon in the Universe, it must be on other planets. But how to find it? For a long time there was the only method of searching for life, which has not yet given positive results - by radio signals from aliens. At the end of the 20th century, a new idea arose - using telescopes to look for planets outside the solar system. The hunt for exoplanets has begun. In 1995, the first specimen was caught: a planet with a mass of half the Jupiter, rapidly orbiting the 51st star in the constellation Pegasus. As a result of almost 10 years of searching, 118 planetary systems containing 141 planets were discovered. None of these systems is similar to the Solar, none of the planets - to the Earth. The found exoplanets are close in mass to Jupiter, that is, they are much larger than the Earth. Distant giants are uninhabitable due to the peculiarities of their orbits. Some of them rotate very close to their star, which means that their surfaces are hot and there is no liquid water in which life develops. The rest of the planets - their minority - move in an elongated elliptical orbit, which dramatically affects the climate: the change of seasons there must be very sharp, and this is detrimental to organisms.

The fact that no solar-type planetary system has been discovered has caused pessimistic statements by some scientists. Perhaps small stone planets are very rare in the Universe, or our Earth is generally the only one of its kind, or perhaps we simply lack the accuracy of measurements. But hope dies last, and astronomers continue to hone their methods. Now they are looking for planets not by direct observation, but by indirect signs, because the resolution of telescopes is not enough. Thus, the position of Jupiter-like giants is calculated from the gravitational perturbation that they exert on the orbits of their stars. In 2006, the European Space Agency will launch the Korot satellite, which will search for terrestrial-mass planets by dimming a star as it passes across its disk. The same way to hunt for planets will be NASA's Kepler satellite, starting in 2007. In another 2 years, NASA will organize a space interferometry mission - a very sensitive method for detecting small planets by their impact on bodies of greater mass. Only by 2015 will scientists build devices for direct observation - it will be a whole fleet of space telescopes called "Earth-like planet hunter", capable of simultaneously looking for signs of life.

When Earth-like planets are discovered, a new era will begin in science, and scientists are preparing for this event now. From a great distance, one must be able to recognize traces of life in the atmosphere of the planet, even if its most primitive forms - bacteria or the simplest multicellular organisms. The probability of discovering primitive life in the Universe is higher than making contact with green men, because life has existed on Earth for more than 4 billion years, of which only one century falls on a developed civilization. Before the appearance of man-made signals, it was possible to learn about our existence only by the presence of special compounds in the atmosphere - biomarkers. The main biomarker is ozone, which indicates the presence of oxygen. Water vapor means the presence of liquid water. Carbon dioxide and methane are emitted by some types of organisms. Searching for biomarkers on distant planets will be entrusted to the Darwin mission, which European scientists will launch in 2015. Six infrared telescopes will orbit 1.5 million kilometers from Earth and survey several thousand nearby planetary systems. By the amount of oxygen in the atmosphere, the Darwin project is able to determine very young life, several hundred million years old.

If in the radiation of the planet's atmosphere there are spectral lines of three substances - ozone, water vapor and methane - this is additional evidence in favor of the presence of life. The next step is to establish its type and degree of development. For example, the presence of chlorophyll molecules would mean that there are bacteria and plants on the planet that use photosynthesis for energy. The development of next generation biomarkers is a very promising task, but it is still a distant future.

organic source

If there were no conditions on Earth for the synthesis of prebiological organics, then they could be in space. Back in 1961, the American biochemist John Oro published an article on the cometary origin of organic molecules. The young Earth, not protected by a dense atmosphere, was subjected to massive bombardment by comets, which consist mainly of ice, but also contain ammonia, formaldehyde, hydrogen cyanide, cyanoacetylene, adenine and other compounds necessary for the abiogenic synthesis of amino acids, nucleic and fatty acids - the main components cells. According to astronomers, 1,021 kg of cometary matter fell on the Earth's surface. The water of the comets formed the oceans, where life flourished hundreds of millions of years later.

Observations confirm that there are simple organics and even amino acids in cosmic bodies and interstellar dust clouds. Spectral analysis showed the presence of adenine and purine in the tail of the Haley-Bopp comet, and pyrimidine was found in the Murchison meteorite. The formation of these compounds in outer space does not contradict the laws of physics and chemistry.

