Chemical evolution of living things. From hydrogen, nitrogen and carbon, in the presence of free energy on Earth, simple molecules should first have arisen: ammonia, methane and similar compounds. And in the future, these simple molecules in the primary ocean could enter into new bonds with each other and with other substances.
Apparently, the processes of growth of molecules proceeded with particular success in the presence of the –N=C=N– group. This group is fraught with great chemical opportunities for growth, both by attaching an oxygen atom to the carbon atom, and by reacting with a nitrogenous base.
From a certain stage of chemical evolution, the participation of oxygen in this process became necessary. In the Earth's atmosphere, oxygen could accumulate as a result of the decomposition of water and water vapor. under the influence of the ultraviolet rays of the sun. It took at least 1–1.2 billion years for the transformation of the reduced atmosphere of the primary Earth into an oxidized one (Fig. 5.1). With the accumulation of oxygen in the atmosphere, the reduced compounds should have been oxidized, namely: NH 3 - to NO 3, CH 4 - to CO 2, H 2 S - to SO 3. In some cases, the oxidation of CH 4 could form methyl alcohol, formaldehyde, formic acid, etc., which, together with rainwater, fell into the primary ocean. These substances, reacting with ammonia and hydrogen cyanide, could give rise to amino acids and compounds such as adenine.
Rice. 5.1. Evolution of the Biosphere and Atmosphere (from Yu. Odum, 1975). The left part of the curve should be extended, apparently, up to 2.5 billion years.
In the course of such and similar reactions, the waters of the primary ocean were saturated with various substances, forming a primary soup.
The possibility of synthesizing amino acids and other low molecular weight organic compounds from inorganic elements and compounds has been experimentally proven. Thus, by passing electric discharges or ultraviolet radiation through a mixture of methane and ammonia gases, in the presence of water vapor, it will be possible to obtain such relatively complex compounds as glycine, alanine, aspartic acid, γ-aminobutyric, succinic and lactic acids and other low molecular weight organic compounds of all four basic classes: amino acids, nucleotides, sugars and fatty acids. The possibility of such a synthesis has been proven in numerous experiments using other ratios of initial gases and types of energy source.
Experiments in this direction turned out to be promising for elucidating the origin of other substances. Adenine, guanine, adenosine, adenosine monophosphate, adenosine diphosphate and adenosine triphosphate have been synthesized. More complex molecules, such as proteins, lipids, nucleic acids, and their derivatives, could be formed from simple molecules by the polymerization reaction.
Without dwelling on other features of the initial stages of chemical evolution, we note that one of its most important steps should be recognized as the combination of the ability for self-reproduction of polynucleotides with the catalytic activity of polypeptides. When life arose, the participation of both polynucleotides and polypeptides was necessary. The properties of each needed to be complemented by the properties of the other. The catalytic abilities of RNA molecules (A.S. Spirin), which probably played an important role in the course of prebiological evolution, were enhanced catalytic functions protein molecules. In addition, the synthesis of proteins themselves by lengthening the peptide chain would not have been very successful without the transfer of stability by storing "information" about it in nucleic acids. In the course of prebiological selection, those complexes had the greatest chances for preservation, in which the ability for metabolism was combined with the ability for self-reproduction.
For this stage of prebiological evolution, a fraction of macromolecules of polynucleotides or polypeptides is distinguished as an elementary object of evolution, and a stable “collective” of macromolecules (connected by processes of synthesis, catalysis, etc.) is distinguished as an elementary evolving unit.
In the further complication of metabolism in such systems, catalysts (various organic and non-organic) should have played a significant role. organic matter) and space-time separation of the initial and final reaction products. Probably, all this could not have arisen before the appearance of membranes. The formation of a membrane structure is considered one of the "difficult" stages of prebiological evolution. Although the possibility of self-assembly of the system was achieved to some extent by combining polynucleotides and polypeptides, however, the true being could not take shape before the appearance of the membrane structure and enzymes.
Rice. 5.2. Possible ways of formation: A - membranes during the formation of coacervates in the primary broth (from M. Calvin, 1971); B - formation of mitochondria; B - formation of eukaryotic cells (according to E. Wolpe, 1981)
Biological membranes, as is known, are aggregates of proteins and lipids capable of separating substances from the environment and imparting strength to the packaging of molecules. The membranes could have arisen either during the formation of coacervates (Fig. 5.2), which are formed in water when two weakly interacting polymers come into contact, or during the adsorption of polymers on the surface of clays (see below).
Chemical evolution.
Chemical evolution: initial stages.
The central parts of the Sun and other stars have almost no real chemical elements and are formed primarily from plasma. Plasma is a completely ionized gas, consisting of randomly moving positively charged (atomic nuclei) and negatively charged (electrons) particles.
The structure of the matter of stars is determined by the degree of ionization (the percentage of matter in the plasma state). In the central part of the Sun, the temperature ranges from 3 to 20 million degrees. At this temperature, the degree of ionization reaches 100%; All substances are in the plasma state. At a depth equal to 0.1 of the radius of the Sun, the temperature drops to 400,000* C, and on the surface of the Sun, the temperature drops to 5500* C. In this case, the degree of ionization decreases to 0.01%, i.e. 99.99% of the substances on the surface of the Sun are in the form of atoms with electron shells.
Spectral analyzes on the surface of the Sun revealed about 60 chemical elements, among which hydrogen and helium predominate. This is due to the fact that other elements with a higher atomic mass and a more complex structure of the atomic nucleus and electron shell cannot exist for a long time at a high temperature. The number of hydrogen atoms in the solar atmosphere is 4-5 times greater than the number of helium atoms; the number of atoms of all other elements is 1000 times less than the number of hydrogen.
In the depths of the Sun and stars, in the plasma, complex nuclei are formed from the simplest ones due to the capture of protons and neutrons. The formation of a helium nucleus from hydrogen occurs in three stages. From a hydrogen nucleus (proton) and a neutron, a heavy hydrogen nucleus (deuterium - D) is formed - a deuteron. When a deuteron combines with another proton, the nucleus of the light isotope of helium, He |, is formed. As a result of the fusion of two nuclei of light helium, a nucleus of ordinary, heavy helium - He2 is formed and two protons are released.
In the course of thermonuclear reactions, the nuclei of new elements are created. When three helium nuclei combine, a carbon isotope nucleus is formed.
