The origin of life on Earth is a key and unresolved problem of natural science, often serving as a ground for a clash between science and religion. If the existence of the evolution of living matter in nature can be considered proven, since its mechanisms were discovered, archaeologists discovered ancient, more simply arranged organisms, then no hypothesis of the origin of life has such an extensive evidence base. We can observe evolution with our own eyes, at least in selection. No one has been able to create a living thing from an inanimate one.
Despite the large number of hypotheses about the origin of life, only one of them has an acceptable scientific explanation. It's a hypothesis abiogenesis- a long chemical evolution that took place in the special conditions of the ancient Earth and preceded biological evolution. At the same time, simple organic substances were first synthesized from inorganic substances, of which more complex ones, then biopolymers appeared, the following stages are more speculative and hardly proven. The hypothesis of abiogenesis has many unresolved problems, different views on certain stages of chemical evolution. However, some of its points were confirmed empirically.
Other hypotheses for the origin of life - panspermia(introduction of life from space), creationism(creation by the creator), spontaneous generation(living organisms suddenly appear in inanimate matter), steady state(life has always existed). The impossibility of spontaneous generation of life in the inanimate was proved by Louis Pasteur (XIX century) and a number of scientists before him, but not so categorically (F. Redi - XVII century). The panspermia hypothesis does not solve the problem of the origin of life, but transfers it from Earth to outer space or to other planets. However, it is difficult to refute this hypothesis, especially those of its representatives who claim that life was brought to Earth not by meteorites (in this case, living things could burn out in the layers of the atmosphere, be subjected to the destructive action of cosmic radiation, etc.), but by intelligent beings. But how did they get to Earth? From the point of view of physics (the huge size of the Universe and the inability to overcome the speed of light), this is hardly possible.
For the first time, possible abiogenesis was substantiated by A.I. Oparin (1923-1924), later this hypothesis was developed by J. Haldane (1928). However, the idea that life on Earth could be preceded by the abiogenic formation of organic compounds was expressed by Darwin. The theory of abiogenesis has been finalized and is being finalized by other scientists to this day. Its main unresolved problem is the details of the transition from complex non-living systems to simple living organisms.
In 1947, J. Bernal, based on the developments of Oparin and Haldane, formulated the theory of biopoiesis, distinguishing three stages in abiogenesis: 1) the abiogenic occurrence of biological monomers; 2) formation of biopolymers; 3) the formation of membranes and the formation of primary organisms (protobionts).
Abiogenesis
The hypothetical scenario of the origin of life according to the theory of abiogenesis is described below in general terms.
The age of the Earth is about 4.5 billion years. Liquid water on the planet, so necessary for life, according to scientists, appeared no earlier than 4 billion years ago. At the same time, life on Earth already existed 3.5 billion years ago, which is proved by the discovery of rocks of such ages with traces of the vital activity of microorganisms. Thus, the first simple organisms arose relatively quickly - in less than 500 million years.
When the Earth first formed, its temperature could reach 8000 °C. When the planet cooled, metals and carbon, as the heaviest elements, condensed and formed the earth's crust. At the same time, volcanic activity was taking place, the crust was moving and contracting, folds and ruptures formed on it. Gravitational forces led to the compaction of the crust, while energy was released in the form of heat.
Light gases (hydrogen, helium, nitrogen, oxygen, etc.) were not retained by the planet and escaped into space. But these elements remained in the composition of other substances. Until the temperature on Earth dropped below 100°C, all water was in a vapor state. After the temperature dropped, evaporation and condensation repeated many times, there were heavy showers with thunderstorms. Hot lava and volcanic ash, once in the water, created different environmental conditions. In some, certain reactions could take place.
Thus, the physical and chemical conditions on the early Earth were favorable for the formation of organic substances from inorganic ones. The atmosphere was of a reducing type, there was no free oxygen and no ozone layer. Therefore, ultraviolet and cosmic radiation penetrated the Earth. Other sources of energy were the warmth of the earth's crust, which has not yet cooled down, erupting volcanoes, thunderstorms, radioactive decay.
Methane, carbon oxides, ammonia, hydrogen sulfide, cyanide compounds, and water vapor were present in the atmosphere. A number of the simplest organic substances were synthesized from them. Further, amino acids, sugars, nitrogenous bases, nucleotides and other more complex organic compounds could be formed. Many of them served as monomers for future biological polymers. The absence of free oxygen in the atmosphere favored the reactions.
