Valery Spiridonov, the first candidate for a head transplant, for RIA Novosti

For many years, mankind has been trying to unravel the true cause and history of the appearance of life on our planet. A little more than a hundred years ago, in almost all countries, people did not even think to question the theory of divine intervention and the creation of the world by a higher spiritual being.

The situation changed after the publication in November 1859 of the greatest work of Charles Darwin, and now there is a lot of controversy around this topic. The number of supporters of the Darwinian theory of evolution in Europe and Asia is more than 60-70%, approximately 20% in the USA and about 19% in Russia according to the data of the end of the last decade.

In many countries today there is a call to exclude Darwin's work from school curriculum or at least study it on a par with other possible theories. Apart from the religious version, to which most of the world's population is inclined, today there are several main theories of the origin and evolution of life, describing its development at various stages.

Panspermia

Proponents of the idea of ​​panspermia are convinced that the first microorganisms were brought to Earth from outer space. This was the opinion of the famous German scientist-encyclopedist Hermann Helmholtz, the English physicist Kelvin, the Russian scientist Vladimir Vernadsky and the Swedish chemist Svante Arrhenius, who is considered today the founder of this theory.

It has been scientifically confirmed that meteorites from Mars and other planets have been repeatedly found on Earth, possibly from comets that could even come from alien star systems. No one doubts this today, but it is not yet clear how life could have arisen on other worlds. In fact, panspermia apologists transfer "responsibility" for what is happening to alien civilizations.

The primordial soup theory

The birth of this hypothesis was facilitated by the experiments of Harold Urey and Stanley Miller, conducted in the 1950s. They were able to recreate almost the same conditions that existed on the surface of our planet before the origin of life. Through a mixture of molecular hydrogen, carbon monoxide and methane, small electric discharges and ultraviolet light were passed.

As a result, methane and other primitive molecules turned into complex organic substances, including dozens of amino acids, sugar, lipids, and even the rudiments of nucleic acids.

Relatively recently, in March 2015, scientists at the University of Cambridge led by John Sutherland showed that all types of "molecules of life", including RNA, proteins, fats and carbohydrates, can be obtained during similar reactions, which will involve simple inorganic carbon compounds, hydrogen sulfide, metal salts and phosphates.

Clay breath of life

One of the main problems of the previous version of the evolution of life is that many organic molecules, including sugars, DNA and RNA, are too fragile to accumulate in sufficient quantities in the waters of the Earth's primordial ocean, where, as previously believed most evolutionists, the first living beings arose.

Scientists have found out in what environment lived the most ancient ancestors of peopleLarge-scale excavations in the Olduvai Gorge helped paleontologists find out that our first ancestors lived in groves of palms and acacias, under the shade of which they could butcher the carcasses of the giraffes, antelopes and other ungulates they killed from the African savannas.

The British chemist Alexander Cairns-Smith believes that life has a "clay" and not water origin - the optimal environment for the accumulation and complexity of complex organic molecules can be found inside the pores and crystals in clay minerals, and not in Darwin's "primary pond" or the ocean of Miller-Urey theories.

In fact, evolution began at the level of crystals, and only then, when the compounds became sufficiently complex and stable, did the first living organisms go into "open swimming" in the primary ocean of the Earth.

Life at the bottom of the ocean

This idea competes with the popular idea today that life did not originate on the surface of the ocean, but in the deepest regions of its bottom, in the vicinity of "black smokers", underwater geysers, and other geothermal sources.

Their emissions are rich in hydrogen and other substances, which, according to scientists, could accumulate on the slopes of the rocks and give the first life all the necessary food resources and reaction catalysts.

Evidence of this can be recognized as modern ecosystems that exist in the vicinity of such sources at the bottom of all the oceans of the Earth - they include not only microbes, but even multicellular living beings.

RNA universe

The theory of dialectical materialism is based on the simultaneous unity and endless struggle of a pair of principles. We are talking about the heredity of information and structural biochemical changes. The version of the origin of life, in which RNA plays a key role, has passed long way development from its inception in the 1960s until the end of the 1980s, when it acquired its modern features.

On the one hand, RNA molecules are not as efficient at storing information as DNA, but they are able to simultaneously accelerate chemical reactions and collect your own copies. At the same time, one must understand that scientists have not yet been able to show how the entire chain of evolution of RNA life worked, and therefore this theory has not yet received universal recognition.

Protocells

Another important question in the evolution of life is the mystery of how such molecules of RNA or DNA and proteins "fenced off" from the outside world and turned into the first isolated cells, the contents of which are protected by a flexible membrane or semipermeable hard shell.

A well-known Soviet chemist Alexander Oparin became a pioneer in this field, showing that water droplets surrounded by a double layer of fat molecules can have similar properties.