As a result of the addition of other helium particles to the carbon nucleus, isotopes of oxygen, neon, magnesium and other elements arise. Thus, the emergence of atoms of chemical elements is the initial stage of inorganic evolution. Hydrogen, carbon, oxygen, nitrogen, phosphorus (the so-called biogenic elements) are widely distributed in space and had a great opportunity to react with each other with the formation of the simplest inorganic compounds - the next stage of inorganic evolution. This was facilitated by the presence of energy in space in the form of electromagnetic radiation and heat emitted by stars. The predominance of hydrogen, oxygen, nitrogen and phosphorus in living systems is not accidental: hydrogen is a good reducing agent, it easily forms hydrogen bonds with oxygen and nitrogen, which have great importance in the formation of biological structures and for life processes. Oxygen has a high oxidizing activity, and phosphorus is characterized by the formation of macroergic bonds, in which energy is stored during chemical reactions.
The third stage of chemical evolution - the formation of the simplest organic compounds - is associated with the specific valency of carbon - the main carrier of organic life, its ability to combine with almost all elements, to form chains and cycles, with its catalytic activity and other properties. The simplest organic molecules are widely distributed in the interstellar medium.
The first stage of chemical evolution on Earth.
Chemical evolution is a set of processes that took place in the Cosmos and in the early stages of the Earth's existence, which led to the emergence of life. At the first stage, the lithosphere, hydrosphere, and atmosphere were formed. The lithosphere arose as a result of volcanism. Every year, volcanoes eject about 1 km to the surface of the Earth. During the existence of the Earth, with the current activity of volcanoes, such an amount of lava was thrown out, which is enough to form the Earth's crust.
The hydrosphere is also created by volcanoes: 3% of the mass of lava is water vapor. The steam has condensed. This led to the appearance of precipitation and the Primary Ocean. The atmosphere was formed during the degassing of lavas. In the beginning, the Earth had a primordial atmosphere. But the mass of the young Earth was not enough to hold the gases, and they evaporated. The Earth has increased its mass due to cosmic dust and meteorites: 107 kg of dust falls on the Earth every year. In addition, the Earth, passing through a dust cloud, could receive 10 "tons of organic material with cosmic dust. The secondary atmosphere also arose due to the degassing of lavas and consisted of CO, CO3, H3, H3O, N, MN3. Oxygen appeared in the atmosphere due to photolysis - the decomposition of water vapor in the upper atmosphere by sunlight. Later, the atmosphere was enriched with oxygen through photosynthesis. Two and a half billion years ago, gold-uranium conglomerates, which form only in the absence of oxygen, disappeared. At the same period, red flowers appear, which form only in the presence of oxygen.
The second stage of chemical evolution on Earth.
At this stage, the formation of low molecular weight organic compounds (amino acids, alcohols, carbohydrates, organic acids) took place. Life on Earth is based on carbon compounds. Why did carbon become the basis of life? First, because carbon forms compounds in the form of large molecular chains. Secondly, carbon compounds interact slowly. Thirdly, carbon forms complex compounds with a special structure that is essential for the course of the most important life processes.
Chemical evolution began long before the appearance of the Earth - it began in the Cosmos. More than 50 organic compounds have been discovered in interstellar space. In space, formaldehyde, carbon monoxide, water, ammonia, hydrogen cyanide are common. These substances, as experiments have shown, can be precursors of amino acids and other organic compounds. Hydrocarbons, aldehydes, ethers, amino acids, nucleotides, and aromatic compounds have been found in extraterrestrial space. A substance containing 18 carbon atoms has been discovered. The synthesis of primitive hydrocarbons, which began in space, continued during the formation of the solar system and the earth.
Assumptions about the processes of the second stage of chemical evolution have experimental confirmation. In 1850, the German chemist A. Stekker carried out the chemical synthesis of amino acids from ammonia, aldehydes, hydrocyanic acid. In 1861, A. M. Butlerov, by heating formaldehyde in a strong alkaline solution, obtained a mixture of sugars. D. I. Mendeleev obtained carbohydrates by exposing carbides to the action of water vapor. In 1953, a student at the University of Chicago, S. L. Miller, for his thesis work, carried out under the guidance of S. Fox, assembled a special apparatus to test the possibility of abiogenetic synthesis of organic compounds. In this hermetic device, a mixture of gases circulated in a closed circuit for a week, which, according to the general opinion, were most likely contained in the early atmosphere of the Earth: CH4, H, NH. Boiling water - a source of water vapor - and a refrigerator kept the gas mixture circulating. Sparks were continuously passed through the device at a voltage of 60,000 volts. After that, the water was subjected to chromatographic and chemical analysis. 6 amino acids were found (glycine, alanine, aspartic and glutamic acids, etc.), urea, lactic, succinic, acetic acids. A total of 11 organic acids were found.
The fact that abiogenetic synthesis of organics is possible is confirmed by the following fact: one volcanic eruption is currently accompanied by the release of up to 15 tons of organic matter. In addition, the Earth, passing through a dust cloud, could receive 108 tons of organic material with cosmic dust. All this, presumably, could create the "broth" about which A. Oparin and J. Haldane wrote.
The initial stages of biological evolution.
The formation of primary cellular organisms marked the beginning of biological evolution. It is believed that the selection of coacervates and the boundary stage of chemical and biological evolution lasted about 750 Ma. At the end of this period, the first primitive nuclear-free cells appeared - prokaryotes. The first living organisms - heterotrophs - used organic compounds dissolved in the waters of the primary ocean as a source of energy (food). Since there was no free oxygen in the Earth's atmosphere, heterotrophs had an anaerobic (oxygen-free) type of metabolism, the efficiency of which is low. The increase in the number of heterotrophs led to the depletion of the waters of the primary ocean, where there were less and less ready-made organic substances that could be used for food.In a more advantageous position were organisms that have developed the ability to use the energy of solar radiation for the synthesis of organic substances from inorganic - photosynthesis. Thus, a fundamentally new power source appeared. For example, modern photosynthetic purple bacteria, due to solar radiation, oxidize hydrogen sulfide to sulfates. The hydrogen released as a result of the oxidation reaction is used to reduce carbon dioxide to carbohydrates with the formation of water. The use of organic compounds as a source (donor) of hydrogen led to the emergence of autotrophic organisms (capable of synthesizing all organic substances necessary for life from inorganic substances).