Chemical experiments (for the first time in 1953 by S. Miller and G. Urey), simulating the conditions of the ancient Earth, proved the possibility of abiogenic synthesis of organic substances from inorganic ones. By passing electric discharges through a gas mixture that imitated the primitive atmosphere, in the presence of water vapor, amino acids, organic acids, nitrogenous bases, ATP, etc. were obtained.
It should be noted that in the ancient atmosphere of the Earth, the simplest organic substances could be formed not only abiogenically. They were also brought from space, contained in volcanic dust. Moreover, it could be quite large amounts of organic matter.
Low molecular weight organic compounds accumulated in the ocean, creating the so-called primordial soup. Substances were adsorbed on the surface of clay deposits, which increased their concentration.
Under certain conditions of the ancient Earth (for example, on clay, the slopes of cooling volcanoes), polymerization of monomers could occur. This is how proteins and nucleic acids were formed - biopolymers, which later became the chemical basis of life. In an aqueous environment, polymerization is unlikely, since depolymerization usually occurs in water. Experience has proven the possibility of synthesizing a polypeptide from amino acids in contact with pieces of hot lava.
The next important step towards the origin of life is the formation of coacervate drops in water ( coacervates) from polypeptides, polynucleotides, other organic compounds. Such complexes could have a layer on the outside that imitated a membrane and preserved their stability. Coacervates were obtained experimentally in colloidal solutions.
Protein molecules are amphoteric. They attract water molecules to themselves so that a shell forms around them. Colloidal hydrophilic complexes are obtained, isolated from the water mass. As a result, an emulsion is formed in water. Further, the colloids merge with each other and form coacervates (the process is called coacervation). The colloidal composition of the coacervate depended on the composition of the medium in which it was formed. In different reservoirs of the ancient Earth, coacervates of different chemical composition were formed. Some of them were more stable and could, to a certain extent, carry out selective metabolism with the environment. There was a kind of biochemical natural selection.
Coacervates are able to selectively absorb certain substances from the environment and release into it some products of chemical reactions occurring in them. It's like metabolism. With the accumulation of substances, the coacervates grew, and when they reached a critical size, they broke up into parts, each of which retained the features of the original organization.
In the coacervates themselves, chemical reactions could take place. During the absorption of metal ions by coacervates, enzymes could be formed.
In the process of evolution, only such systems remained that were capable of self-regulation and self-reproduction. This marked the onset of the next stage in the origin of life - the emergence protobionts(according to some sources, this is the same as coacervates) - bodies that have a complex chemical composition and a number of properties of living beings. Protobionts can be considered as the most stable and successful coacervates.
The membrane could be formed in the following way. Fatty acids combine with alcohols to form lipids. Lipids formed films on the surface of water bodies. Their charged heads face into the water, while the non-polar ends face out. Protein molecules floating in water were attracted to the heads of lipids, resulting in the formation of double lipoprotein films. From the wind, such a film could bend, and bubbles formed. Coacervates may have been accidentally trapped in these vesicles. When such complexes again appeared on the surface of the water, they were already covered with a second lipoprotein layer (due to hydrophobic interactions of non-polar ends of lipids facing each other). The general layout of the membrane of today's living organisms is two layers of lipids inside and two layers of proteins located at the edges. But over millions of years of evolution, the membrane became more complex due to the inclusion of proteins immersed in the lipid layer and penetrating it, protrusion and protrusion of individual sections of the membrane, etc.
Coacervates (or protobionts) could get already existing nucleic acid molecules capable of self-reproduction. Further, in some protobionts, such a rearrangement could occur that the nucleic acid began to encode the protein.