The comet hypothesis is also popular among cosmologists because it explains the appearance of life on Earth after the formation of the Moon. As is commonly believed, about 4.5 billion years ago, the Earth collided with a huge cosmic body. Its surface melted, part of the substance splashed into orbit, where a small satellite, the Moon, was formed from it. After such a catastrophe, no organics and water should have remained on the planet. Where did they come from? Comets brought them back.

The problem of polymers

Cellular proteins, DNA, RNA are all polymers, very long molecules, like threads. The structure of polymers is quite simple, they consist of parts that repeat in a certain order. For example, cellulose is the most common molecule in the world, which is part of plants. One cellulose molecule consists of tens of thousands of carbon, hydrogen and oxygen atoms, but at the same time it is nothing more than a multiple repetition of shorter glucose molecules linked together, like in a necklace. Proteins are a chain of amino acids. DNA and RNA - a sequence of nucleotides. And in total, these are very long sequences. Thus, the decoded human genome consists of 3 billion pairs of nucleotides.

In the cell, polymers are constantly produced by complex matrix chemical reactions. To get a protein, one amino acid needs to detach the hydroxyl group OH from one end and the hydrogen atom from the other, and only after that “glue” the next amino acid. It is easy to see that water is formed in this process, and again and again. The release of water, dehydration, is a very ancient process, key to the origin of life. How did it happen when there was no cell with its protein factory? There is also a problem with a warm shallow pond - the cradle of living systems. Indeed, during polymerization, water must be removed, but this is impossible if there is a lot of it around.

clay gene

There had to be something in the primordial soup that helped the living system to be born, accelerated the process and supplied energy. In the 1950s, the English crystallographer John Bernal suggested that ordinary clay, which is abundantly covered with the bottom of any reservoir, could serve as such an assistant. Clay minerals contributed to the formation of biopolymers and the emergence of the mechanism of heredity. Bernal's hypothesis has grown stronger over the years and attracted many followers. It turned out that ultraviolet-irradiated clay particles store the resulting energy reserve, which is spent on the biopolymer assembly reaction. In the presence of clay, the monomers assemble into self-replicating molecules, sort of like RNA.

Most clay minerals are structurally similar to polymers. They consist of a huge number of layers interconnected by weak chemical bonds. Such a mineral ribbon grows by itself, each next layer repeats the previous one, and sometimes defects occur - mutations, as in real genes. Scottish chemist A.J. Kearns-Smith claimed that the clay gene was the first organism on Earth. Getting between the layers of clay particles, organic molecules interacted with them, adopted the way of storing information and growth, one might say, they learned. For a while, minerals and proto-life coexisted peacefully, but soon there was a break, or genetic takeover, according to Kearns-Smith, after which life left the mineral home and began its own development.

The most ancient microbes

The 3.5 billion year old black shales of Western Australia contain the remains of the most ancient organisms ever discovered on Earth. Visible only under a microscope, the balls and fibers belong to prokaryotes - microbes in whose cell there is still no nucleus and the DNA helix is ​​laid directly in the cytoplasm. The oldest fossils were discovered in 1993 by the American paleobiologist William Schopf. The volcanic and sedimentary rocks of the Pilbara Complex, west of the Great Sandy Desert in Australia, are some of the oldest rocks on earth. By a happy coincidence, these formations have not changed so much under the influence of powerful geological processes and have preserved the remains of early creatures in the interlayers.

It turned out to be difficult to make sure that tiny balls and fibers were living organisms in the past. A row of small beads in a rock can be anything: minerals, non-biological organics, an optical illusion. In total, Schopf counted 11 types of fossils related to prokaryotes. Of these, 6, according to the scientist, are cyanobacteria, or blue-green algae. Similar species still exist on Earth in fresh water and oceans, in hot springs and near volcanoes. Schopf counted six signs by which suspicious objects in black shales should be considered alive.

These are the signs:
1. Fossils are composed of organic matter
2. They have a complex structure - the fibers consist of cells of various shapes: cylinders, boxes, disks
3. There are many objects - only 200 fossils include 1,900 cells
4. Objects are similar to each other, like modern representatives of the same population
5. These were organisms well adapted to the conditions of the early Earth. They lived at the bottom of the sea, protected from ultraviolet radiation by a thick layer of water and mucus.
6. The objects multiplied like modern bacteria, as evidenced by the findings of cells in the division stage.