His ideas were brought to life by Canadian biologists led by Jack Szostak, winner of the 2009 Nobel Prize in Physiology or Medicine. His team was able to "pack" the simplest set of self-replicating RNA molecules into a membrane of fatty molecules by adding magnesium ions and citric acid inside the first "protocell".

Endosymbiosis

Another mystery of the evolution of life is how multicellular creatures arose and why the cells of humans, animals and plants include special bodies such as mitochondria and chloroplasts, which have an unusually complex structure.

The diets of the ancestors of humans and chimpanzees "diverge" 3 million years agoPaleontologists have compared the proportions of carbon isotopes in Australopithecus tooth enamel and found that the ancestors of humans and chimpanzees switched to different diets 3 million years ago, 1.5 million years earlier than previously thought.

For the first time, the German botanist Andreas Schimper thought about this problem, suggesting that chloroplasts in the past were independent organisms, similar to cyanobacteria, which "made friends" with the cells of plant ancestors and began to live inside them.

This idea was later developed by the Russian botanist Konstantin Merezhkovsky and the American evolutionist Lynn Margulis, who showed that mitochondria and potentially all other complex organelles of our cells have a similar origin.
As in the case with the theories of the "RNA world" and the "clay" evolution of life, the idea of ​​endosymbiosis initially caused a lot of criticism from most scientists, but today almost all evolutionists do not doubt its correctness.

Who is right and who is wrong?

In favor of the Darwinian hypotheses, many scientific works and specialized studies have been found, in particular in the field of "transitional forms". Darwin did not have the necessary number of archaeological artifacts in his hands to confirm scientific works, since for the most part he was guided by personal guesses.

For example, in just the last ten years, scientists have found the remains of several such "lost links" of evolution, such as Tiktaalik (Tiktaalik) and Indohyus (Indohyus), which allow us to draw a line between land animals and fish, and whales and hippos.
On the other hand, skeptics often argue that such animal species are not true transitional forms, which gives rise to constant endless disputes between supporters of Darwinism and their opponents.

On the other hand, experiments on ordinary Escherichia coli and on various multicellular creatures clearly show that evolution is real, and that animals can quickly adapt to new living conditions, acquiring new features that their ancestors did not have 100-200 generations ago.

It should be remembered, however, that a large part modern society still inclined to believe in the existence of a higher divine mind or extraterrestrial civilizations who founded life on earth. So far, the only true theory does not exist, and humanity has yet to answer this question in the future.

How did life originate on Earth? The details are unknown to mankind, but the cornerstone principles have been established. There are two main theories and many minor ones. So, according to the main version, the organic components came to Earth from outer space, according to another, everything happened on Earth. Here are some of the most popular teachings.

Panspermia

How did our Earth come about? The biography of the planet is unique, and people are trying to unravel it in different ways. There is a hypothesis that the life that exists in the universe is distributed with the help of meteoroids (celestial bodies intermediate in size between interplanetary dust and an asteroid), asteroids and planets. It is assumed that there are life forms that can withstand exposure (radiation, vacuum, low temperatures and etc.). They are called extremophiles (including bacteria and microorganisms).

They get into debris and dust, which are thrown into space after saving, thus, life after the death of small bodies of the solar system. Bacteria can travel at rest for long periods of time before another random collision with other planets.

They can also mix with protoplanetary disks (dense gas cloud around a young planet). If in a new place "persistent but sleepy soldiers" fall into favorable conditions, they become active. The process of evolution begins. History is unraveled with the help of probes. Data from instruments that have been inside comets indicate that in the vast majority of cases, the likelihood is confirmed that we are all "a little alien", since the cradle of life is space.

Biopoiesis

And here is another opinion on how life originated. On Earth there is living and non-living. Some sciences welcome abiogenesis (biopoesis), which explains how, in the course of natural transformation, biological life emerged from inorganic matter. Most amino acids (also called the building blocks of all living organisms) can be formed using natural chemical reactions that are not related to life.

This is confirmed by the Muller-Urey experiment. In 1953, a scientist ran electricity through a mixture of gases and produced several amino acids in laboratory conditions that mimic those of the early Earth. In all living beings, amino acids are transformed into proteins under the influence of nucleic acids, the genetic memory custodians.

The latter are synthesized independently by biochemical means, and proteins accelerate (catalyze) the process. Which of the organic molecules is the first? And how did they interact? Abiogenesis is in the process of finding an answer.

Cosmogonic trends

This is the doctrine of space. In a certain context of space science and astronomy, the term refers to the theory of creation (and study) of the solar system. Attempts to gravitate toward naturalistic cosmogony do not stand up to scrutiny. First, the existing scientific theories cannot explain the main thing: how did the Universe itself appear?

