According to modern ideas, life on Earth began more than 3.5 billion years ago. It was not at all the planet we know today: a hot rocky ball without oxygen, shaken by the violent activity of young volcanoes, over which the sun and stars rushed at crazy speed - after all, the day lasted only about 6 hours. There are a great many theories about the origin of the first forms of life, and then more complex ones - including intelligent design. We will get acquainted with the basic scientific ideas, the understanding of which also allows us to assume where and under what conditions extraterrestrial life can exist.

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 “embryos of life” from other planets. This hypothesis was put forward by the German scientist G. Richter in 1865, who meant the transfer of microorganism spores either by meteorites or under the influence of light pressure. Later, cosmic radiation was discovered, which affects living organisms no less destructively than the decay of uranium. And the theory of panspermia fell into dust until the first flight to the Moon - when living microorganisms from Earth were found on the landing Surveyor 3 probe, which safely survived the long flight in outer space.

In 2006, the presence of both water and simple organic compounds in the cometary substance was discovered. Funnily enough, this means that a small meteorite with a luminous trail that is approaching a much larger globe of the planet is something like the cosmic analogue of female and male reproductive cells, together giving rise to new life.

Some followers of panspermia believe that an exchange of bacteria occurred between Earth and Mars during the period when the Red Planet still flourished and was partially covered by oceans. Moreover, this is not necessarily caused by meteorites - perhaps the bacteria were brought here by intelligent visitors (but this is a separate topic). But even if such events took place in history, we will be forced to figure out where life came from on another planet.

Electricity and the primordial broth

The famous Miller-Urey experiment in 1953 proved that electric sparks could 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 have created 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 reservoirs of the planet under the influence of electrical discharges, cosmic radiation and high liquid temperatures. At first, its composition was dominated by nucleotides, polypeptides, nitrogenous bases and amino acids. Then, over millions of years, more complex molecules were formed in the primordial broth until the simplest single-celled organisms, bacteria, were formed.

Clay life

According to religious sources, Adam was created from the dust of the ground, 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 from the University of Glasgow in Scotland, this may not be a simple allegory: the first molecules of life may have formed on clay. Initially, primitive carbon compounds did not have DNA, which means they could not reproduce their own kind - “reproduction” could only be stimulated by sources from the external environment.

Such a source could be clay rock, which is not just a certain mass of earth - it is an organized, orderly 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 organize themselves. Subsequently, they became more complex: 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 undersea 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.

This is how bacteria were born that formed the world-famous geological wonder - stromatolites (from “stromatos” - carpet and “litos” - stone). These formations have survived to this day in a fossilized form. And underwater sources of this type continue to play an important role in maintaining diverse marine ecosystems in our time.

Cold is a catalyst for evolution

Whichever scientist is right, simple single-celled bacteria still populated the planet - and in this form they invariably existed for more than a billion years. Then an incredibly rapid explosion occurred by the standards of evolution - much more complex forms of life 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 entire 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 with).

Living conditions for the simplest microorganisms have changed dramatically - but, on the other hand, hardy extremophile bacteria had to adapt under the thickness of the ice! It was during this “incubation” period that the primary division of bacteria took place according to their methods of survival: some of them learned to obtain energy from sunlight, others drew strength by processing substances dissolved in water. This marked the beginning of the kingdoms of living nature - the former in the future will become plants and single-celled photosynthetic animals, the latter - multicellular animals and fungi.

But one day, hot volcanoes awakened again and released huge amounts of carbon dioxide into the atmosphere, which caused a powerful greenhouse effect. The planet warmed up, the ice melted and released “matured” bacteria. The process of photosynthesis occurring in cyanobacteria (blue-green algae) gave a new reaction - and the atmosphere was quickly saturated with oxygen. And the fragments of mineral rocks brought by the glacier that fell into the ocean gave new variants of chemical reactions. This, as is already becoming clear, allowed animals to evolve. Soon, instead of dividing bacteria into two new ones, they began to divide without going “free swimming” and form the first multicellular structures. An example is the oldest multicellular animals without a nervous, blood or digestive system - 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 group 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 following our own path, and as our sun begins to age and become brighter, evolution will also prevail over the eternal cold.

Lifeless mountains, rocks and water, a huge moon in the sky and 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 inevitably faces a 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 currently existing hypotheses, nor has it been possible to come up with a third solution.

The first hypothesis about the origin of life on Earth is old, it includes respectable figures of European science: 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 failures of experimenters to synthesize living from non-living things lead scientists to the camp of adherents of this approach. There are numerous variations on exactly how life got to Earth, and the most famous of them is the theory of panspermia. According to it, life is widespread in interstellar space, but since there are no conditions for development there, 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 development developed.