The evolution of protobionts is no longer chemical, but prebiological evolution. It led to an improvement in the catalytic function of proteins (they began to play the role of enzymes), membranes and their selective permeability (which makes the protobiont a stable set of polymers), the emergence of matrix synthesis (transfer of information from nucleic acid to nucleic acid and from nucleic acid to protein).
| Evolution | results | |
|---|---|---|
| 1 | Chemical evolution - synthesis of compounds |
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| 2 | Prebiological evolution - chemical selection: the most stable, self-reproducing protobionts remain |
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| 3 | Biological evolution - biological selection: the struggle for existence, the survival of the most adapted to environmental conditions |
|
One of the biggest mysteries about the origin of life is how RNA came to code for the amino acid sequence of proteins. The question refers to RNA, not DNA, since it is believed that at first ribonucleic acid played not only a role in the implementation of hereditary information, but was also responsible for its storage. DNA replaced it later, emerging from RNA by reverse transcription. DNA is better at storing information and is more stable (less prone to reactions). Therefore, in the process of evolution, it was she who was left as the custodian of information.
In 1982, T. Chek discovered the catalytic activity of RNA. In addition, RNA can be synthesized under certain conditions even in the absence of enzymes, and also form copies of themselves. Therefore, it can be assumed that RNAs were the first biopolymers (the RNA world hypothesis). Some sections of RNA could accidentally encode peptides useful for the protobiont, while other sections of RNA became excised introns in the course of evolution.
A feedback appeared in protobionts - RNA encodes enzyme proteins, enzyme proteins increase the amount of nucleic acids.
Beginning of biological evolution
Chemical evolution and the evolution of protobionts lasted more than 1 billion years. Life arose, and its biological evolution began.
Some protobionts gave rise to primitive cells, which include the totality of the properties of living things that we observe today. They implemented the storage and transmission of hereditary information, its use to create structures and metabolism. Energy for vital processes was provided by ATP molecules, and membranes typical of cells appeared.
The first organisms were anaerobic heterotrophs. They obtained the energy stored in ATP through fermentation. An example is glycolysis - the oxygen-free breakdown of sugars. These organisms ate at the expense of organic substances of the primary broth.
But the reserves of organic molecules were gradually depleted, as the conditions on the Earth changed, and the new organics were almost no longer synthesized abiogenically. Under conditions of competition for food resources, the evolution of heterotrophs accelerated.
The advantage was gained by bacteria, which turned out to be able to fix carbon dioxide with the formation of organic substances. Autotrophic synthesis of nutrients is more complex than heterotrophic nutrition, so it could not have arisen in early life forms. From some substances, under the influence of the energy of solar radiation, compounds necessary for the cell were formed.
The first photosynthetic organisms did not produce oxygen. Photosynthesis with its release most likely appeared later in organisms similar to the current blue-green algae.
The accumulation of oxygen in the atmosphere, the appearance of an ozone screen, and a decrease in the amount of ultraviolet radiation led to the almost impossibility of the abiogenic synthesis of complex organic substances. On the other hand, emerging life forms have become more resilient under such conditions.
Oxygen respiration spread on Earth. Anaerobic organisms have survived only in a few places (for example, there are anaerobic bacteria living in hot underground springs).
Water, constantly evaporating from the surface of the Earth, condensed in the upper layers of the atmosphere and again fell in the form of rain on the hot earth's surface. The gradual decrease in temperature led to the fact that downpours fell on the Earth, accompanied by continuous thunderstorms. Water bodies began to form on the earth's surface.
Atmospheric gases and those substances that were washed out of the earth's crust were dissolved in hot water. In the atmosphere, under the influence of frequent and strong electrical lightning discharges, powerful ultraviolet radiation coming from the Sun, and active volcanic activity, which was accompanied by emissions of radioactive compounds, the simplest organic substances (formaldehyde, glycerin, amino acids, urea, lactic acid) were formed.
Since there was no free oxygen in the atmosphere yet, these compounds, getting into the waters of the ancient ocean, were not oxidized and could accumulate, becoming more complex in structure and forming a concentrated "primal broth" is a term introduced by . Organic matter, accumulating for millions of years in the water of the ancient ocean, formed a concentrated solution, or "primary soup".
Formation of biological polymers and coacervates
The first stage of biochemical evolution was confirmed by numerous experiments, but what happened at the next stage, scientists can only guess based on the knowledge of chemistry and molecular biology.
Apparently, the formed simplest organic substances interacted with each other and with inorganic compounds entering water bodies. Fatty acids, reacting with alcohols, formed lipids, which formed fatty films on the surface of water bodies. Amino acids combine with each other to form peptides. An important event of this stage was the emergence of nucleic acids - molecules capable of reduplication.