The next step in evolution is associated with the development in photosynthetic organisms of the ability to use water as a source of hydrogen for the synthesis of organic molecules. The assimilation of carbon dioxide by such organisms was accompanied by the release of oxygen and the incorporation of carbon into organic compounds. So oxygen began to accumulate in the Earth's atmosphere. The first photosynthetic organisms that release oxygen into the atmosphere were cyanobacteria (cyanoea).
The transition from the primary atmosphere to an environment containing oxygen is a major event both in the evolution of living beings and in the transformation of minerals. Firstly, oxygen released into the atmosphere in its upper layers under the influence of powerful ultraviolet radiation from the Sun turns into active ozone (O3), which is able to absorb most of the hard short-wave ultraviolet rays that destroy complex organic compounds. Secondly, in the presence of free oxygen, an oxygen type of metabolism is possible, which is energetically more favorable. The formation of free oxygen has given rise to numerous new forms of aerobic living organisms and their wider use of resources. environment.
As a result of mutually beneficial symbiosis of various prokaryotic (not possessing a formed cell nucleus) cells, nuclear, or eukaryotic, organisms (eukaryotes) arose. The basis of the symbiosis was probably a heterotrophic amoeba-like cell. It was fed by smaller cells and, in particular, oxygen-breathing aerobic bacteria that can also function inside the host cell, producing energy. Those large amoeboid cells, in the body of which aerobic bacteria remained unharmed, turned out to be in a more advantageous position than cells that received energy by anaerobic means - fermentation. Later, symbiont bacteria turned into mitochondria (cell organelles where reactions take place that provide cells with energy). When the second group of symbionts, flagella-like bacteria similar to modern spirochetes, attached to the surface of the host cell, the mobility and ability to find food of such an organism increased dramatically. This is how primitive animal cells arose - the predecessors of the current flagellated protozoa.
The resulting mobile eukaryotes, by symbiosis with photosynthetic (possibly cyanobacteria) organisms, gave an algae, or a plant, and the structure of the pigment complex in photosynthetic anaerobic bacteria is similar to the pigments of green plants. This similarity indicates the possibility of evolutionary transformation of the photosynthetic apparatus of anaerobic bacteria into a similar apparatus of green plants.
The stated hypothesis about the emergence of eukaryotic cells through a series of successive symbioses was accepted by many modern scientists, since it is well substantiated. Firstly, unicellular algae even now easily enter into an alliance with animals - eukaryotes; for example, the alga chlorella lives in the body of the shoe ciliates. Secondly, some cell organelles - mitochondria and plastids - are very similar in DNA structure to prokaryotic bacteria and cyanobacteria.
The ability of eukaryotes to use the environment is significantly higher than that of prokaryotes, since they have a diploid (double) set of genes. In prokaryotes, any mutation immediately manifests itself in the form of a trait. If the mutation is beneficial, the organism continues to exist; if it is harmful, it dies; prokaryotes continuously adapt to environmental changes, but are deprived of the opportunity to form major structural changes. The appearance of a double set of genes in eukaryotes made possible the accumulation of non-manifesting phenotypic mutations and, consequently, the formation of a reserve of hereditary variability - the basis of evolutionary transformations.
The possibilities of unicellular organisms in the development of the habitat were limited, since the respiration and nutrition of the protozoa are carried out through the surface of the body. With an increase in the size of a cell of a unicellular organism, its surface increases according to a quadratic law, and its volume increases according to a cubic law, so the biological membrane surrounding the cell could not provide oxygen to an organism that was too large. A different evolutionary path was realized later, about 2.6 billion years ago, when multicellular organisms appeared, the evolutionary possibilities of which are much wider.
The first hypothesis about the origin of multicellular organisms belongs to E. Haeckel (second half of the 19th century). When constructing it, he proceeded from studies of the embryonic development of the lancelet (a genus of animals of the class of non-cranial animals) conducted by A.O. Kovalevsky and other zoologists. Haeckel believed that the initial stage of development of the embryo (the zygote stage) corresponds to unicellular ancestors, and the stage of development of the embryo of multicellular animals in the process of blastulation (the final phase of the egg crushing period) corresponds to a spherical flagellate colony. Later, according to this hypothesis, an invagination (invagination) of one of the sides of the spherical colony occurred and a hypothetical two-layer organism was formed, which Haeckel called gastraea. Haeckel's theory played an important role in the history of science, contributing to the establishment of monophyletic (i.e., from one root) ideas about the origin of multicellular organisms.
The basis of modern ideas about the emergence of multicellular organisms is the hypothesis of phagocytella I.I. Mechnikov. According to his ideas, multicellular organisms descended from colonial protozoa - flagellates. An example of such an organization is the now existing colonial flagellates of the Volvox type. Among the cells of the colony, moving ones are distinguished, equipped with flagella, phagocytic prey and carrying it inside the colony, and sexual, whose function is reproduction. So the colony turned into a primitive, but integral multicellular organism. The validity of the phagocytella hypothesis is evidenced by the structure of a primitive multicellular organism - trichoplax, which in structure corresponds to a hypothetical phagocytella and therefore should be distinguished into a special type of animal - phagocytella-like, filling the gap between multicellular and unicellular organisms.
Thus, at present, most researchers in the field of natural science recognize that the emergence of life on Earth is associated with a long process of chemical evolution. The formation of a structure that delimits the organism from the environment - a membrane with its inherent properties contributed to the emergence of living organisms and marked the beginning of biological evolution. Both the simplest living organisms that arose about 3 billion years ago, and those with a more complex structure, have a cell at the core of their structural organization.
The main directions of biological evolution.
In the Proterozoic era, many algae lived in the seas. Initial links
etc.................
Chemical evolution
Also, these terms denote the theory of the emergence and development of those molecules that are of fundamental importance for the emergence and development of living matter.
Everything that is known about the chemistry of matter makes it possible to limit the problem of chemical evolution to the framework of the so-called "water-carbon chauvinism", postulating that life in our Universe is presented in the only possible way: as a "mode of existence of protein bodies", feasible due to a unique combination of polymerization the properties of carbon and the depolarizing properties of the liquid-phase aqueous medium, as both necessary and/or sufficient (?) conditions for the emergence and development of all life forms known to us. This implies that, at least within one formed biosphere, there can be only one code of heredity common to all living beings of a given biota, but the question remains open whether there are other biospheres outside the Earth and whether other variants of the genetic apparatus are possible.