Sperm are small embryos that can withstand large temperature changes, cosmic radiation and other environmental factors destructive to living things. As the English astronomer F. Hoyle suggested, interstellar dust particles, which may include bacteria in a graphite shell, are suitable for the role of sperm. To date, no sperm have been found in space. But even if they were found, such an amazing discovery would only shift the problem of the origin of life from our planet to another place. And we would not avoid asking where sperm came to Earth from and how they were born. The second part of the dilemma - how life arose from inorganic matter - is not so romantic, since it is based on the laws of physics and chemistry. This narrow, mechanistic approach, called the theory of abiogenesis, incorporates the efforts of many specialists. Perhaps because of its specificity, this approach turned out to be fruitful and over the course of half a century advanced entire branches of biochemistry, evolutionary biology and cosmology.

According to scientists, the synthesis of a living cell is just around the corner; it is a matter of technology and a matter of time. But will a cell born in a test tube be the answer to the question of how life originated on Earth? Hardly. The synthetic cell will only prove that abiogenesis is somehow possible. But 4 billion years ago on Earth, everything could have happened differently. For example, like this. The Earth's surface cooled 4.5 billion years ago. The atmosphere was thin, and comets actively bombarded the Earth, delivering organic matter in abundance. Extraterrestrial matter settled in shallow warm reservoirs heated by volcanoes: lava flowed at the bottom, islands grew, and hot springs erupted - fumaroles. 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 rotated faster, the days were shorter, the tidal waves were higher, and the storms were more severe. Above it all lay steel-colored skies, darkened by dust storms, clouds of volcanic ash, and shards of rock dislodged by meteorite impacts. An atmosphere rich in nitrogen, carbon dioxide and water vapor gradually developed. The abundance of greenhouse gases has caused the climate of the entire planet to warm. In such extreme conditions, the synthesis of living matter took place. Was this a miracle, an accident that occurred contrary to the evolution of the Universe, or is this the only way life can appear? Already at the early stages one of the main features of living matter appeared - adaptability to environmental conditions. The early atmosphere contained little free oxygen, ozone was in short supply, and the earth was bathed in ultraviolet rays, fatal to living things. The planet would have remained uninhabited if cells had not invented a mechanism for protecting against ultraviolet radiation. This scenario for the emergence of life in general does not differ from that proposed by Darwin. New details were added - we learned something by studying ancient rocks and experimenting, and guessed something. While this scenario is the most reasonable, it is also the most controversial. Scientists struggle with every 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 an 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 in abiogenic synthesis. He created the conditions of the early Earth in a laboratory test tube and, as a result of a chemical reaction, obtained a set of amino acids. Thus, Oparin’s theory began to receive experimental confirmation.

Oparin and the priest

According to the recollections 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 scientist’s views and personality 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, somewhere 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 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 Oparin’s hand, 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 arose 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, from it - planets, and life arose on the planets. Living things arise from non-living things, obeying some higher law, the essence of which is still unknown to us. Life could not help but arise 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 and often contradict 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. The structure of a living cell and its functions are surprising in their complexity. What is the general probability of the origin of life? Here are some examples.

Proteins consist only of so-called “left-handed” amino acids, that is, asymmetric molecules that rotate the polarization of light passing through them to the left. Why only left-handed amino acids are used in protein construction is unknown. Maybe this happened by chance and somewhere in the Universe there are living beings consisting of right-handed amino acids. Most likely, in the primordial broth, where the synthesis of the original proteins took place, there was an equal amount of left- and right-handed amino acids. And only the appearance of a truly living “left-handed” structure broke this symmetry and the biogenic synthesis of amino acids followed the “left-handed” path.

The calculation that Fred Hoyle gives in his book “Evolution from Space” is impressive. The probability of randomly obtaining 2,000 cell enzymes, each consisting of 200 amino acids, is 10 -4000 - an absurdly small value, even if the entire cosmos were an organic soup.

