Modern hypotheses for the origin of life

The hypothesis of the origin of life on Earth, proposed by the famous Russian biochemist Academician A. I. Oparin (1894-1980) and the English biochemist J. Haldane (1892-1964), received the greatest recognition and distribution in the 20th century. The essence of their hypothesis, formulated by them independently of each other in 1924-1928. and developed in the subsequent time, is reduced to the existence on Earth of a long period of abiogenic formation of a large number of organic compounds. These organic substances saturated the waters of the ancient oceans, forming (according to J. Haldane) the so-called "primary soup". Subsequently, due to numerous processes of local shallowing and drying up of the oceans, the concentration of the "primary soup" could increase by tens and hundreds of times. These processes took place against the background of intense volcanic activity, frequent lightning discharges in the atmosphere, and powerful cosmic radiation. Under these conditions, there could be a gradual complication of the molecules of organic substances, the appearance of simple proteins, polysaccharides, lipids, nucleic acids. Over many hundreds and thousands of years, they could form clots of organic matter (coacervates). Under the conditions of the reducing atmosphere of the Earth, the coacervates did not collapse, their gradual complication took place, and at a certain moment of development, the first primitive organisms (probionts) could form from them. This hypothesis was accepted and further developed by many scientists from different countries, and in 1947 the English scientist John Bernal formulated the hypothesis of biopoiesis. He identified three main stages in the formation of life: 1) the abiogenic occurrence of organic monomers; 2) formation of biological polymers; 3) the development of membrane structures and the first organisms.

Let us briefly consider the processes and stages of biopoiesis.

The first stage of biopoiesis was a series of processes called chemical evolution, which led to the appearance of probionts - the first living beings. Its duration is estimated by different scientists from 100 to 1000 million years. This is the prehistory of life on our planet.

The Earth as a planet arose about 4.5 billion years ago (according to other sources - about 13 billion years ago, but they do not yet have solid evidence). The cooling of the Earth began about 4 billion years ago, and the age of the earth's crust is estimated at about 3.9 billion years. By this time, the ocean and the Earth's primary atmosphere are also formed. The earth at that time was quite warm due to the release of heat during the solidification and crystallization of the components of the crust and active volcanic activity. Water was in a vapor state for a long time, evaporating from the Earth's surface, condensing in the upper atmosphere and falling again onto a hot surface. All this was accompanied by almost constant thunderstorms with powerful electrical discharges. Later, reservoirs and primary oceans begin to form. The ancient atmosphere of the Earth did not contain free oxygen and was saturated with volcanic gases, which included oxides of sulfur, nitrogen, ammonia, oxides and carbon dioxide, water vapor and a number of other components. Powerful cosmic radiation and radiation from the Sun (there was no ozone layer in the atmosphere yet), frequent and strong electrical discharges, active volcanic activity, accompanied by emissions of large masses of radioactive components, led to the formation of organic compounds such as formaldehyde, formic acid, urea, lactic acid, glycerin, glycine, some simple amino acids, etc. Since there was no free oxygen in the atmosphere, these compounds were not oxidized and could accumulate in warm and even boiling water bodies and gradually become more complex in structure, forming the so-called "primary broth". The duration of these processes was many millions and tens of millions of years. Thus, the first stage of biopoiesis was realized - the formation and accumulation of organic monomers.

Stage of polymerization of organic monomers

A significant part of the resulting monomers was destroyed under the influence of high temperatures and numerous chemical reactions that took place in the "primary soup". Volatile compounds passed into the atmosphere and practically disappeared from water bodies. Periodic drying of water bodies led to a multiple increase in the concentration of dissolved organic compounds. Against the background of high chemical activity of the medium, the processes of complication of these compounds took place, and they could enter into compounds with each other (reactions of condensation, polymerization, etc.). Fatty acids, combining with alcohols, could form lipids and form fatty films on the surface of water bodies. Amino acids could combine with each other, forming more and more complex peptides. Other types of compounds could also be formed - nucleic acids, polysaccharides, etc. The first nucleic acids, as modern biochemists believe, were small RNA chains, since they, like oligopeptides, could be synthesized spontaneously in an environment with a high content of mineral components, without the participation of enzymes . Polymerization reactions could be noticeably activated with a significant increase in the concentration of the solution (drying of the reservoir) and even in wet sand or when the reservoirs completely dried out (the possibility of such reactions occurring in a dry state was shown by the American biochemist S. Fox). Subsequent rains dissolved the molecules synthesized on land and moved them with water currents to water bodies. Such processes could be cyclical, leading to even greater complication of organic polymers.