It is also unknown when and where chemical evolution began. Any terms are possible after the end of the second cycle of star formation, which occurred after the condensation of products of explosions of primary supernovae, supplying heavy elements (with an atomic mass of more than 26) into interstellar space. The second generation of stars, already with planetary systems enriched in heavy elements, which are necessary for the implementation of chemical evolution, appeared 0.5-1.2 billion years after big bang. Under certain quite probable conditions, almost any environment can be suitable for launching chemical evolution: the depths of the oceans, the bowels of planets, their surfaces, protoplanetary formations and even clouds of interstellar gas, which is confirmed by the widespread detection in space by astrophysics methods of many types of organic substances - aldehydes, alcohols, sugars, and even the amino acid glycine, which together can serve as the starting material for chemical evolution, which has as its end result the emergence of life.
Methodology for the study of chemical evolution (theory)
The study of chemical evolution is complicated by the fact that at present knowledge about the geochemical conditions of the ancient Earth is not sufficiently complete.
Therefore, in addition to geological data, astronomical data are also involved. Thus, the conditions on Venus and Mars are considered as close to those on Earth at various stages her evolution.
The main data on chemical evolution were obtained as a result of model experiments, during which it was possible to obtain complex organic molecules by simulating various chemical compositions atmosphere, hydrosphere and lithosphere and climatic conditions.
Based on the available data, a number of hypotheses have been put forward about the specific mechanisms and direct driving forces of chemical evolution.
Abiogenesis
Abiogenesis - the formation of organic compounds common in wildlife, outside the body without the participation of enzymes.
In a broad sense, abiogenesis is the emergence of living things from non-living things, that is, the initial hypothesis of the modern theory of the origin of life
There is also a theory of hypercycles; according to which the first manifestations of life were, respectively, in the form of hypercycles - a complex of complex catalytic reactions, the output products of which are catalysts for subsequent reactions.
In 2008, American biologists took an important step towards understanding the initial stages of the origin of life. They managed to create a "protocell" with a shell of simple lipids and fatty acids, capable of drawing in activated nucleotides from the environment - the "building blocks" necessary for DNA synthesis. In 2011, Japanese scientists reported that they had succeeded in creating a synthetic cell with a shell and DNA elements inside, capable of reproducing when the "primordial broth" was heated to 94 degrees Celsius.
Evolution
Biological evolution is a natural process of development of living nature, accompanied by a change in the genetic composition of populations, the formation of adaptations, speciation and extinction of species, the transformation of ecosystems and the biosphere as a whole.
There are several evolutionary theories that explain the mechanisms underlying evolutionary processes. At the moment, the synthetic theory of evolution (STE), which is a development of Darwin's theory, is generally accepted. STE makes it possible to explain the relationship between the substrate of evolution (genes) and the mechanism of evolution (natural selection). As part of STE evolution is a process of changing hereditary traits in populations of organisms over a period of time exceeding the life span of one generation.
Charles Darwin was the first to formulate the theory of evolution by natural selection. Evolution by natural selection is a process that follows from three facts about populations: 1) more offspring are born than can survive; 2) in different organisms different traits, which leads to differences in survival and the likelihood of having offspring; 3) these traits are inherited. Thus, in the next generation, the number of such individuals will increase, the features of which contribute to survival and reproduction in this environment. Natural selection is the only known cause of adaptations, but not the only cause of evolution. Non-adaptive causes include genetic drift, gene flow, and mutations.
Despite the ambiguous perception in society, the fact of evolution is one of the most proven in biology. Discoveries in evolutionary biology have had a huge impact not only on traditional areas of biology, but also on other academic disciplines such as anthropology and psychology.
Introduction
Evolution occurs over a period of time exceeding the lifetime of one generation and consists in changing the inherited traits of an organism. The first step in this process is to change the allele frequencies of genes in a population. In an ideal population, in which there is no environmental influence, drift and gene flow, according to the Hardy-Weinberg law, the allele frequency will be unchanged from generation to generation. Mutations increase variability in a population due to the emergence of new allelic variants of genes - mutational variability. In addition to mutational, there is also combinative variability due to recombination, but it does not lead to changes in allele frequencies, but to their new combinations. Another factor leading to changes in allele frequencies is gene flow.
Two other evolutionary factors, natural selection and genetic drift, "sort out" the variability created by mutations and gene flow, leading to the establishment of a new allele frequency in the population. Genetic drift is a probabilistic process of changing gene frequencies and is most pronounced in relatively small populations. Drift can lead to the complete disappearance of certain alleles from the population. Natural selection is the main creative factor in evolution. Under its influence, individuals with a certain phenotype (and a certain set of hereditary traits) will be more successful than others, that is, they will have a higher probability of surviving and leaving offspring. Thus, the proportion of such organisms in the population that have hereditary traits with a selective advantage will increase. The mutual influence of drift and natural selection is difficult to unambiguously assess, but in general it probably depends on the size of the population and the intensity of selection. In addition to the above factors, horizontal gene transfer can also be important, which can lead to the appearance of completely new genes for a given organism.
Natural selection increases the fitness of organisms, leading to the formation of adaptations. Evolutionary processes proceeding for a long time can lead both to the formation of new species and their further divergence, and to the extinction of entire species.
Heredity
Heredity is the property of organisms to repeat in a number of generations similar types of metabolism and individual development in general. The evolution of organisms occurs through changes in the hereditary characteristics of the organism. An example of a hereditary trait in a person is the brown color of the eyes, inherited from one of the parents. Hereditary traits are controlled by genes. The totality of all the genes of an organism forms its genotype.
Heritability can also occur on a larger scale. For example, ecological inheritance through niche construction. Thus, the descendants inherit not only genes, but also the ecological features of the habitat created by the activity from the ancestors. Other examples of inheritance not under the control of genes are the inheritance of cultural traits and symbiogenesis.
Variability
The phenotype of an organism is determined by its genotype and environmental influences. A significant part of the phenotype variations in populations is caused by differences in their genotypes. In STE, evolution is defined as the change over time in the genetic structure of populations. The frequency of one of the alleles changes, becoming more or less common among other forms of this gene. Operating Forces evolution leads to changes in the allele frequency in one direction or the other. The change disappears when the new allele reaches the point of fixation - completely replaces the ancestral allele or disappears from the population.