The probability of synthesizing one protein consisting of 300 amino acids is one chance in 2x10,390. Again, negligible. Let us reduce the number of amino acids in a protein to 20, then the number of possible combinations of the synthesis of such a protein will be 1,018 - just an order of magnitude greater than the number of seconds in 4.5 billion years. It is not difficult to see that evolution simply did not have time to sort through all the options and choose the best. 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 accidentally 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 specific 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 these probability calculations. They point out that calculating the chances of protein synthesis by randomly trying combinations is incorrect, since molecules have preferences, and some chemical bonds are always more likely than others. According to Australian biochemist Ian Musgrave, calculating the probability of abiogenesis is generally pointless. Firstly, the formation of polymers from monomers is not accidental, but obeys the laws of physics and chemistry. Secondly, it is incorrect to calculate the formation of modern protein molecules, DNA or RNA because they were not part of the first living systems. Perhaps there is nothing left of past times in the structure of organisms that exist today. 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 increasing in complexity. Nature did not even try to build a Boeing 747 right away. Thirdly, there is no need to be afraid of low probability. One chance in a million million? And so what, it might fall out on the first try.

What is life

Not only philosophers are engaged in the search for a definition of life. This definition is necessary for biochemists to understand: what happened in the test tube - living or non-living? Paleontologists studying ancient rocks in search of the beginning of life. Exobiologists searching for organisms of extraterrestrial origin. Defining life is not easy. In the words of the Great Soviet Encyclopedia, “a strictly scientific distinction between living and inanimate objects encounters certain difficulties.” Indeed, what is characteristic only of a living organism? Maybe a set of external signs? Something white, soft, moves, makes sounds. This primitive definition does not include plants, microbes and many other organisms, because they are silent and do not move. We 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 incorrect. A better definition, on which there is general 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, even in its smallest particle - a cell. This 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 the 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 secret will be awarded the Nobel Prize. In addition to the reproduction function, there are a number of necessary but insufficient properties of a system in order to be called living. A living organism can adapt to environmental changes at the genetic level. This is very important for survival. Variability enabled life to survive on the early Earth, through catastrophes and 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 selection of necessary substances from the environment, their processing and the production of energy necessary for further life activity. 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 living.

The decisive experiment

There is no hope that one day a cell would emerge by itself from 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 cell synthesis was as incredible as assembling a Boeing like a hurricane passing through a parts dump. And yet 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 over a billion years. At least 3.5 billion years ago, living cells 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 electric discharges and cooled it. After a week, a brown liquid containing seven amino acids, including glycine, alanine and aspartic acid, which are part of cellular proteins, formed in the flask. Miller's experiment showed how prebiological organic matter could be formed - substances that are involved in the synthesis of more complex cell components. Since then, biologists have considered this issue resolved, despite the serious problem. The fact is that abiogenic synthesis of amino acids occurs only under reducing conditions, which is why Oparin believed that the atmosphere of the early Earth was methane-ammonia. But geologists do not agree with this conclusion.

The problem of the early atmosphere

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 under the influence of 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 creates a chemically neutral environment. This is evidenced by the composition of the oldest rocks that were smelted from the mantle at that time. The oldest rocks on the planet, 3.9 billion years old, were discovered in Greenland. These are the so-called gray gneisses - highly altered igneous rocks of average composition. These rocks changed over millions of years under the influence of carbon dioxide fluids in the mantle, which simultaneously saturated the atmosphere. Under such conditions, abiogenic synthesis is impossible.

Academician E.M. is trying to solve the problem of the Earth's early atmosphere. Galimov, director of the Institute of Geochemistry and Analytical Chemistry named after. 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 this case, the mantle really should 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 influence of ultraviolet radiation could lead to the creation of an aerosol of organic particles in the upper layers of the atmosphere. These particles absorbed solar radiation and protected the planet's restorative environment. True, this mechanism was developed for Mars, but it also applies to the early Earth.

Suitable conditions for the formation of prebiological organic matter did not last long on Earth. Over the next 200-300 million years, the mantle began to oxidize, leading to the release of carbon dioxide 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

First life could have originated around volcanoes. Imagine on the still fragile bottom of the oceans numerous faults and cracks, oozing magma and bubbling with gases. In such zones saturated with hydrogen sulfide vapor, deposits of metal sulfides are formed: iron, zinc, copper. What if the synthesis of primary organic matter 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 does not contradict geological data and is based on the assumption that early organisms lived in extreme conditions, like modern chemosynthetic bacteria. In the 60s of the 20th century, researchers discovered underwater volcanoes - black smokers - at the bottom of the Pacific Ocean. There, in clouds of poisonous gases, without access to sunlight and oxygen, at a temperature of +120°, there are colonies of microorganisms. Conditions similar to black smokers existed on Earth already 2.5 billion years ago, as evidenced by layers of stromatolites - traces of the activity of blue-green algae. Forms similar to these microbes are also found 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 these conditions. Groups of biochemists from the USA, Germany, England and Russia are working in this direction, 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 pyrite disk (FeS 2) was placed in an autoclave filled with a saline solution, serving as a cathode; Carbon dioxide and electric current passed through the system. A day later, formic acid appeared in the autoclave - the simplest organic matter that participates in the metabolism of living cells and serves as a material for the abiogenic synthesis of more complex biological substances.