The discovery of such ancient cyanobacteria means that almost 3.5 billion years ago there were organisms that consumed carbon dioxide and produced oxygen, were able to hide from solar radiation and recover from injuries, as modern species do. The biosphere has already begun to take shape. For science, this is a piquant moment. As William Schopf admits, in such respectable breeds he would prefer to find more primitive creatures. After all, the discovery of the most ancient cyanobacteria pushes back the beginning of life for a period erased from geological history forever, it is unlikely that geologists will ever be able to detect and read it. The older the rocks, the longer they were under pressure, temperature, weathered. In addition to Western Australia, only one place with very ancient rocks has survived on the planet where fossils can be found - in the east of South Africa in the kingdom of Swaziland. But African breeds have undergone dramatic changes over billions of years, and traces of ancient organisms have been lost.

Currently, geologists have not found the beginning of life in the rocks of the Earth. Strictly speaking, they cannot name the interval of time when there were no living organisms at all. Nor can they trace the early - up to 3.5 billion years ago - stages of the evolution of the living. Largely due to the lack of geological evidence, the mystery of the origin of life remains unsolved.

Realist and Surrealist

The first conference of the International Society for the Study of the Origin of Life (ISSOL) was held in 1973 in Barcelona. The emblem for this conference was drawn by Salvador Dali. Here is how it was. John Oro, an American biochemist, was friendly with the artist. In 1973, they met in Paris, dined at Maxim's, and went to a lecture on holography. After the lecture, Dali unexpectedly invited the scientist to come to his hotel the next day. Oro came and Dali handed him a drawing symbolizing the problem of chirality in living systems. Two crystals grow from the oozing puddle in an inverted hourglass pattern, hinting at the end time of evolution. A female figure sits on the left, a man stands on the right and holds a butterfly wing, a DNA worm winds between the crystals. The left and right quartz crystals shown in the figure are taken from Oparin's 1957 book The Origin of Life on Earth. To the scientist's surprise, Dali kept this book in his room! After the conference, the Oparins went to visit Dali, on the coast of Catalonia. Both celebrities were dying from the desire to communicate. A long conversation ensued between the realist and the surrealist, animated by the language of facial expressions and gestures - after all, Oparin spoke only in Russian.

RNA world

In the theory of abiogenesis, the search for the origin of life leads to the idea of ​​a system that is simpler than a cell. The modern cell is extraordinarily complex, its work rests on three pillars: DNA, RNA and proteins. DNA stores hereditary information, proteins carry out chemical reactions according to the scheme laid down in DNA, information from DNA to proteins is transmitted by RNA. What can be included in a simplified system? Some one of the components of the cell, which can, at least, reproduce itself and regulate metabolism.

The search for the most ancient molecule, with which, in fact, life began, has been going on for almost a century. Like geologists reconstructing the history of the earth from rock layers, biologists discover the evolution of life according to the structure of the cell. A series of discoveries in the 20th century led to the hypothesis of a spontaneously born gene that became the progenitor of life. It is natural to think that the DNA molecule could be such a primary gene, because it stores information about its structure and changes in it. Gradually, they found out that DNA cannot itself transmit information to other generations, for this it needs helpers - RNA and proteins. When new properties of RNA were discovered in the second half of the 20th century, it turned out that this molecule was more suitable for the main role in the play about the origin of life.

The RNA molecule is simpler in structure than DNA. It is shorter and consists of one thread. This molecule can serve as a catalyst, that is, carry out selective chemical reactions, for example, connect amino acids together, and in particular, carry out its own replication, that is, reproduction. As is known, selective catalytic activity is one of the main properties inherent in living systems. In modern cells, only proteins perform this function. Perhaps this ability passed to them over time, and once this was done by RNA.

To find out what else RNA is capable of, scientists began to breed it artificially. In a solution saturated with RNA molecules, its own life boils. The inhabitants exchange parts and reproduce themselves, that is, information is transmitted to descendants. The spontaneous selection of molecules in such a colony resembles natural selection, which means that it can be controlled. As breeders grow new breeds of animals, they also began to grow RNA with desired properties. For example, molecules that help stitch nucleotides into long chains; high temperature molecules, and so on.