Modern biochemists believe that short chains of RNA were the first to form, which could be synthesized independently, without the participation of special enzymes. The formation of nucleic acids and their interaction with proteins has become a necessary prerequisite for the emergence of life, which is based on the reactions of matrix synthesis and metabolism.
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The purpose of the lesson: To form a system of knowledge among students about different views on the origin of life on Earth.
Lesson objectives:
I. Educational:
- Show the role of experiment in resolving scientific disputes about the origin of life.
- Learn to analyze the main scientific hypotheses about the origin of life.
II. Developing:
- Continue to develop the desire for independent cognitive activity.
- Continue the formation of formal-logical skills of explanation, concretization, definition, generalization.
III. Educational:
- Intellectual - to continue the formation of a scientific worldview.
- Ecological - consolidation of knowledge about the relationship of animate and inanimate nature.
- Moral - the formation of students' knowledge and beliefs about human responsibility for maintaining the integrity of the biosphere of our planet.
Motivation:
The origin of life on our planet is the subject of centuries-old discussions in which more than one generation of mankind participated. This is an interesting field of knowledge, which has scientific, philosophical and ideological significance, still attracts the attention of researchers in various fields.
The study of various theories about the origin of life on Earth is necessary to compile a holistic view of the historical path of the development of living nature, the formation of a scientific worldview.
Learners should know:
- The main provisions of theories about the origin of life;
- Modern ideas about the origin of life on Earth (the theory of biochemical evolution).
Learners should be able to:
- To reveal the key provisions of the main theories about the origin of life on Earth;
- Give a description of the experiments of F. Redi, L. Spallanzani, L. Pasteur, S. Miller, reveal their significance for solving the issue of the origin of life;
- To reveal the main provisions of modern ideas about the origin of life on Earth (theories of biochemical evolution);
- Formulate the main provisions of the theory of A.I. Oparin.
Lesson equipment:
- lesson plan;
- abstract;
- Handout;
- tasks for control;
- presentation;
- notebook;
- multimedia projector;
- screen.
Interdisciplinary connections:
a) physics (design of instruments, physical phenomena);
b) chemistry (composition of the atmosphere, chemicals);
c) history (development of science);
d) philosophy (formation of scientific outlook);
e) foreign language (translation of terms).
Literature for the teacher:
- Sivoglazov V.I., Agafonov I.B. General Biology 10-11. - M .: Bustard, 2005
- Sivoglazov V.I., Sukhova T.S., Kozlova T.A. General biology. A guide for the teacher. – M.: IRIS PRESS, 2004
- Sukhova T.S. Biology lesson. Developmental learning technology. – M.: Ventana-Graf, 2001
Literature for students:
1. Sivoglazov V.I., Agafonov I.B. General biology 10-11.– M.: Bustard, 2005
Lesson timeline:
1. Organizational moment
Greeting, checking those present according to the list, wishing successful work in class.
2. Control of the initial level of knowledge (standards of correct answers are indicated in brackets)
Goals:
- Determine the level of knowledge of students.
- Adjust the level of complexity of the presentation of new material.
1. By what main features (criteria) can a living object be distinguished from an inanimate one?
(The unity of the chemical composition of living organisms, metabolism, irritability, growth, reproduction, development, adaptability to the environment, self-regulation).
2. Where and when did the first living organisms arise? What were they like? (The first organisms appeared about 3 billion years ago in the aquatic environment, they were unicellular prokaryotes, fed on the organic matter of the ocean, anaerobes.)
3. What stages in the development of plants on Earth can you name? (unicellular, multicellular; the emergence of photosynthesis, the sexual process; access to land, the development of terrestrial vegetation.)
4. What stages in the development of animals on Earth can you name? (Single-celled, colonial, multicellular; the appearance of the sexual process; the appearance of invertebrates and vertebrates; access to land; complication in structure due to the terrestrial lifestyle.)
5. What substances are part of living organisms?
(Inorganic (water, mineral salts) and organic (amino acids, proteins, fats, carbohydrates, etc.))
3. Studying new material (explanation of new material is accompanied by a presentation, slide numbers are indicated in the text)
3.1. Formulation of the problem
Life has existed on Earth for billions of years. It fills all corners of our planet.
From ancient times to our time, a huge number of hypotheses about the origin of life have been expressed. The specificity of living things determines a number of questions that need to be answered when solving the problem of the origin of life:
- How did life originate and develop on our planet?