Cyanobacteria capable of assimilating atmospheric nitrogen

Hunters for habitable planets

Both theories of the origin of life, panspermia and abiogenesis, admit that life is not a unique phenomenon in the Universe, it must exist on other planets. But how to detect it? For a long time, there was only one method of searching for life, which has not yet yielded positive results - using radio signals from aliens. At the end of the 20th century, a new idea arose - using telescopes to search for planets outside the solar system. The hunt for exoplanets has begun. In 1995, the first specimen was caught: a planet half the mass of Jupiter, rapidly rotating around the 51st star of the constellation Pegasus. As a result of almost 10 years of searching, 118 planetary systems containing 141 planets were discovered. None of these systems are similar to the Solar system, none of the planets are similar to Earth. The exoplanets found are close in mass to Jupiter, that is, they are much larger than the Earth. Distant giants are unsuitable for life due to the characteristics of their orbits. Some of them rotate very close to their star, which means their surfaces are hot and there is no liquid water in which life develops. The remaining planets - their minority - move along 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, admit that life is not a unique phenomenon in the Universe, it must exist on other planets. But how to detect it? For a long time, there was only one method of searching for life, which has not yet yielded positive results - using radio signals from aliens. At the end of the 20th century, a new idea arose - using telescopes to search for planets outside the solar system. The hunt for exoplanets has begun. In 1995, the first specimen was caught: a planet half the mass of Jupiter, rapidly rotating around the 51st star of the constellation Pegasus. As a result of almost 10 years of searching, 118 planetary systems containing 141 planets were discovered. None of these systems are similar to the Solar system, none of the planets are similar to Earth. The exoplanets found are close in mass to Jupiter, that is, they are much larger than the Earth. Distant giants are unsuitable for life due to the characteristics of their orbits. Some of them rotate very close to their star, which means their surfaces are hot and there is no liquid water in which life develops. The remaining planets - their minority - move along 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 not a single solar-type planetary system has been discovered has led to pessimistic statements from some scientists. Perhaps small rocky 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 planets are searched 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 disturbance that they exert on the orbits of their stars. In 2006, the European Space Agency will launch the Korot satellite, which will search for Earth-mass planets by reducing the brightness of a star as they pass across its disk. NASA's Kepler satellite will hunt for planets in the same way starting in 2007. In another 2 years, NASA will organize a space interferometry mission - a very sensitive method of detecting small planets by their impact on bodies of larger mass. Only by 2015 will scientists build instruments for direct observation - this will be a whole flotilla of space telescopes called “Earth-type Planet Hunter”, capable of simultaneously searching 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, you need to be able to recognize traces of life in the planet’s atmosphere, even its most primitive forms - bacteria or protozoan multicellular organisms. The probability of discovering primitive life in the Universe is higher than coming into contact with little green men, because life on Earth has existed for more than 4 billion years, of which only one century has been spent on a developed civilization. Before the advent of man-made signals, it was possible to find out 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 indicates the presence of liquid water. Carbon dioxide and methane are released by some species of organisms. The Darwin mission, which European scientists will launch in 2015, will be tasked with searching for biomarkers on distant planets. Six infrared telescopes will orbit 1.5 million kilometers from Earth and survey several thousand nearby planetary systems. Based on the amount of oxygen in the atmosphere, the Darwin project is able to determine very young life, several hundred million years old.

If the radiation of the planet's atmosphere contains 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 to produce energy. The development of the next generation of biomarkers is very promising, but it is still a long way off.

Organic source

If there were no conditions on Earth for the synthesis of prebiological organics, then they could be in space. Back in 1961, 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 material fell onto the Earth's surface. The water of comets formed oceans where life flourished after hundreds of millions of years.

Observations confirm that cosmic bodies and interstellar dust clouds contain simple organic matter and even amino acids. Spectral analysis showed the presence of adenine and purine in the tail of comet Haley-Bopp, and pyrimidine was found in the Murchison meteorite. The formation of these compounds in 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. It is generally accepted that approximately 4.5 billion years ago the Earth collided with a huge cosmic body. Its surface melted, part of the substance splashed into orbit, where it formed a small satellite - the Moon. After such a catastrophe, there should have been no organic matter or water left on the planet. Where did they come from? Comets brought them again.

