The unity of the origin of life on Earth and the reasons for the heterogeneity and diversity of living organisms
Performed:
student naturally -
faculty of Geography
gr. BI - 11
Frolova Alla Alexandrovna
Ulyanovsk, 2014
Chapter I. Unity of origin. 3
1. 1. Prebiological (chemical) evolution. 3
1. 2. The main stages of chemical evolution. 3
Chapter II. Reasons for heterogeneity and diversity. 7
Used Books. ten
Chapter I. Unity of origin.
Prebiological (chemical) evolution.
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-dust cloud rotating around the Sun.
The reductive nature of the primary atmosphere of the Earth is extremely important for the origin of life, since substances in a reduced state 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 (almost 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 be impossible.
The 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, and hard space radiation and, finally, ultraviolet radiation from the Sun, from which the Earth is not yet protected by an ozone shield.
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 into a diluted “primary broth”. Apparently, it is in the primary ocean that the processes of formation from simple organic molecules - monomers of complex organic molecules - biopolymers begin.
The formation of biopolymers (in particular, proteins from amino acids) could also occur in the atmosphere at a temperature of about 180 ° C. In addition, it is possible that on ancient Earth, amino acids were concentrated in drying up water bodies and polymerized in dry form under the influence of ultraviolet light and the heat of lava flows.
Polymerization of nucleotides is easier than polymerization of amino acids. It was shown that in solutions with a high concentration of salts, individual nucleotides polymerize spontaneously, turning into nucleic acids.
The life of all modern living things is a process of continuous interaction of the most important biopolymers of a living cell - proteins and nucleic acids.
Thus, the secret of the origin of life is the secret of the origin of the mechanism of interaction between proteins and nucleic acids.
Where did the complex process of interaction between proteins and nucleic acids develop? According to the theory of A.I. Oparin, the so-called coacervate drops became the birthplace of life.
Hypothesis of the occurrence of interaction between proteins and nucleic acids:
The phenomenon of coacervation is that under certain conditions (for example, in the presence of electrolytes) high molecular weight 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 disintegrates 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 physicochemical forces, they can selectively absorb some substances from solution and release their decay products into the environment. Due to the selective concentration of substances from the environment, they can grow, but upon reaching a certain size, they begin to "multiply", budding off small droplets, which, in turn, can grow and "bud".
The coacervate drops formed as a result of the concentration of protein solutions during mixing under the action of waves and wind can be covered with a lipid shell: a single one that resembles soap micelles (with a single separation of the drop from the water surface covered with a lipid layer), or double, resembling a cell membrane (with repeated falling of a drop covered with a single-layer lipid membrane onto a lipid film covering the surface of the reservoir).
The processes of emergence of coacervate drops, their growth and “budding”, as well as “dressing” them with a membrane from a double lipid layer, are easily simulated in laboratory conditions.
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 to accurately reproduce, self-copy. Obviously, the precursors of living cells were such coacervate drops, which included complexes of replicator molecules (RNA or DNA) and proteins encoded by them. It is possible that the RNA-protein complexes existed for a long time outside the coacervate drops in the form of the so-called "free-living gene", or perhaps their formation took place directly inside some coacervate drops.
From a historical point of view, the extremely complex process of the emergence of life on Earth, not fully understood by modern science, passed extremely quickly. For 3.5 billion years, the so-called. chemical evolution ended with the appearance of the first living cells and biological evolution began . (URL: http: //www.grandars.ru/shkola/geografiya/proishozhdenie-zhizni (date accessed: 09/28/2014).
Chapter 3. Origin of life: chemical evolution
An insignificant nothing is the beginning of all beginnings.
Theodor Roethke, "Lust"
The theory of chemical evolution - the modern theory of the origin of life - also relies on the idea of \u200b\u200bspontaneous generation. However, it is based not on the sudden (de novo) emergence of living beings on Earth, but on the formation of chemical compounds and systems that make up living matter. She considers the chemistry of the ancient Earth, primarily the chemical reactions that took place in the primitive atmosphere and in the surface layer of water, where, in all likelihood, the light elements that make up the basis of living matter were concentrated, and a huge amount of solar energy was absorbed. This theory tries to answer the question: how could organic compounds spontaneously arise and form into a living system in that distant era?