Colonies of molecules in Petri dishes - this is the world of RNA, only artificial. The natural world of RNA could have arisen 4 billion years ago in warm puddles and small lakes, where spontaneous reproduction of molecules took place. Gradually, the molecules began to gather in communities and compete with each other for a place under the sun, the fittest survived. True, the transfer of information in such colonies is inaccurate, and the newly acquired features of an individual "individual" may be lost, but this shortcoming is covered by a large number of combinations. The selection of RNA was very fast, and in half a billion years a cell could have arisen. Giving impetus to the emergence of life, the world of RNA did not disappear, it continues to exist inside all organisms on Earth.

The world of RNA is almost alive, it has only one step left to complete revival - to produce a cell. The cell is separated from the environment by a strong membrane, which means that the next stage in the evolution of the RNA world is the conclusion of colonies, where molecules are related to each other, into a fatty membrane. Such a protocell could have happened by accident, but in order to become a full-fledged living cell, the membrane had to be reproduced from generation to generation. With the help of artificial selection in the colony, it is possible to remove the RNA that is responsible for the growth of the membrane, but did this really happen? The authors of the experiments from the Massachusetts Institute of Technology USA emphasize that the results obtained in the laboratory will not necessarily be similar to the real assembly of a living cell, and may be far from the truth. However, it has not yet been possible to create a living cell in a test tube. The world of RNA has not fully revealed its secrets.

The science

According to scientists, life on earth began about 3 billion years ago: during this time, the simplest organisms have evolved into complex forms of life. However, it is still a mystery to scientists how life began on the planet, and they put forward several theories to explain this phenomenon:

1. Electrical sparks

In the famous Miller-Urey Experiment, scientists proved that lightning could have contributed to the creation of the basic substances necessary for the origin of life: electrical sparks form amino acids in an atmosphere made up of huge amounts of water, methane, ammonia and hydrogen. Then more complex forms of life evolved from the amino acids. This theory was somewhat changed after the researchers found that the atmosphere of the planet billions of years ago was poor in hydrogen. Scientists have suggested that methane, ammonia and hydrogen were contained in volcanic clouds saturated with electrical charges.

2. Clay

The chemist Alexander Graham Cairns-Smith of the University of Glasgow, Scotland, theorized that, at the dawn of life, clay contained many organic compounds that were close together, and that clay helped organize these substances into structures similar to our genes.

DNA stores information about the structure of molecules, and the genetic sequence of DNA indicates how amino acids should be built into proteins. Cairns-Smith suggests that clay crystals helped to organize organic molecules into ordered structures, and later the molecules themselves began to do this, "without the help" of clay.

3. Deep sea vents

According to this theory, life originated in underwater hydrothermal vents that ejected molecules rich in hydrogen. On their rocky surface, these molecules could come together and become mineral catalysts for the reactions that led to the birth of life. Even now, such hydrothermal vents, rich in chemical and thermal energy, are home to a fairly large number of living beings.

4. Ice start

3 billion years ago, the Sun did not shine as brightly as it does now, and, accordingly, less heat reached the Earth. It is quite possible that the surface of the earth was covered with a thick layer of ice that protected the fragile organic matter in the water below it from ultraviolet rays and cosmic exposure. In addition, the cold helped the molecules survive longer, allowing the reactions that led to the birth of life.

5. World of RNA

DNA needs proteins to form, and proteins need DNA to form. How could they form without each other? Scientists suggested that RNA was involved in this process, which, like DNA, stores information. From RNA, respectively, proteins and DNA were formed., which replaced it in view of its greater efficiency.

Another question arose: "How did RNA appear?". Some believe that it spontaneously appeared on the planet, while others deny such a possibility.

6. "Simple" theory

Some scientists have suggested that life did not develop from complex molecules like RNA, but from simple ones that interacted with each other. They may have been in simple shells similar to cell membranes. As a result of the interaction of these simple molecules, complex that react more efficiently.

7. Panspermia

Finally, life could have originated not on our planet, but brought from space: in science, this phenomenon is called panspermia. This theory has a very solid foundation: due to cosmic impact, fragments of stones are periodically separated from Mars, which reach the Earth. After scientists discovered Martian meteorites on our planet, they suggested that these objects brought bacteria with them. If you believe them, then we are all martians. Other researchers have suggested that comets from other star systems brought life. Even if they are right, humanity will look for an answer to another question: "How did life originate in space?".