The polymer problem

Cellular proteins, DNA, RNA are all polymers, very long molecules, like threads. The structure of polymers is quite simple; they consist of parts repeated in a certain order. For example, cellulose is the most common molecule in the world that is found in 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 multiple repetitions of shorter glucose molecules linked together, like in a necklace. Proteins are a chain of amino acids. DNA and RNA are a sequence of nucleotides. Moreover, in total these are very long sequences. Thus, the deciphered human genome consists of 3 billion nucleotide pairs.

Within the cell, polymers are continually produced through complex matrix chemical reactions. To obtain a protein, you need to remove the OH hydroxyl group from one amino acid from one end and the hydrogen atom from the other, and only then “glue” the next amino acid. It is easy to see that water is formed in this process, over and over again. Liberation from 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 production factory? A problem also arises with a warm shallow pond - the cradle of living systems. After all, during polymerization, water must be removed, but this is impossible if there is a lot of it around.

Clay gene

There must have been something in the primordial broth that helped the living system to be born, accelerated the process and provided energy. The English crystallographer John Bernal suggested in the 50s of the 20th century that ordinary clay, which is abundantly covered in the bottom of any body of water, could serve as such an assistant. Clay minerals contributed to the formation of biopolymers and the emergence of a mechanism of heredity. Bernal's hypothesis has grown stronger over the years and attracted many followers. It turned out that clay particles irradiated with ultraviolet energy 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, something like RNA.

Most clay minerals are similar in structure to polymers. They consist of a huge number of layers connected to each other 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 argued that the first organism on Earth was precisely the “clay gene.” Getting between the layers of clay particles, organic molecules interacted with them, adopted the method of storing information and growing, one might say, they learned. For a time, minerals and protolife coexisted peacefully, but soon there was a rupture, 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 oldest organisms ever discovered on Earth. The balls and fibers visible only under a microscope belong to prokaryotes - microbes whose cells do not yet have a nucleus and the DNA helix is ​​laid directly in the cytoplasm. The oldest fossils were discovered in 1993 by American paleobiologist William Schopf. The volcanic and sedimentary rocks of the Pilbara complex, west of Australia's Great Sandy Desert, are some of the oldest rocks on Earth. By a lucky coincidence, these formations did not change so much under the influence of powerful geological processes and preserved the remains of early creatures in the interlayers.

It has been difficult to verify that the tiny balls and filaments were living organisms in the past. A series of small beads in a rock could be anything: minerals, non-biological organic matter, 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 bodies 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 - fibers consist of cells of different shapes: cylinders, boxes, disks
3. There are many objects - a total of 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 stage of division.

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 injury, 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 venerable breeds he would prefer to find more primitive creatures. After all, the discovery of ancient cyanobacteria pushes back the beginning of life to a period that has been erased from geological history forever; it is unlikely that geologists will ever be able to discover and read it. The older the rocks, the longer they were under pressure, temperature, and weathered. Besides Western Australia, there is only one place on the planet with very ancient rocks where fossils can be found - in the east of South Africa in the kingdom of Swaziland. But African rocks 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 generally cannot name the time interval when living organisms did not yet exist. They cannot trace the early stages of the evolution of living things, up to 3.5 billion years ago. 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) took place 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 friends 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 arrived and Dali handed him a drawing symbolizing the problem of chirality in living systems. Two crystals grow from an oozing pool in the shape of an inverted hourglass, hinting at the finite time of evolution. A female figure sits on the left, a man stands on the right holding a butterfly wing, and a DNA worm curls 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 to chat. A long conversation ensued between the realist and the surrealist, animated by the language of facial expressions and gestures - after all, Oparin spoke only Russian.

RNA world

In the theory of abiogenesis, the search for the origin of life leads to the idea of ​​a system simpler than a cell. The modern cell is extremely complex; its work is based on three pillars: DNA, RNA and proteins. DNA stores hereditary information, proteins carry out chemical reactions according to the scheme embedded in DNA, and RNA transmits information from DNA to proteins. What can be included in a simplified system? One of the components of a cell that can, at a minimum, reproduce itself and regulate metabolism.

The search for the most ancient molecule, with which life actually began, has been going on for almost a century. Just as geologists reconstruct the history of the Earth from rock layers, biologists discover the evolution of life from the structure of the cell. A series of discoveries in the 20th century led to the hypothesis of a spontaneously generated gene, which became the progenitor of life. It is natural to think that such a first gene could be a DNA molecule, because it stores information about its structure and changes in it. It was gradually discovered that DNA itself cannot 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, connecting amino acids with each other, 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 upon a time 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 is boiling. The inhabitants exchange parts and reproduce themselves, that is, information is transferred to descendants. Spontaneous selection of molecules in such a colony resembles natural selection, which means it can be controlled. Just as breeders grow new breeds of animals, they also began to grow RNA with specified properties. For example, molecules that help stitch nucleotides into long chains; molecules resistant to high temperatures, and so on.