Formation of coacervates

The next stage in the origin of life was the formation of coacervates, that is, large accumulations of complex organic polymers. The causes and mechanisms of this phenomenon are largely unclear. The coacervates of this period were still a mechanical mixture of organic compounds, devoid of any signs of life. In a certain period of time, bonds arose between RNA molecules and peptides, reminiscent of the reactions of matrix protein synthesis. However, it is still unclear how RNA came to encode peptide synthesis. Later, DNA molecules appeared, which, due to the presence of two helices and the possibility of more accurate (compared to RNA) self-copying (replication), became the main carriers of information on peptide synthesis, transferring this information to RNA. Such systems (coacervates) already resembled living organisms, but they were not yet such, since they did not have an ordered internal structure inherent in living organisms, and were not able to reproduce. After all, certain reactions of peptide synthesis can also occur in non-cellular homogenates.

The emergence of biological membranes

Ordered biological structures are impossible without biological membranes. Therefore, the next stage in the formation of life was the formation of precisely these structures that isolate and protect coacervates from the environment, turning them into autonomous formations. The membranes could be formed from lipid films that appeared on the surface of water bodies. Peptides brought by rain streams to water bodies or formed in these water bodies could be attached to lipid molecules. When water bodies were disturbed or precipitation fell on their surface, bubbles surrounded by membrane-like compounds could appear. For the emergence and evolution of life, those vesicles that surrounded the coacervates with protein-nucleide complexes were important. But even such formations were not yet living organisms.

The emergence of probionts - the first self-reproducing organisms

Only those coacervates that were capable of self-regulation and self-reproduction could turn into living organisms. How these abilities arose is also not yet clear. Biological membranes provided autonomy and protection to coacervates, which contributed to the emergence of a significant orderliness of biochemical reactions occurring in these bodies. The next step was the emergence of self-reproduction, when nucleic acids (DNA and/or RNA) began not only to ensure the synthesis of peptides, but also to regulate the processes of self-reproduction and metabolism with its help. This is how a cellular structure appeared, which has a metabolism and the ability to reproduce itself. It is these forms that could be preserved in the process of natural selection. So coacervates turned into the first living organisms - probionts.

The stage of chemical evolution has ended, and the stage of biological evolution of already living matter has begun. It happened 3.5-3.8 billion years ago. The appearance of a living cell is the first major aromorphosis in the evolution of the organic world.

The first living organisms were similar in structure to prokaryotes, did not yet have a strong cell wall and some intracellular structures (they were covered with a biological membrane, the internal bends of which performed the functions of cellular structures). Perhaps the first probionts had hereditary material represented by RNA, and genomes with DNA appeared later in the evolutionary process. There is an opinion that the further evolution of life went from a common ancestor, from which the first prokaryotes originated. This is what ensured the great similarity in the structure of all prokaryotes, and subsequently eukaryotes.

The impossibility of spontaneous generation of life in modern conditions

The question is often asked: why is there no spontaneous generation of living beings at the present time? After all, if living organisms do not appear now, then on what basis can we create hypotheses about the origin of life in the distant past? Where is the probability criterion for this hypothesis? The answers to these questions can be as follows: 1) the above hypothesis of biopoiesis is in many respects only a logical construction, it has not yet been proven, it contains many contradictions and unclear points (although there is a lot of data, both paleontological and experimental, suggesting just such a development of biopoiesis ); 2) this hypothesis, for all its incompleteness, nevertheless tries to explain the emergence of life, based on specific earthly conditions, and this is precisely its value; 3) self-formation of new living beings at the present stage of development of life is impossible for the following reasons: a) organic compounds must exist in the form of accumulations for a long time, gradually becoming more complex and transforming; in the conditions of the oxidizing atmosphere of the modern Earth, this is impossible, they will be quickly destroyed; b) in modern conditions there are many organisms that can very quickly use even insignificant accumulations of organic matter for their nutrition.