Variation is made up of mutations, gene flow, and recombination of genetic material. Variation is also increased by gene exchanges between different types such as horizontal gene transfer in bacteria, hybridization in plants. Despite the constant increase in variability due to these processes, most of the genome is identical in all representatives of this species. However, even relatively small changes in the genotype can cause huge differences in the phenotype, for example, the genomes of chimpanzees and humans differ by only 5%.
Mutations
Random mutations constantly occur in the genomes of all organisms. These mutations create genetic variation. Mutations are changes in the DNA sequence. They are caused by radiation, viruses, transposons, mutagens, and errors that occur during DNA replication or meiosis. Mutations may have no effect, may change the gene product, or interfere with its function. Studies done on Drosophila have shown that if a mutation changes a protein produced by a gene, then in about 70% of cases this will have harmful effects, and in other cases, neutral or weakly positive effects. To reduce the negative effect of mutations in cells, there are DNA repair mechanisms. The optimal mutation rate is a balance between high level harmful mutations and the cost of maintaining the repair system. RNA viruses have a high level of mutability, which seems to be an advantage in helping to avoid defensive responses of the immune system.
Mutations can involve large sections of chromosomes. For example, with duplication, which causes the appearance of additional copies of a gene in the genome. These copies become the basic material for the emergence of new genes. This is an important process because new genes develop within a gene family from a common ancestor.
Recombination
In asexual organisms, genes during reproduction cannot mix with the genes of other individuals. In contrast, in sexually reproducing organisms, offspring receive random mixtures of chromosomes from their parents. This is due to the process of homologous recombination, during which there is an exchange of sections of two homologous chromosomes. During recombination, there is no change in the frequency of alleles, but the formation of their new combinations occurs. Thus, sexual reproduction usually increases hereditary variability and can accelerate the rate of evolution of the organism. However, asexual reproduction is often advantageous and may develop in animals with sexual reproduction. This may allow the two sets of alleles in the genome to diverge to acquire new functions.
Recombination allows even alleles that are close to each other in DNA to be inherited independently. However, the level of recombination is low - about two recombinations per chromosome per generation.
gene flow
Gene flow is the transfer of alleles of genes between populations. The flow of genes can be carried out by the migration of individuals between populations in the case of mobile organisms, or, for example, by the transfer of pollen or seeds in the case of plants. The rate of gene flow is highly dependent on the mobility of organisms.
The extent to which gene flow influences variability in populations is not entirely clear. There are two points of view, one of them is that gene flow can have a significant impact on large population systems, homogenizing them and, accordingly, acting against the processes of speciation; second, that the rate of gene flow is only sufficient to affect local populations.
Mechanisms of evolution
Natural selection
Evolution by natural selection is the process by which mutations are fixed that increase the fitness of organisms. Natural selection is often referred to as a "self-evident" mechanism because it follows from facts such as:
- Hereditary changes exist in populations of organisms;
- Organisms produce more offspring than can survive;
- These offspring differ in that they have different survival rates and ability to reproduce.
Such conditions create competition between organisms for survival and reproduction. Thus, organisms with inherited traits that give them a competitive advantage are more likely to pass them on to their offspring than organisms with inherited traits that do not.
The central concept of the concept of natural selection is the fitness of organisms. Fitness is defined as the ability of an organism to survive and reproduce, which determines the size of its genetic contribution to the next generation. However, the main thing in determining fitness is not the total number of offspring, but the number of offspring with a given genotype (relative fitness). Natural selection for traits that can vary over a range of values (such as the size of an organism) can be divided into three types:
Description of work
Chemical evolution or prebiotic evolution is the first stage in the evolution of life, during which organic, prebiotic substances arose from inorganic molecules under the influence of external energy and selection factors and due to the deployment of self-organization processes inherent in all relatively complex systems, which are undoubtedly all carbon-containing molecules. .
According to most scientists (primarily astronomers and geologists), the Earth was formed as a celestial body about 5 billion years ago. by condensation of particles of a gas and dust cloud rotating around the Sun.
Under the influence of compressive forces, the particles from which the Earth is formed release a huge amount of heat. Thermonuclear reactions begin in the bowels of the Earth. As a result, the Earth gets very hot. Thus, 5 billion years ago The Earth was a hot ball rushing through outer space, the surface temperature of which reached 4000-8000°C (Fig. 2.4.1.1).
Gradually, due to the radiation of thermal energy into outer space, the Earth begins to cool. About 4 billion years ago The earth cools so much that a hard crust forms on its surface; at the same time, light, gaseous substances escape from its bowels, rising up and forming the primary atmosphere. The composition of the primary atmosphere was significantly different from the modern one. Apparently, there was no free oxygen in the atmosphere of the ancient Earth, and its composition included substances in a reduced state, such as hydrogen (H 2), methane (CH 4), ammonia (NH 3), water vapor (H 2 O ), and possibly also nitrogen (N 2), carbon monoxide and carbon dioxide (CO and CO 2).
The reducing nature of the Earth's primary atmosphere is extremely important for the origin of life, since substances in a reduced state are highly reactive and, under certain conditions, are able to interact with each other, forming organic molecules. The absence of free oxygen in the atmosphere of the primary Earth (practically all of the Earth's oxygen was bound in the form of oxides) is also an important prerequisite for the emergence of life, since oxygen easily oxidizes and thereby destroys organic compounds. Therefore, in the presence of free oxygen in the atmosphere, the accumulation of a significant amount of organic matter on the ancient Earth would have been impossible.
About 5 billion years ago- the origin of the earth celestial body; surface temperature - 4000-8000°С
About 4 billion years ago - formation of the earth's crust and primary atmosphere
At 1000°C- synthesis of simple organic molecules begins in the primary atmosphere
The energy for synthesis is given by:
The temperature of the primary atmosphere is below 100 ° C - the formation of the primary ocean -
Synthesis of complex organic molecules - biopolymers from simple organic molecules:
simple organic molecules - monomers
complex organic molecules - biopolymers
Rice. 2.1. Main stages of chemical evolution
When the temperature of the primary atmosphere reaches 1000°C, the synthesis of simple organic molecules begins in it, such as amino acids, nucleotides, fatty acids, simple sugars, polyhydric alcohols, organic acids, etc. The energy for synthesis is supplied by lightning discharges, volcanic activity, hard space radiation and, finally, the ultraviolet radiation of the Sun, from which the Earth is not yet protected by the ozone screen, and it is ultraviolet radiation that scientists consider the main source of energy for abiogenic (that is, passing without the participation of living organisms) synthesis of organic substances.