Colonies of molecules in Petri dishes are the world of RNA, only artificial. The natural world of RNA may have arisen 4 billion years ago in warm puddles and shallow lakes where molecules spontaneously multiplied. Gradually, molecules began to gather in communities and compete with each other for a place in the sun, the fittest surviving. True, the transfer of information in such colonies occurs inaccurately, and the newly acquired characteristics of an individual “individual” may be lost, but this deficiency is covered by a large number of combinations. RNA selection proceeded very quickly, and a cell could have arisen in half a billion years. Having given impetus to the emergence of life, the RNA world did not disappear; it continues to exist inside all organisms on Earth.

The RNA world is almost alive; there is only one step left for it to be fully revived - 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 enclosure of colonies, where the molecules are related to each other, in a fatty membrane. Such a protocell could have arisen by chance, but in order to become a full-fledged living cell, the membrane had to reproduce from generation to generation. Using artificial selection, the RNA that is responsible for membrane growth can be bred into colonies, but has this actually happened? The authors of the experiments from the Massachusetts Institute of Technology in the 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 completely far from the truth. However, it has not yet been possible to create a living cell in a test tube. The RNA world has not fully revealed its secrets.

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 the synthesis of proteins, 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 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 resolved in the 1980s, when Thomas Check and Sidney 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 microbiologist Carl Woese in 1968 and finally formulated by Nobel Prize-winning biochemist 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 were fixed through the process of natural selection.

At the same time, “repositories” of genetic information—DNA—were formed. Ribonucleic acids have been 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.

At the 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 mixture containing only nucleotides and replicase, and in 2009, researchers from the University of Manchester showed that uridine and cytidine - the constituent parts of ribonucleic acid - could be synthesized under 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 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 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 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 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.

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

almost all religions. Although there is still no exact scientific answer to this question, some facts allow us to make more or less reasonable hypotheses. Researchers found a rock sample in Greenland

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

What did the primitive Earth look like?

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

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

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

Charles Darwin and his contemporaries believed that life could have arisen in a body of water. Many scientists still adhere to this point of view. In a closed and relatively small reservoir, organic substances brought by the waters flowing into it could accumulate in the required quantities. These compounds were then further concentrated on the inner surfaces of layered minerals, which could catalyze the reactions. For example, two molecules of phosphaldehyde that met on the surface of a mineral reacted with each other to form a phosphorylated carbohydrate molecule, a possible precursor to ribonucleic acid (Fig. 2).

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

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

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

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

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

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

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

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

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

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

Thus, the most primitive living system could be represented by RNA molecules duplicating, undergoing mutations and being subject to natural selection. In the course of evolution, based on RNA, specialized DNA molecules arose - the custodians of genetic information - and no less specialized protein molecules, which took on the functions of catalysts for the synthesis of all currently known biological molecules.

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

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

Space objects falling to Earth could have played a central role in the emergence of life on our planet, since, according to a number of researchers, cells similar to bacteria could have arisen on another planet and then reached Earth along with asteroids. One piece of evidence supporting the theory of extraterrestrial origins of life was found inside a meteorite shaped like a potato and named ALH84001. This meteorite was originally a piece of Martian crust, which was then thrown into space as a result of an explosion when a huge asteroid collided with the surface of Mars, which occurred about 16 million years ago. And 13 thousand years ago, after a long journey within the solar system, this fragment of Martian rock in the form of a meteorite landed in Antarctica, where it was recently discovered. A detailed study of the meteorite revealed rod-shaped structures resembling fossilized bacteria inside it, which gave rise to heated scientific debate about the possibility of life deep in the Martian crust. It will be possible to resolve these disputes no earlier than 2005, when the US National Aeronautics and Space Administration will implement a program to fly an interplanetary spacecraft to Mars to take samples of the Martian crust and deliver samples to Earth. And if scientists manage to prove that microorganisms once inhabited Mars, then we can speak with a greater degree of confidence about the extraterrestrial origin of life and the possibility of life being brought from outer space (Fig. 5).

Rice. 5. Our origin is from microbes.

What have we inherited from ancient life forms? The comparison below of single-celled organisms with human cells reveals many similarities.

1. Sexual reproduction Two specialized algae reproductive cells - gametes - mate to form a cell that carries genetic material from both parents. This is remarkably reminiscent of the fertilization of a human egg by a sperm.