Secondly, there is no physical model that explains the earliest moments of the existence of the universe. In the mentioned theory, there is no concept of quantum gravity. Although string theorists say that elementary particles arise as a result of vibrations and interactions of quantum strings), who investigate the origin and consequences of the Big Bang (loop quantum cosmology), do not agree with this. They believe they have formulas to describe the model in terms of field equations.

With the help of cosmogonic hypotheses, people explained the uniformity of the movement and composition of celestial bodies. Long before life appeared on Earth, matter filled all space and then evolved.

Endosymbiont

The endosymbiotic version was first formulated by the Russian botanist Konstantin Merezhkovsky in 1905. He believed that some organelles originated as free-living bacteria and were taken into another cell as endosymbionts. Mitochondria evolved from proteobacteria (specifically Rickettsiales or close relatives) and chloroplasts from cyanobacteria.

This allows us to assume that plural forms bacteria entered into symbiosis with the formation of a eukaryotic cell (eukaryotes are cells of living organisms containing a nucleus). The horizontal transfer of genetic material between bacteria is also facilitated by symbiotic relationships.

The emergence of a variety of life forms may have been preceded by the Last Common Ancestor (LUA) of modern organisms.

Spontaneous birth

Until the early 19th century, people generally dismissed "suddenness" as an explanation for how life began on Earth. The unexpected spontaneous generation of certain forms of life from inanimate matter seemed implausible to them. But they believed in the existence of heterogenesis (a change in the method of reproduction), when one of the forms of life comes from another species (for example, bees from flowers). Classical ideas about spontaneous generation boil down to the following: some complex living organisms appeared due to the decomposition of organic substances.

According to Aristotle, this was an easily observable truth: aphids arise from dew that falls on plants; flies - from spoiled food, mice - from dirty hay, crocodiles - from rotting logs at the bottom of reservoirs, and so on. The theory of spontaneous generation (refuted by Christianity) secretly existed for centuries.

It is generally accepted that the theory was finally refuted in the 19th century by the experiments of Louis Pasteur. The scientist did not study the origin of life, he studied the appearance of microbes in order to be able to fight infectious diseases. However, Pasteur's evidence was no longer controversial, but strictly scientific.

Clay Theory and Sequential Creation

The emergence of life on the basis of clay? Is that possible? A Scottish chemist named A.J. Kearns-Smith from the University of Glasgow in 1985 is the author of such a theory. Based on similar assumptions by other scientists, he argued that organic particles, being between the layers of clay and interacting with them, adopted the way of storing information and growing. Thus, the scientist considered the “clay gene” to be primary. Initially, the mineral and the emerging life existed together, and on certain stage"ran away".

The idea of ​​destruction (chaos) in the emerging world paved the way for the theory of catastrophism as one of the forerunners of the theory of evolution. Its proponents believe that the Earth has been affected by sudden, short-lived, turbulent events in the past, and that the present is the key to the past. Each next catastrophe destroyed the existing life. The subsequent creation revived it already different from the previous one.

materialistic doctrine

And here is another version of how life originated on Earth. It was put forward by the materialists. They believe that life appeared as a result of gradual chemical transformations extended in time and space, which, in all likelihood, took place almost 3.8 billion years ago. This development is called molecular, it affects the area of ​​deoxyribonucleic and ribonucleic acids and proteins (proteins).

As a scientific trend, the doctrine arose in the 1960s, when active research was carried out affecting molecular and evolutionary biology, population genetics. Scientists then tried to understand and validate recent discoveries regarding nucleic acids and proteins.

One of the key topics that stimulated the development of this field of knowledge was the evolution of the enzymatic function, the use of nucleic acid divergence as a "molecular clock". Its disclosure contributed to a deeper study of the divergence (branching) of species.

organic origin

About how life appeared on Earth, supporters of this doctrine argue as follows. The formation of species began a long time ago - more than 3.5 billion years ago (the number indicates the period in which life exists). Probably, at first there was a slow and gradual process of transformation, and then a fast (within the Universe) stage of improvement began, a transition from one static state to another under the influence of existing conditions.

Evolution, known as biological or organic, is the process of changing over time one or more inherited traits found in populations of organisms. Hereditary traits are special distinguishing features, including anatomical, biochemical and behavioral, that are transmitted from one generation to another.

Evolution has led to diversity and diversified development all living organisms (diversification). Our colorful world was described by Charles Darwin as "endless forms, the most beautiful and the most wonderful." One gets the impression that the origin of life is a story without beginning or end.

special creation

According to this theory, all forms of life that exist today on planet Earth are created by God. Adam and Eve are the first man and woman created by the Almighty. Life on Earth began with them, believe Christians, Muslims and Jews. Three religions agreed that God created the universe within seven days, making the sixth day the culmination of labor: he created Adam from the dust of the earth and Eve from his rib.