- How did the cell, the structural unit of life, come into existence?
- How did all the substances and structures specific to living things come about?
- How was the existing metabolism formed? Etc.
We have to get acquainted with the hypotheses of the origin of life, analyze them and form an idea of how life arose and developed on Earth.
3.2. Development of ideas about the origin of life on Earth (slide number 1)
Since time immemorial, the origin of life has been a mystery to mankind. From the moment of its appearance, thanks to labor, a person begins to stand out among other living beings.
But the ability to ask yourself the question “where are we from?” a person receives relatively recently - 7-8 thousand years ago.
Until that time, man had difficulty separating himself from other animals (man was both a hunter and a kind of game), but gradually he began to delimit himself from nature with his inner spiritual world. The first primitive forms of belief in unreal, supernatural or divine forces arose already 35-40 thousand years ago.
3.3. The main theories of the origin of life on Earth (Slide #2)
According to this theory, life arose as a result of some supernatural event in the past, which most often means a divine creation. The idea of the creation of the world as a “creative act” of God arose, and this myth underlies all religions.
"These are the facts - living things can arise not only as a result of the mating of animals, but also the decomposition of the soil ... Some plants develop from seeds, while others spontaneously arise under the influence of the forces of nature from the decaying earth or certain parts of plants ..."
However, with the advent of Christianity, especially in the Middle Ages, the theory of spontaneous generation came under the yoke of the Church. She was considered an attribute of witchcraft and a manifestation of the devil. However, she continued to exist.
At the turn of the XVI-XVII centuries. Van Helmont (1579 - 1644) described an experiment in which he managed to get mice from dirty linen and wheat placed in a dark closet. Van Helmont considered human sweat to be the active beginning of the birth of a mouse. (Slide number 5)- To concept of biogenesis (from the Greek "bios" - life, "genesis" - origin). (Slide number 6)
In 1668 the Italian physician Francesco Redi (1626-1698) proved that the white worms found in meat are fly larvae; if meat or fish are closed up while they are fresh, and the access of flies is prevented, they, although they rot, will not produce worms. From this, F. Redi concluded that the emergence of the living only from the living). (Slide number 7) In 1765, Lazzardo Spallanzani (1729-1799) boiled meat and vegetable concoctions and immediately sealed them. A few days later, he examined the decoctions and found no signs of life. From this, he concluded that the heat had destroyed all living things, and nothing new could arise. (Slide number 8)
J. Needham - supporter vitalism (from lat. vita - life), explained the negative results obtained by L. Spallanzani by the fact that he subjected his infusions to too harsh processing, as a result of which their "life force" was destroyed. (Slide number 9) According to the vitalists, the "life force" is present everywhere. It is enough just to “breathe” it, and the inanimate will become alive.
In 1862 the great French scientist Louis Pasteur (1822-1895) publishes his observations on the problem of arbitrary spontaneous generation. He proves that the sudden appearance (“spontaneous spontaneous generation”) of microbes in various types of rotting tinctures or extracts is not the emergence of life. Rotting and fermentation is the result of the vital activity of microorganisms introduced from the outside. His research finally destroyed the age-old prejudices about spontaneous spontaneous generation.
Fig.1. Experience of L. Pasteur in flasks with S-shaped necks:
1 - flask with sugared yeast water; after sterilization and cooling remains sterile for a long time;
2 - the same flask 48 hours after removal of the curved neck; growth of microorganisms is observed. (slides №10,11)
- Steady State Theory (Slide number 12)
According to this theory, the Earth has existed forever, never having arisen, has always been able to support life, and any changes on it are completely insignificant. This theory does not stand up to scrutiny at the present time.
- Panspermia theory (Slide number 13)
In the 5th century BC. Greek philosopher Anaxagoras expressed the idea of cosmic seeding - panspermia(from the Greek "pan" - everything and "sperma" - seed). According to him, life arose from a seed that exists "always and everywhere." According to this theory, the germs of life are brought to Earth by meteorites or cosmic dust. This theory does not offer any mechanism for the emergence of life, simply putting forward the postulate of its extraterrestrial origin. It is argued that life could arise repeatedly at different times and in different places in the universe.