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

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

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

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

The evolution of life on Earth: from simple to complex

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

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

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

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

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

It is believed that initially there was no free oxygen in the earth’s atmosphere and the oceans, and under these conditions only anaerobic microorganisms lived and developed. A special step in the evolution of living things was the emergence of photosynthetic bacteria, which, using light energy, converted carbon dioxide into carbohydrate compounds that served as food for other microorganisms. If the first photosynthetics produced methane or hydrogen sulfide, then the mutants that appeared once began to produce oxygen during photosynthesis. As oxygen accumulated in the atmosphere and waters, anaerobic bacteria, for which it is harmful, occupied oxygen-free niches.

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

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

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

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

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

Studies of fossil remains of organisms in rocks of different geological ages have shown that for hundreds of millions of years after their origin, eukaryotic life forms were represented by microscopic spherical single-celled organisms such as yeast, and their evolutionary development proceeded at a very slow pace. But a little over 1 billion years ago, many new species of eukaryotes emerged, marking a dramatic leap in the evolution of life.

First of all, this was due to the emergence of sexual reproduction. And if bacteria and single-celled eukaryotes reproduced by producing genetically identical copies of themselves and without the need for a sexual partner, then sexual reproduction in more highly organized eukaryotic organisms occurs as follows. Two haploid sex cells of the parents, having a single set of chromosomes, fuse to form a zygote that has a double set of chromosomes with the genes of both partners, which creates opportunities for new gene combinations. The emergence of sexual reproduction led to the emergence of new organisms, which entered the arena of evolution.

Three quarters of the entire existence of life on Earth was represented exclusively by microorganisms, until a qualitative leap in evolution occurred, leading to the emergence of highly organized organisms, including humans. Let's trace the main milestones in the history of life on Earth in a descending line.

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

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

2 billion years ago, complex eukaryotic cells appeared when single-celled organisms complicated their structure by absorbing other prokaryotic cells. Some of them - aerobic bacteria - turned into mitochondria - energy stations for oxygen respiration. Others - photosynthetic bacteria - began to carry out photosynthesis inside the host cell and became chloroplasts in algae and plant cells. Eukaryotic cells, which have these organelles and a clearly distinct nucleus containing genetic material, make up all modern complex life forms - from molds to humans.

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

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

A.L. Prokhorov
Based on an article by Richard Monasterski
in National Geographic magazine, 1998 No. 3

The origin of life on Earth is one of the most difficult and at the same time relevant and interesting questions in modern natural science.

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. A prehistoric Ocean formed in the depressions of the earth's surface. The original life arose in it approximately 3.8 billion years ago.

The emergence of life on Earth

How did the planet itself originate 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 the chemical elements known in nature, 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. The water, which had previously evaporated into the atmosphere and turned into its constituent part, continuously fell to the Earth. Powerful showers fell from the clouds with thunder and lightning.

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 possibly 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.

What did the primitive Earth look like?

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

Theories of the origin of life on Earth

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

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.

Biogenesis theory

In the development of doctrines about the origin of life, the theory of biogenesis - the origin of living things only from living things - occupies a significant place. But many consider it untenable, since it fundamentally contrasts the living with the inanimate and affirms the idea of ​​​​the eternity of life, rejected by science. Abiogenesis - the idea of ​​the origin of living things from non-living things - is the initial hypothesis of the modern theory of the origin of life. In 1924, the famous biochemist A.I. Oparin suggested that with powerful electrical discharges in the earth’s atmosphere, which 4-4.5 billion years ago consisted of ammonia, methane, carbon dioxide and water vapor, the simplest organic compounds could arise, necessary for the emergence of life. Academician Oparin's prediction came true. In 1955, American researcher S. Miller, passing electrical charges through a mixture of gases and vapors, obtained the simplest fatty acids, urea, acetic and formic acids and several amino acids. Thus, in the middle of the 20th century, the abiogenic synthesis of protein-like and other organic substances was experimentally carried out under conditions reproducing the conditions of the primitive Earth.

Panspermia theory

The theory of panspermia is the possibility of transferring organic compounds and spores of microorganisms from one cosmic body to another. But it does not answer the question at all: how did life originate in the Universe? There is a need to substantiate the emergence of life at that point in the Universe, the age of which, according to the Big Bang theory, is limited to 12-14 billion years. Before this time there were not even elementary particles. And if there are no nuclei and electrons, there are no chemical substances. Then, within a few minutes, protons, neutrons, electrons appeared, and matter entered the path of evolution.