4. Do your own work"Analysis and evaluation of various hypotheses of the origin of life on Earth"

Record the results in a tableHypotheses of the origin of life on Earth.

The hypothesis of the abiogenic origin of life in the process of biochemical evolution is the most developed from a scientific point of view. However, the unresolved question is when and where the abiogenic synthesis of organic compounds took place and, most importantly, how the jump from non-living to living occurred.

MAIN STAGES OF DEVELOPMENT OF LIFE ON EARTH.

1. Fill in the table " The main stages of the development of life on Earth from the standpoint of the theory of biopoiesis.

2. What hypotheses exist for the origin of eukaryotes?

Most scientists believe that eukaryotes evolved from prokaryotic cells. There are two hypotheses for the origin of eukaryotes:

The eukaryotic cell and its organelles were formed by invagination of the cell membrane;
Symbiotic hypothesis that mitochondria, plastids, basal bodies of cilia and flagella were once free prokaryotes. They became organelles in the process of symbiosis.

3. What facts support the hypothesis of the symbiotic origin of the eukaryotic cell?

Answer: This hypothesis is supported by the presence of its own RNA and DNA in mitochondria and chloroplasts. In their structure, chloroplast RNA is similar to cyanobacteria RNA, mitochondrial RNA is similar to purple bacteria RNA. COMPLICATION OF LIVING ORGANISMS ON EARTH IN THE PROCESS OF EVOLUTION.

1. Give definitions of concepts.

An era is a section of the geochronological scale, a large Earth.
A period is a section of the geochronological scale that divides an era into several parts.

2. What are the main reasons for the diversity of species of organisms on Earth?

Answer: The reasons for the diversity of species are the result of the interaction of the driving forces of evolution: hereditary variability, the struggle for existence, natural selection. There are various habitats on Earth. In this regard, each species has adapted to the conditions of life, each in its own environment. A large variety of species in nature reduces the chances of extinction.

3. Complete the table "Complication of living organisms on Earth.

Topic 4.2. Modern evolutionary teaching Topic 4.4. Human Origins

CCE question 42

Hypotheses for the origin of life on earth

1. Creationism

2. Spontaneous (spontaneous) generation

3. Panspermia hypothesis

4. Hypothesis of biochemical evolution

5. Stationary state

1. creationism. According to this concept, life and all species of living beings inhabiting the Earth are the result of a creative act of a higher being at some specific time. The main provisions of creationism are set out in the Bible, in the Book of Genesis. The process of the divine creation of the world is conceived as having taken place only once and therefore inaccessible to observation. This is enough to take the whole concept of divine creation out of the scope of scientific research. Science deals only with observable phenomena and therefore will never be able to either prove or reject this concept.

2. Spontaneous (spontaneous) generation. The ideas of the origin of living beings from inanimate matter were widespread in Ancient China, Babylon, and Egypt. The largest philosopher of ancient Greece, Aristotle, suggested that certain “particles” of matter contain some kind of “active principle”, which, under suitable conditions, can create a living organism.

Van Helmont (1579-1644), a Dutch physician and natural philosopher, described an experiment in which he allegedly created mice in three weeks. For this, a dirty shirt, a dark closet and a handful of wheat were needed. Van Helmont considered human sweat to be the active principle in the process of the birth of a mouse. And until the appearance in the middle of the tenth century of the work of the founder of microbiology, Louis Pasteur, this doctrine continued to find adherents.

The development of the idea of spontaneous generation refers, in essence, to the era when religious ideas dominated the public consciousness. Those philosophers and naturalists who did not want to accept the Church's teaching on the "creation of life", with the then level of knowledge, easily came to the idea of its spontaneous generation. To the extent that, in contrast to the belief in creation, the idea of the natural origin of organisms was emphasized, the idea of spontaneous generation was at a certain stage of progressive significance. Therefore, this idea was often opposed by the Church and theologians.

3. Panspermia hypothesis. According to this hypothesis, proposed in 1865. by the German scientist G. Richter and finally formulated by the Swedish scientist Arrhenius in 1895, life could be brought to Earth from space. The most likely hit of living organisms of extraterrestrial origin with meteorites and cosmic dust. This assumption is based on data on the high resistance of some organisms and their spores to radiation, high vacuum, low temperatures, and other influences. However, there are still no reliable facts confirming the extraterrestrial origin of microorganisms found in meteorites. But even if they got to Earth and gave rise to life on our planet, the question of the original origin of life would remain unanswered.