4. Modern ideas about the origin of life
(Slide 14)The modern theory of the origin of life is based on the idea that biological molecules could have originated in the distant geological past in an inorganic way.
The greatest distribution in the twentieth century. received the theory of biochemical evolution, proposed independently by the Russian chemist A.I. Oparin (1894 - 1980) and the English biologist D. Haldane (1892 - 1964).
- Theory of biochemical evolution (Slide number 15)
Stage 1 - abiogenic occurrence of organic monomers Our planet arose about 4.6 billion years ago. The gradual compaction of the planet was accompanied by the release of a huge amount of heat, radioactive compounds decayed, and a stream of hard ultraviolet radiation came from the Sun. After 500 million years, the slow cooling of the Earth began. The formation of the earth's crust was accompanied by active volcanic activity. It is believed that the primary atmosphere consisted mainly of ammonia, water, methane, carbon monoxide and carbon dioxide. The absence of oxygen gave it reducing properties. On May 3, 1924, at a meeting of the Russian Botanical Society, the young scientist A.I. Oparin expressed the opinion that in the conditions of the Earth's primary atmosphere, which differs significantly from the current one, a synthesis of all precursor substances necessary for the origin of life could take place.
Under such conditions, organic substances could be created much more easily and could be preserved without undergoing decay for a long time. A.I. Oparin believed that complex substances could be synthesized from simpler ones under ocean conditions. The energy necessary for the reactions was brought by solar radiation, since. the protective ozone screen did not yet exist; synthesis also took place under conditions of lightning discharges.
Conditions on primeval earth (slides No. 16,17):
The variety of simple compounds found in the ocean and the large time scales suggest the possibility of the accumulation of a large amount of organic matter in the ocean, which formed the "primordial soup" in which life could originate.
The scheme of formation of the “primary broth”
This theory was confirmed in the experiments of S. Miller conducted in 1953. (Slide 18)
Fig.2. Scheme of the device S. Miller:
1 - reaction flask; 2 - tungsten electrodes; 3 - spark discharge; 4 - a flask with boiling water; 5 - refrigerator; 6 - trap; 7 - a tap through which a gas mixture is supplied to the apparatus
Through a gas mixture containing methane, ammonia, molecular hydrogen and water vapor, i.e., simulating the atmospheric composition of the primitive Earth, he passed electrical discharges, and then analyzed the resulting reaction products. Tungsten electrodes were mounted in a reaction flask containing a mixture of gases. Spark discharges with a voltage of 60,000 V were passed during the week. In another flask (small), water was kept in a state of boiling. Water vapor passed through the reaction flask and condensed in the refrigerator. In the process of circulation, they captured the reaction products from the reaction flask and transferred them to a trap, where they were concentrated. When identifying the reaction products, organic compounds were found: urea, lactic acid and some amino acids.
Stage 2 - the formation of biological polymers and coacervates (Slide No. 19)
A.I. Oparin believed that the decisive role in the transformation of the inanimate into the living belongs to proteins. Protein molecules formed complexes with water molecules surrounding them. The merger of such complexes with each other led to their separation from the aquatic environment; coacervates(from lat. "coacervus" - clot). Drops-coacervates were able to: exchange substances with the environment, accumulate various compounds. Absorption of metal ions by coacervates led to the formation of enzymes. Proteins in coacervates protected nucleic acids from the damaging effects of ultraviolet radiation. In the drops themselves, further chemical transformations of the substances that got there took place. Lipid molecules lined up at the boundary of the droplets with the external environment, forming a primitive membrane that increased the stability of the entire system.
Stage 3 - the formation of membrane structures and primary organisms (probionts) Around the coacervates, rich in organic compounds, layers of lipids arose that separated the coacervate from the surrounding aquatic environment. Lipids were transformed in the course of evolution into the outer membrane, which significantly increased the viability and resistance of organisms. This is how probionts arose - primitive heterotrophic organisms that fed on organic substances of the primary broth. It happened 3.5 - 3.8 billion years ago. Chemical evolution is over.
The essence of the theory of A.I. Oparin can be formulated in the form of three postulates:
1. Life is one of the stages in the evolution of the Universe. 2. The emergence of life is a natural result of the chemical evolution of carbon compounds. 3. For the transition from chemical to biological evolution, the formation and natural selection of integral multimolecular systems isolated from the environment, but constantly interacting with it, which were called probionts, are necessary.