To substantiate this theory, multiple sightings of UFOs, rock paintings of objects resembling rockets and “astronauts,” and reports of alleged encounters with aliens are used. When studying the materials of meteorites and comets, many “precursors of life” were discovered in them - substances such as cyanogens, hydrocyanic acid and organic compounds, which may have played the role of “seeds” that fell on the bare Earth.

Proponents of this hypothesis were Nobel Prize laureates F. Crick and L. Orgel. F. Crick was based on two indirect evidence: the universality of the genetic code: the need for the normal metabolism of all living beings of molybdenum, which is now extremely rare on the planet.

The origin of life on Earth is impossible without meteorites and comets

A researcher from Texas Tech University, after analyzing a huge amount of collected information, put forward a theory about how life could form on Earth. The scientist is confident that the appearance of early forms of the simplest life on our planet would have been impossible without the participation of comets and meteorites that fell on it. The researcher shared his work at the 125th annual meeting of the Geological Society of America, held on October 31 in Denver, Colorado.

The author of the work, a professor of geoscience at Texas Tech University (TTU) and curator of the university's museum of paleontology, Sankar Chatterjee, said that he came to this conclusion after analyzing information about the early geological history of our planet and comparing this data with various theories of chemical evolution.

The expert believes that this approach makes it possible to explain one of the most hidden and incompletely studied periods in the history of our planet. According to many geologists, the bulk of space “bombardments”, in which comets and meteorites took part, occurred about 4 billion years ago. Chatterjee believes that the earliest life on Earth formed in craters left by falling meteorites and comets. And most likely this happened during the “Late Heavy Bombardment” period (3.8-4.1 billion years ago), when the collision of small space objects with our planet increased sharply. At that time, there were several thousand cases of comet falls. Interestingly, this theory is indirectly supported by the Nice Model. According to it, the real number of comets and meteorites that should have fallen to the Earth at that time corresponds to the real number of craters on the Moon, which in turn was a kind of shield for our planet and did not allow the endless bombardment to destroy it.

Some scientists suggest that the result of this bombardment is the colonization of life in the Earth's oceans. However, several studies on this topic indicate that our planet has more water reserves than it should. And this excess is attributed to comets that came to us from the Oort Cloud, which is supposedly located one light year away from us.

Chatterjee points out that the craters created by these collisions were filled with melted water from the comets themselves, as well as the necessary chemical building blocks needed to form simple organisms. At the same time, the scientist believes that those places where life did not appear even after such a bombardment simply turned out to be unsuitable for this.

“When the Earth was formed about 4.5 billion years ago, it was completely unsuitable for living organisms to appear on it. It was a real boiling cauldron of volcanoes, poisonous hot gas and meteorites constantly falling on it,” writes the online magazine AstroBiology, citing the scientist.

“And after one billion years, it became a quiet and peaceful planet, rich in huge reserves of water, inhabited by various representatives of microbial life - the ancestors of all living things.”

Life on Earth could have arisen thanks to clay

A group of scientists led by Dan Luo from Cornell University came up with a hypothesis that ordinary clay could serve as a concentrator for ancient biomolecules.

Initially, the researchers were not concerned with the problem of the origin of life - they were looking for a way to increase the efficiency of cell-free protein synthesis systems. Instead of allowing the DNA and its supporting proteins to float freely in the reaction mixture, the scientists tried to force them into hydrogel particles. This hydrogel, like a sponge, absorbed the reaction mixture, sorbed the necessary molecules, and as a result, all the necessary components were locked in a small volume - similar to what happens in a cell.

The study authors then tried to use clay as an inexpensive hydrogel substitute. Clay particles turned out to be similar to hydrogel particles, becoming a kind of microreactors for interacting biomolecules.

Having received such results, scientists could not help but recall the problem of the origin of life. Clay particles, with their ability to sorb biomolecules, could actually serve as the very first bioreactors for the very first biomolecules, before they yet acquired membranes. This hypothesis is also supported by the fact that the leaching of silicates and other minerals from rocks to form clay began, according to geological estimates, just before, according to biologists, the oldest biomolecules began to unite into protocells.

In water, or more precisely in a solution, little could happen, because the processes in a solution are absolutely chaotic, and all compounds are very unstable. Modern science considers clay - more precisely, the surface of particles of clay minerals - as a matrix on which primary polymers could form. But this is also only one of many hypotheses, each of which has its own strengths and weaknesses. But to simulate the origin of life on a full scale, you really need to be God. Although in the West today articles with the titles “Cell Construction” or “Cell Modeling” are already appearing. For example, one of the last Nobel laureates, James Szostak, is now actively attempting to create effective cell models that multiply on their own, reproducing their own kind.