On the seventh day God rested. Then he breathed in and sent to look after the garden called Eden. In the center grew the Tree of Life and the Tree of the Knowledge of Good. God allowed the fruits of all the trees in the garden to be eaten, except for the Tree of Knowledge (“for on the day that you eat them you will die”).

But the people disobeyed. The Qur'an says that Adam offered to taste the apple. God forgave sinners and sent both of them to earth as his representatives. And yet... Where did life come from on Earth? As you can see, there is no single answer. Although modern scientists are increasingly inclined towards the abiogenic (inorganic) theory of the origin of all living things.

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.

Sperm are small embryos that can withstand large temperature fluctuations, cosmic radiation and other environmental factors that are detrimental to living things. As suggested by the English astronomer F. Hoyle, interstellar dust particles, among which there may be bacteria in a graphite shell, are suitable for the role of sperm. To date, no sperm has been found in space. But even if they were found, so amazing discovery would only shift the problem of the origin of life from our planet to another place. And we would not have avoided the questions of where the sperm arrived on Earth and how they originated. The second part of the dilemma - how life arose from inorganic matter - is not so romantic, since it relies 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 has proved fruitful and has advanced whole sections of biochemistry, evolutionary biology, and cosmology over the course of half a century.

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? Unlikely. 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 organic matter 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 dust storms, clouds of volcanic ash and fragments of rocks knocked out by meteorite impacts. An atmosphere rich in nitrogen, carbon dioxide and water vapor gradually developed. abundance greenhouse gases 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 Soviet man and knowing full well the biased attitude of the foreign intelligentsia towards the USSR, he 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.

Proteins consist only of so-called "left" 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 it happened by chance and somewhere in the universe there are living beings, consisting of the right amino acids. Most likely, in the primary broth, where the synthesis of the initial proteins took place, there were equally left and right amino acids. And only the appearance of a really living "left" structure broke this symmetry and the biogenic synthesis of amino acids went along the "left" path.

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. Big words 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 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 them. 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 the clubs 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 more 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 not 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 planets would be NASA satellite Kepler since 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 advanced civilization takes only one century. 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 are the main components of the cell. 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 complex structure- 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, there is only one place on the planet with very ancient rocks where fossils can be found - in the east 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 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 the molecules are related to each other, into a fatty membrane. Such a protocell could happen by chance, but 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 question of when life appeared on Earth has always worried not only scientists, but all people. Answers to it

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

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

What did the primitive Earth look like?

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

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

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

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

Or maybe life arose in areas of volcanic activity? Immediately after its formation, the Earth was a fire-breathing ball of magma. During volcanic eruptions and with gases released from molten magma, various chemical substances necessary for the synthesis of organic molecules. Thus, carbon monoxide molecules, being on the surface of the pyrite mineral, 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 do research in prebiotic (pre-life) chemistry at the University of California. The installation on which the first experiment was carried out was a system of flasks, in one of which it was possible to obtain a powerful electric discharge at a voltage of 100,000 V.

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

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

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

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

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

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

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

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

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

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

For many years, scientists believed that life could hardly have arisen and developed during the period when the Earth was constantly subjected to collisions with large comets and meteorites, and this period ended about 3.8 billion years ago. Recently, however, traces of complex cellular structures dating back to at least 3.86 billion years have been found 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 that fell to Earth could play a central role in the emergence of life on our planet, since, according to some researchers, cells like bacteria could originate on another planet and then fall to Earth along with asteroids. One of the pieces of evidence in favor of the extraterrestrial origin of life was found inside a potato-shaped meteorite named ALH84001. Initially, this meteorite was a piece of the Martian crust, which was then ejected into space as a result of an explosion when a huge asteroid collided with the surface of Mars, which occurred about 16 million years ago. And 13 thousand years ago, after a long journey within the solar system, this fragment of Martian rock in the form of a meteorite landed in Antarctica, where it was recently discovered. A detailed study of the meteorite inside it revealed rod-shaped structures resembling fossilized bacteria in shape, which gave rise to heated scientific debate about the possibility of life in the depths of the Martian crust. It will not be possible to resolve these disputes until 2005, when the US National Aeronautics and Space Administration will implement an interplanetary spacecraft mission to Mars to take samples of the Martian crust and deliver samples to Earth. And if scientists manage to prove that microorganisms once inhabited Mars, then it will be possible to speak with a greater degree of certainty about the extraterrestrial origin of life and the possibility of bringing life from space (Fig. 5).

Rice. 5. Our origin is from microbes.

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

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

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

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

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

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

The evolution of life on Earth: from simple to complex

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The modern concept of the origin of life on Earth is the result of a broad synthesis 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.