4. Hypothesis of biochemical evolution. In 1924, the biochemist A. I. Oparin, and later the English scientist J. Haldane (1929), formulated a hypothesis that considers life as the result of a long evolution of carbon compounds.

Currently, in the process of the formation of life, four stages are conventionally distinguished:

1. Synthesis of low molecular weight organic compounds (biological monomers) from gases of the primary atmosphere.

2. Formation of biological polymers.

3. Formation of phase-separated systems of organic substances separated from the external environment by membranes (protobionts).

4. The emergence of the simplest cells that have the properties of a living thing, including the reproductive apparatus, which ensures the transfer of the properties of parental cells to daughter cells.

"PRIMARY SOFT" (optional)

In 1923, the Russian scientist Alexander Ivanovich Oparin suggested that, under the conditions of the primitive Earth, organic substances arose from the simplest compounds - ammonia, methane, hydrogen and water. The energy necessary for such transformations could be obtained either from ultraviolet radiation, or from frequent lightning electrical discharges - lightning. Perhaps these organic substances gradually accumulated in the ancient ocean, forming the primordial soup in which life originated.

According to the hypothesis of A.I.

Oparin, in the primary broth, long filamentous protein molecules could fold into balls, “stick together” with each other, becoming larger. Thanks to this, they became resistant to the destructive action of the surf and ultraviolet radiation. Something similar happened to what can be observed by pouring mercury from a broken thermometer onto a saucer: the mercury, crumbling into many small droplets, gradually collects into slightly larger drops, and then into one large ball. Protein "balls" in the "primary broth" attracted to themselves, bound water molecules, as well as fats. Fats settled on the surface of protein bodies, enveloping them with a layer, the structure of which remotely resembled a cell membrane. Oparin called this process coacervation (from Latin coacervus - “clot”), and the resulting bodies were called coacervate drops, or simply coacervates. Over time, coacervates absorbed more and more portions of the substance from the solution surrounding them, their structure became more complicated until they turned into very primitive, but already living cells.

5. Stationary state

According to the steady state theory, the Earth never came into being, but existed forever; it has always been capable of sustaining life, and if it has changed, it has changed very little. According to this version, species also never arose, they always existed, and each species has only two possibilities - either a change in numbers or extinction.

The problem of the origin and evolution of life is one of the most interesting and at the same time the least studied issues related to philosophy and religion. Almost throughout almost the entire history of the development of scientific thought, it was believed that life is a self-generating phenomenon.

Main theories:

1) life was created by the Creator at a certain time - creationism (from lat. creation - creation);

2) life arose spontaneously from inanimate matter;

3) life has always existed;

4) life was brought to Earth from space;

5) life arose as a result of biochemical evolution.

According to the theory creationism , the origin of life refers to a specific event in the past that can be calculated. The organisms that inhabit the Earth today are descended from separately created basic types of living beings. The created species were from the very beginning excellently organized and endowed with the capacity for some variability within certain boundaries (microevolution).

Theory of spontaneous origin of life existed in Babylon, Egypt and China as an alternative to creationism. It goes back to Empedocles and Aristotle: certain “particles” of matter contain some kind of “active principle”, which, under certain conditions, can create a living organism. Aristotle believed that the active principle is in a fertilized egg, sunlight, rotting meat. For Democritus, the beginning of life was in silt, for Thales, in water, for Anaxagoras, in air.

With the spread of Christianity, the ideas of spontaneous generation were declared heretical, and for a long time they were not remembered. But Helmont came up with a recipe for getting mice from wheat and dirty laundry. Bacon believed that decay is the germ of a new birth. The ideas of spontaneous generation of life were supported by Copernicus, Galileo, Descartes, Harvey, Hegel, Lamarck, Goethe, Schelling.

L. Pasteur in 1860 finally showed that bacteria can appear in organic solutions only if they were brought there earlier. And to get rid of microorganisms, sterilization is necessary, called pasteurization . Hence, the idea was strengthened that a new organism can only be from a living one.