Conclusions. (Slide number 20)
In the 20-30s of the XX century. science returned to the idea of spontaneous generation, taking into account the criticism that the concept of abiogenesis was subjected to in the 19th century. Spontaneous generation of life is impossible under modern conditions, but it could have taken place in a time long past, when conditions on Earth were different. At the beginning of the XX century. the belief prevailed that the organic substances underlying life (proteins, fats, carbohydrates) under natural conditions can only arise biogenically, i.e. by their own synthesis. In the twenties A.I. Oparin and J. Haldane experimentally showed that in solutions of high-molecular organic compounds, zones of their increased concentration can appear - coacervate drops- which in a sense behave like living objects: spontaneously grow, divide and exchange matter with the surrounding liquid through a compacted interface.
Soviet biochemist A.I. Oparin (1894-1980) 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 necessary for the emergence of life could arise . A.I.'s prediction Oparin received wide recognition and was confirmed by experiments. The experiments of G. Urey and S. Miller (1955), carried out at the University of Chicago, gained particular fame. Passing electric discharges up to 60,000 V through a mixture of carbon dioxide, methane, ammonia, hydrogen and water vapor under a pressure of several pascals at a temperature of + 80 * C, they obtained the simplest fatty acids, urea, acetic and formic acids and several amino acids, in including glycine and alanine. The diagram of the Miller device is shown in rice. 49 As you know, amino acids are the "bricks" from which protein molecules are built. After some time, S. Fox managed to combine the latter into short irregular chains - a matrixless synthesis of polypeptides; similar polypeptide chains were later actually found, among other simple organic matter, in meteorite matter. Experimental evidence of the possibility of the formation of amino acids from inorganic compounds gave reason to assume that the first step towards the emergence of life on Earth was the abiogenic synthesis of organic substances ( rice. 39).
At present, abiogenic synthesis of many biologically important monomers has been carried out in various laboratories. Much information has been obtained regarding the abiogenic synthesis of amino acids ( tab. 14). The amino acids listed in the table are formed in gas or water mixtures of simple composition as a result of exposure to different energy sources. With some complication of the reaction mixture by introducing into it C2-, C3-hydrocarbons, acetaldehyde, hydroxylamine, hydrazine and other compounds, the formation of which easily occurs under the conditions of the primitive Earth, a significantly larger number of amino acids are synthesized, including those that have not been found as reaction products in gaseous and aqueous mixtures of simple composition. It has been experimentally proven that almost all the amino acids that make up natural proteins can be obtained in the laboratory by simulating the conditions of the primitive Earth.
An important step on the path of chemical evolution is the synthesis of nucleosides and nucleotides, and primarily adenine ones. The American biochemist K. Ponnamperuma managed to show that when a mixture of aqueous solutions of adenine and ribose is irradiated with UV at a temperature of 40 degrees C in the presence of phosphoric acid, a condensation reaction occurs, leading to the formation of adenosine. If the reaction is carried out by adding ethyl metaphosphate to the reaction mixture, the formation of nucleotides also takes place: AMF , ADP , ATP. The function of phosphorus compounds in these chemical syntheses is twofold: they play a catalytic role and can be directly included in the reaction products. The abiogenic synthesis of ATP, which is the result of several relatively simple chemical reactions, indicates the possible early appearance of this compound. The first living structures could receive ATP from the environment.
The next stage of prebiological evolution is the further complication of organic compounds associated with the polymerization of monomers. All living cells are composed of four main types of macromolecules: proteins, nucleic acids, lipids and polysaccharides. Of these, proteins and nucleic acids are the most complex substances of the cell.
S. Fox (S. Fox) carried out abiogenic synthesis of polypeptides, consisting of 18 natural amino acids, with a molecular weight of 3,000 to 10,000 Da. A feature of the primary structure of these polymers was a certain sequence of amino acid residues found in them in the chain, probably due to the structural features of the amino acids themselves. The resulting polymers had many properties similar to natural proteins: they served as a source of nutrition for microorganisms, were hydrolyzed by proteinases, gave a mixture of amino acids upon acid hydrolysis, had catalytic activity and the ability to form microsystems separated from the environment by membrane-like surface layers. Due to the great similarity with natural proteins, the polypeptides synthesized by S. Fox were named