The recognition and wide dissemination of the theory of A.I. Oparin was greatly facilitated by the fact that the processes of abiogenic synthesis of organic molecules are easily reproduced in model experiments.
The possibility of synthesizing organic substances from inorganic substances has been known since the beginning of the 19th century. Already in 1828, the outstanding German chemist F. Wöhler synthesized an organic substance - urea from inorganic - ammonium cyanate. However, the possibility of abiogenic synthesis of organic substances under conditions close to those of the ancient Earth was first shown in the experiment of S. Miller.
In 1953, a young American researcher, a graduate student at the University of Chicago, Stanley Miller, reproduced in a glass flask with electrodes soldered into it the primary atmosphere of the Earth, which, according to scientists of that time, consisted of hydrogen, methane CH 4, ammonia NH, and water vapor H 2 0 (Fig. 2.4.1.2). Through this gas mixture, S. Miller passed electric discharges simulating thunderstorms for a week. At the end of the experiment, α-amino acids (glycine, alanine, asparagine, glutamine), organic acids (succinic, lactic, acetic, glycocolic), γ-hydroxybutyric acid and urea were found in the flask. When repeating the experiment, S. Miller managed to obtain individual nucleotides and short polynucleotide chains of five to six links.
Rice. 2.2. Installation by S. Miller
In further experiments on abiogenic synthesis conducted by various researchers, not only electrical discharges were used, but also other types of energy characteristic of the ancient Earth - cosmic, ultraviolet and radioactive radiation, high temperatures inherent in volcanic activity, as well as various options for gas mixtures, imitating the original atmosphere. As a result, almost the entire spectrum of organic molecules characteristic of living things was obtained: amino acids, nucleotides, fat-like substances, simple sugars, organic acids.
Moreover, abiogenic synthesis of organic molecules can also occur on Earth at the present time (for example, in the course of volcanic activity). At the same time, not only hydrocyanic acid HCN, which is a precursor of amino acids and nucleotides, but also individual amino acids, nucleotides, and even such complex organic substances as porphyrins can be found in volcanic emissions. Abiogenic synthesis of organic substances is possible not only on Earth, but also in outer space. The simplest amino acids are found in meteorites and comets.
When the temperature of the primary atmosphere dropped below 100 ° C, hot rains fell on the Earth and the primary ocean appeared. With streams of rain, abiogenically synthesized organic substances entered the primary ocean, which turned it, but in the figurative expression of the English biochemist John Haldane, into a dilute "primary soup". Apparently, it is in the primary ocean that the processes of formation from simple organic molecules - monomers of complex organic molecules - biopolymers begin (see Fig. 2.4.1.1).
However, the processes of polymerization of individual nucleoside, amino acids and sugars are condensation reactions, they proceed with the elimination of water, therefore, the aqueous medium does not contribute to polymerization, but, on the contrary, to the hydrolysis of biopolymers (i.e., their destruction with the addition of water).
The formation of biopolymers (in particular, proteins from amino acids) could take place in the atmosphere at a temperature of about 180°C, from where they were washed into the primary ocean with atmospheric precipitation. In addition, it is possible that on the ancient Earth, amino acids were concentrated in drying up reservoirs and polymerized in a dry form under the influence of ultraviolet light and the heat of lava flows.
Despite the fact that water promotes the hydrolysis of biopolymers, the synthesis of biopolymers in a living cell occurs precisely in an aqueous medium. This process is catalyzed by special catalytic proteins - enzymes, and the energy necessary for synthesis is released during the breakdown of adenosine triphosphoric acid - ATP. It is possible that the synthesis of biopolymers in the aquatic environment of the primary ocean was catalyzed by the surface of certain minerals. It has been experimentally shown that a solution of the amino acid alanine can polymerize in an aqueous medium in the presence of a special type of alumina. In this case, the peptide polyalanine is formed. The polymerization reaction of alanine is accompanied by the breakdown of ATP.
The polymerization of nucleotides is easier than the polymerization of amino acids. It has been shown that in solutions with a high salt concentration, individual nucleotides spontaneously polymerize, turning into nucleic acids.
The life of all modern living beings is a process of continuous interaction of the most important biopolymers of a living cell - proteins and nucleic acids.
Proteins are the “working molecules”, “engineer molecules” of a living cell. Characterizing their role in metabolism, biochemists often use such figurative expressions how "the protein works", "the enzyme leads the reaction". The most important function of proteins is catalytic. As you know, catalysts are substances that speed up chemical reactions, but they themselves are not included in the final products of the reaction. Tanks-catalysts are called enzymes. Enzymes in bend and thousands of times accelerate metabolic reactions. Metabolism, and hence life without them, is impossible.
Nucleic acids- these are "molecules-computers", molecules - keepers of hereditary information. Nucleic acids do not store information about all the substances of a living cell, but only about proteins. It is enough to reproduce in the daughter cell the proteins characteristic of the mother cell so that they accurately recreate all the chemical and structural features of the mother cell, as well as the nature and rate of metabolism inherent in it. Nucleic acids themselves are also reproduced due to the catalytic activity of proteins.
Thus, the mystery of the origin of life is the mystery of the emergence of the mechanism of interaction between proteins and nucleic acids. What information does modern science have about this process? What molecules were the primary basis of life - proteins or nucleic acids?
Scientists believe that despite the key role of proteins in the metabolism of modern living organisms, the first "living" molecules were not proteins, but nucleic acids, namely ribonucleic acids (RNA).
In 1982, American biochemist Thomas Check discovered the autocatalytic properties of RNA. He experimentally showed that in a medium containing a high concentration of mineral salts, ribonucleotides spontaneously (spontaneously) polymerize, forming polynucleotides - RNA molecules. On the initial polynucleotide chains of RNA, as on a template, RNA copies are formed by pairing of complementary nitrogenous bases. The RNA template copying reaction is catalyzed by the original RNA molecule and does not require the participation of enzymes or other proteins.