Supporters theories of the eternal existence of life believe that on the ever-existing Earth, some species were forced to become extinct or dramatically change their numbers in certain places due to changes in external conditions. A clear concept on this path has not been developed, since there are some gaps and ambiguities in the paleontological record of the Earth.

The hypothesis about the appearance of life on Earth as a result of the transfer of certain germs of life from other planets was called panspermia (from Greek. pan- all, everyone and sperma- seed). The panspermia theory offers no mechanism for explaining the origin of life and moves the problem elsewhere in the universe. Having originated in space, life was preserved for a long time in anabiosis almost at T= O K and was brought to Earth by meteorites. At the beginning of the XX century. Arrhenius came up with the idea of radiopanspermia. He described how particles of matter, dust particles and living spores of microorganisms leave the inhabited planets for the world space. They, while maintaining viability, fly in the Universe due to light pressure and, when they land on a planet with suitable conditions, they begin a new life.

In the last century, in the study of the substance of meteorites and comets, many "precursors of the living" were discovered - organic compounds, water, formaldehyde, cyanogens. Modern adherents of the concept of panspermia believe that life on Earth was brought by accident or intentionally by space aliens. The panspermia hypothesis is adjoined by the point of view of astronomers Ch.

Vikramasingha (Sri Lanka) and F. Hoyle (Great Britain). They believe that in outer space, mainly in gas and dust clouds, microorganisms are present in large numbers, where, according to scientists, they are formed. Further, these microorganisms are captured by comets, which then, passing near the planets, "sow the germs of life."

The first scientific theory regarding the origin of living organisms on Earth was created by the Soviet biochemist A.I. Oparin. In 1924, he published works in which he outlined ideas about how life could have arisen on Earth. According to this theory, life arose in the specific conditions of the ancient Earth, and is considered as a natural result of the chemical evolution of carbon compounds in the Universe. According to this theory, the process that led to the emergence of life on Earth can be divided into three stages:

1) The emergence of organic substances.

2) The formation of biopolymers (proteins, nucleic acids, polysaccharides, lipids, etc.) from simpler organic substances.

3) The emergence of primitive self-reproducing organisms.

In ideas about the origin of life as a result of biochemical evolution the evolution of the planet itself plays an important role. The earth has existed for almost 4.5 billion years, and organic life for about 3.5 billion years. The young Earth was a hot planet with a temperature of 5 ... 8 103 K. As it cooled, refractory metals and carbon condensed, forming the earth's crust. The atmosphere of the primitive Earth was very different from the modern one. Light gases - hydrogen, helium, nitrogen, oxygen, argon, etc. - were not yet retained by the insufficiently dense planet, while heavier compounds remained (water, ammonia, carbon dioxide, methane).

When the Earth's temperature dropped below 100ºC, water vapor began to condense, forming the oceans. At this time, abiogenic synthesis took place, that is, in the primary terrestrial oceans saturated with various simple chemical compounds, “in the primary soup”, under the influence of volcanic heat, lightning discharges, intense ultraviolet radiation and other environmental factors, the synthesis of more complex organic compounds began, and then biopolymers. The formation of organic substances was facilitated by the absence of living organisms - organic consumers - and the main oxidizing agent - oxygen. Complex amino acid molecules randomly combined into peptides, which in turn created the original proteins. From these proteins, the primary living creatures of microscopic size were synthesized.

The most difficult problem in the modern theory of evolution is the transformation of complex organic substances into simple living organisms. Oparin believed that the decisive role in the transformation of the inanimate into the living belongs to proteins. Apparently, protein molecules, attracting water molecules, formed colloidal hydrophilic complexes. Further merging of such complexes with each other led to the separation of colloids from the aqueous medium (coacervation). On the border between the coacervate (from lat. coacervus- clot, heap) and the environment lined up lipid molecules - a primitive cell membrane. It is assumed that colloids could exchange molecules with the environment (a prototype of heterotrophic nutrition) and accumulate certain substances.

The first organisms on earth were single-celled - prokaryotes. After several billion years, eukaryotes were formed, and with their appearance there was a choice of a plant or animal way of life, the difference between which lies in the method of nutrition and is associated with the process of photosynthesis. It is accompanied by the entry of oxygen into the atmosphere; the current oxygen content in the atmosphere of 21% was reached 25 million years ago as a result of the intensive development of plants.

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