What happened next is fairly well explained by what might be called "natural selection" at the molecular level. During self-copying (self-assembly) of RNA molecules, inaccuracies and errors inevitably arise. The erroneous RNA copies are copied again. When copying again, errors may occur again. As a result, the population of RNA molecules in a certain part of the primary ocean will be heterogeneous.
Since RNA decay processes are also taking place in parallel with the synthesis processes, molecules with either greater stability or better autocatalytic properties will accumulate in the reaction medium (i.e., molecules that copy themselves faster, “multiply” faster).
On some RNA molecules, as on a matrix, self-assembly of small protein fragments - peptides can occur. A protein "sheath" is formed around the RNA molecule.
Along with autocatalytic functions, Thomas Check discovered the phenomenon of self-splicing in RNA molecules. As a result of self-splicing, RNA regions that are not protected by peptides are spontaneously removed from RNA (they are, as it were, “cut out” and “ejected”), and the remaining RNA regions encoding protein fragments “grow together”, i.e. spontaneously combine into a single molecule. This new RNA molecule will already code for a large complex protein (Figure 2.4.1.3).
Apparently, initially protein sheaths performed primarily a protective function, protecting RNA from destruction and thereby increasing its stability in solution (this is the function of protein sheaths in the simplest modern viruses).
Obviously, at a certain stage of biochemical evolution, RNA molecules, which encode not only protective proteins, but also catalytic proteins (enzymes) that sharply accelerate the rate of RNA copying, gained an advantage. Apparently, this is how the process of interaction between proteins and nucleic acids, which we now call life, arose.
In the process of further development, due to the appearance of a protein with the functions of an enzyme - reverse transcriptase, on single-stranded RNA molecules, molecules of deoxyribonucleic acid (DNA) consisting of two strands began to be synthesized. The absence of an OH group in the 2" position of deoxyribose makes DNA molecules more stable with respect to hydrolytic cleavage in slightly alkaline solutions, namely, the reaction of the medium in primary reservoirs was slightly alkaline (this reaction of the medium was also preserved in the cytoplasm of modern cells).
Where did the development of a complex process of interaction between proteins and nucleic acids take place? According to the theory of A.I. Oparin, the so-called coacervate drops became the birthplace of life.
Rice. 2.3. Hypothesis of the occurrence of the interaction of proteins and nucleic acids:
a) in the process of self-copying of RNA, errors accumulate (1 - nucleotides corresponding to the original RNA; 2 - nucleotides that do not correspond to the original RNA - errors in copying); b) on a part of the RNA molecule due to its physical and chemical properties Amino acids “stick” (3 - RNA molecule; 4 - amino acids), which, interacting with each other, turn into short protein molecules - peptides.
As a result of self-splicing inherent in RNA molecules, the parts of the RNA molecule that are not protected by peptides are destroyed, and the remaining ones "grow" into a single molecule encoding a large protein.
The result is an RNA molecule covered with a protein sheath (the most primitive modern viruses, for example, the tobacco mosaic virus, have a similar structure)
The phenomenon of coacervation consists in the fact that under certain conditions (for example, in the presence of electrolytes), macromolecular substances are separated from the solution, but not in the form of a precipitate, but in the form of a more concentrated solution - coacervate. When shaken, the coacervate breaks up into separate small droplets. In water, such drops are covered with a hydration shell that stabilizes them (a shell of water molecules) - fig. 2.4.1.4.
Coacervate drops have some semblance of metabolism: under the influence of purely physical and chemical forces, they can selectively absorb certain substances from the solution and release their decay products into the environment. Due to the selective concentration of substances from the environment, they can grow, but when they reach a certain size, they begin to "multiply", budding small droplets, which, in turn, can grow and "bud".
The coacervate droplets resulting from the concentration of protein solutions in the process of mixing under the action of waves and wind can be covered with a lipid shell: a single membrane resembling soap micelles (with a single detachment of a drop from the surface of water covered with a lipid layer), or a double membrane resembling a cell membrane ( when a drop covered with a single-layer lipid membrane falls again onto the lipid film covering the surface of the reservoir - Fig. 2.4.1.4).
The processes of the emergence of coacervate droplets, their growth and "budding", as well as "clothing" them with a membrane from a double lipid layer are easily modeled in the laboratory.
For coacervate droplets, there is also a process of "natural selection" in which the most stable droplets remain in solution.
Despite the outward resemblance of coacervate drops to living cells, coacervate drops lack the main sign of a living thing - the ability for accurate self-reproduction, self-copying. Obviously, the precursors of living cells were such coacervate drops, which included complexes of replicator molecules (RNA or DNA) and the proteins they encode. Possibly RNA-protein complexes long time existed outside coacervate droplets in the form of the so-called “free-living gene”, and it is possible that their formation took place directly inside some coacervate droplets.
Figure 2.4. Possible way of transition from coacervate drops to primitive flares:
a) the formation of a coat; 6) stabilization of coacervate drops in an aqueous solution; c) - formation of a double lipid layer around the drop, similar to a cell membrane: 1 - coacervate drop; 2 - monomolecular layer of lipid on the surface of the reservoir; 3 - formation of a single lipid layer around the drop; 4 - formation of a double lipid layer around the drop, similar to a cell membrane; d) - a coacervate drop surrounded by a double lipid layer, with a protein-nucleotide complex included in its composition - a prototype of the first living cell
Extremely complex, not fully understood modern science From a historical point of view, the process of the emergence of life on Earth was extremely fast. For 3.5 billion years, the so-called. chemical evolution ended with the appearance of the first living cells and began biological evolution.
2014-05-31Abiogenesis and spontaneous generation. The ancient sages were the first to express their thoughts about how life appeared on Earth. Even then, they assumed that living organisms arose from inorganic matter. In ancient times, the idea of spontaneous generation (spontaneous generation) of living beings from non-living materials was taken for granted. In the Middle Ages, ideas about the origin of life took the form of religious dogma. One of its postulates was the idea of the emergence of living beings from the earth in the process of decay under the influence of a life-giving spirit.
In the Renaissance, the legend of the homunculus actively spread - a tiny little man that can be created from clay, soil or other inanimate matter using magic spells and rituals.
The fallacy of the idea of spontaneous generation of life was documented by the Italian physician Francesco Redi (1626-1698). He conducted a series of experiments that showed that blowflies, contrary to what was then believed, develop from eggs laid by females, and do not originate on their own in rotting meat. So, Redi took two pieces of meat, laid them out in two clay pots, one of which was covered with smoke. After some time, larvae developed in the open pot, and there were no signs of larvae or flies in the closed pot. Therefore, the scientist concluded: flies sit on rotting meat and lay larvae in it, as a result of which new flies are born.
However, in most biologists until the XIX century. there was no doubt that all unicellular organisms possess the property of spontaneous generation. This idea was debunked only in 1865 by the outstanding microbiologist Louis Pasteur (1822-1895). By that time, it was already known that after prolonged boiling in a stoppered flask of any medium, it remains sterile as long as the flask remains uncorked. However, supporters of the idea of spontaneous generation were not convinced by this experience. They believed that clean, not heated air is necessary for spontaneous generation. Therefore, on the order of Pasteur, a flask with a neck curved in the form of a swan's neck was specially made (Fig. 197). The nutrient broth boiled in such a flask did not grow bacteria in the same way as in a flask closed with a stopper. Pasteur explained this by the fact that microorganisms that penetrate into such a flask along with air settle on the bends of the neck. He confirmed his words by shaking the flask so that the broth rinsed the walls of the neck. It was after this that after a while bacteria appeared in the broth. Thus, L. Pasteur proved that in an environment devoid of microorganisms, their generation is impossible even under ideal conditions.
Now the refuge of the idea of spontaneous generation of organisms remains creationism - a religious and philosophical concept, the diversity of living nature, humanity, the Earth and the Universe are considered as an act of divine creation.
The denial of the idea of the possibility of spontaneous origin of organisms in modern conditions does not contradict scientific ideas that life on Earth arose from inorganic matter billions of years ago as a result of chemical or, as it is also called, prebiological evolution. The idea of the prebiological development of nature, which led to the formation of life, was called abiogenesis (from Greek A - not, to bios and genesis). It is now believed that the evolution of life on our planet consists of two stages: abiogenesis and biogenesis - the actual biological evolution, when living organisms come only from living organisms.
Chemical evolution. The material essence of the bodies of living organisms is quite simple. They are built from polymeric organic compounds, which are based on compounds of carbon atoms. The process of vital activity is nothing but a set of ordered, flowing from each other, chemical reactions. Having mentally decomposed the cell into separate structures and macromolecules from which it is built, and the metabolism of the body, first into biochemical cycles, and then into separate reactions, it is easy to imagine the logic of the gradual complication of the structure of chemical compounds and reactions that could occur billions of years ago. Under laboratory conditions that mimic the conditions of the primitive Earth, it is possible to first synthesize the simplest biogenic compounds, then obtain biopolymers from them, have catalytic activity, and then structures resembling a cell membrane. Thus, it is possible to prove the fundamental possibility of chemical evolution - the progressive process of the emergence of new chemical compounds, more complex and highly organized compared to the original substances, took place on Earth before the emergence of life.
The main provisions of the concept of chemical evolution are as follows.
Life on Earth arose naturally from inorganic substances with the expenditure of energy that came from outside.
The emergence of life is the process of the emergence of ever new chemical compounds and chemical reactions.
Chemical evolution is a process that has been going on for billions of years under very specific conditions under the influence of powerful external sources of energy.
An important role in chemical evolution was played by prebiological selection, which contributed to the emergence, first of all, of complex compounds in which the ability to exchange substances was combined with the ability to reproduce itself.
The key factor in the process of chemical evolution was the self-organization factor inherent in all complex systems, which include organic molecules.
Is it possible in modern conditions on Earth to find the limiting state between the inanimate and the living? It turns out you can. These are the same viruses that exhibit the properties of both living and non-living, although, according to most scientists, they have nothing to do with chemical evolution and the origin of life. More interesting find a completely new boundary state between living and non-living - the so-called nanobacteria. These are very small spherical substances that do not exceed viruses in size. they can only be seen in electron microscope. Most scientists consider them biominerals. Nanobacteria are able to reproduce themselves in the presence of certain vitamins. their reproduction in this case occurs by self-copying. Nanobacteria contain neither DNA, nor RNA, nor any proteins. The chemical processes in these substances occur differently than in prokaryotes, and their growth rate is thousands of times slower than in bacteria.
Modern ideas about the main stages of abiogenesis. The formation of organic compounds common in living nature outside the body goes through a number of stages.
1. Synthesis of organic monomers: organic acids, amino acids, carbohydrates, nitrogenous bases. For this, the primitive Earth had all the conditions: the amount of water, methane, ammonia and cyanides, the absence of oxygen and other oxygen oxidation (the atmosphere had a reducing character), as well as an excess of free energy in the form of ultraviolet radiation, electrical discharges and volcanic activity.
The possibility of synthesizing amino acids and other low molecular weight organic compounds from chemical elements and inorganic compounds has been experimentally proven. For this, the components of the atmosphere of the then Earth ( carbon dioxide, methane and ammonia, water vapor) were placed in a closed flask and electric discharges passed through this mixture (Fig. 198). As a result, it was possible to synthesize a number of relatively complex biogenic compounds: amino acids (glycine, alanine, aspartic acid), succinic and lactic acids, and other low molecular weight organic compounds. Similar results have been obtained repeatedly, including for the use of other energy sources, other gases, their different ratios. Considering that dozens of simple organic compounds have now been found in interplanetary space, it can be reasonably assumed that billions of years before the emergence of life, the concentration of organic compounds on Earth could have been quite high in places. Dissolved in water, they formed the so-called "primary broth".
2. The synthesis of organic polymers, carried out from the available monomers, became the next step in chemical evolution. The catalysts could be metal ions, and the matrix could be clay particles. As a result of this process, various polypeptides and the simplest lipids were formed in the "primary broth" (remember, from which the two components are built fats). They combined with each other, forming complex multimolecular complexes - coacervates (from lat. Coacervatus - assembled together), which looked like drops with clear boundaries (Fig. 199). Coacervates were already capable of absorbing various substances, various reactions took place in them, in particular, the polymerization of monomers came from outside. Due to these reactions, the drops could grow—increase in volume, and, after reaching the critical mass, multiply—split into daughter drops.