Oparin - Yuri theory
The general approach to chemical evolution was first formulated by the Soviet biochemist AI Oparin (1894–1980). In 1924, his small book on this issue was published in the USSR: in 1936 a new, supplemented edition of it was published (in 1938 it was translated into English). Oparin drew attention to the fact that modern conditions on the Earth's surface prevent the synthesis of a large number of organic compounds, since free oxygen, which is available in excess in the atmosphere, oxidizes carbon compounds to carbon dioxide (carbon dioxide, CO 2). In addition, he noted that in our time any organic matter, "thrown to the mercy" on earth, is used by living organisms (a similar idea was expressed by Charles Darwin). However, Oparin argued, different conditions prevailed on the primary Earth. It can be assumed that oxygen was absent in the earth's atmosphere at that time, but hydrogen and gases containing hydrogen such as methane (CH 4) and ammonia (NH 3) were abundant. (Such an atmosphere, rich in hydrogen and poor in oxygen, is called reducing, in contrast to the modern, oxidizing, atmosphere, rich in oxygen and poor in hydrogen.) According to Oparin, such conditions created excellent opportunities for the spontaneous synthesis of organic compounds.
Substantiating his idea of \u200b\u200bthe restorative nature of the primitive atmosphere of the Earth, Oparin put forward the following arguments.
1. Hydrogen is abundant in stars (Fig. 6 and Photo 1).
Figure: 6. Lines of hydrogen in the spectrum of the bright star Sirius. This spectrum of the star (white lines on a dark background) is compared with two spectra obtained in the laboratory (dark lines on a light background). All the brightest and widest lines in the spectrum are hydrogen lines. (Photos taken at Mount Palomar Observatory.)
2. Carbon is found in the spectra of comets and cool stars in the composition of CH and CN radicals, while oxidized carbon is rare.
3. Hydrocarbons, ie, compounds of carbon and hydrogen, are found in meteorites.
4. The atmospheres of Jupiter and Saturn are extremely rich in methane and ammonia.
As Oparin pointed out, these four points indicate that the universe as a whole is in a restorative state. Therefore, on the primitive Earth, carbon and nitrogen should have been in the same state.
5. Volcanic gases contain ammonia. This, Oparin believed, suggests that nitrogen was present in the primary atmosphere in the form of ammonia.
6. Oxygen contained in the modern atmosphere is produced by green plants in the process of photosynthesis, and, therefore, by its origin it is a biological product.
Based on these considerations, Oparin came to the conclusion that carbon first appeared on primitive Earth in the form of hydrocarbons, and nitrogen - in the form of ammonia. Further, he suggested that in the course of the currently known chemical reactions on the surface of the lifeless Earth, complex organic compounds arose, which, after a fairly long period of time, apparently gave rise to the first living beings. The first organisms were probably very simple systems capable only of replication (division) due to the organic environment from which they were formed. In modern parlance, they were "heterotrophs", that is, they depended on the environment that supplied them with organic food. At the opposite end of this scale are "autotrophs" - for example, organisms such as green plants, which themselves synthesize all the necessary organic matter from carbon dioxide, inorganic nitrogen and water. According to Oparin's theory, autotrophs appeared only after heterotrophs depleted the supply of organic compounds in the primitive ocean.
J. B.S. Haldane (1892-1964) put forward an idea somewhat similar to Oparin's, which was outlined in a popular essay published in 1929. He suggested that organic matter synthesized during natural chemical processes on the prebiological earth accumulated in ocean, which eventually reached the consistency of "hot diluted broth". According to Haldane, the primitive atmosphere of the Earth was anaerobic (free of oxygen), but he did not claim that reducing conditions were required for the synthesis of organic compounds. Thus, he assumed that carbon could be present in the atmosphere in a fully oxidized form, that is, in the form of dioxide, and not in the composition of methane or other hydrocarbons. At the same time, Haldane referred to the results of experiments (not his own), in which the possibility of the formation of complex organic compounds from a mixture of carbon dioxide, ammonia and water under the influence of ultraviolet radiation was proved. However, in the future, all attempts to repeat these experiments were unsuccessful.
In 1952, Harold Urey (1893–1981), dealing not with the problems of the origin of life itself, but with the evolution of the solar system, independently came to the conclusion that the atmosphere of the young Earth had a restored character. Oparin's approach was qualitative. The problem that Urey was investigating was physico-chemical in nature: using data on the composition of the primary cosmic dust cloud and the boundary conditions determined by the known physical and chemical properties of the moon and planets as a starting point, he set out to develop a thermodynamically acceptable history of the entire solar system generally. Yuri, in particular, showed that by the end of the formation process, the Earth had a highly reduced atmosphere, since its main components were hydrogen and fully reduced forms of carbon, nitrogen and oxygen: methane, ammonia and water vapor. The Earth's gravitational field could not hold light hydrogen - and it gradually escaped into space. A secondary consequence of the loss of free hydrogen was the gradual oxidation of methane to carbon dioxide, and ammonia to gaseous nitrogen, which, after a certain time, turned the atmosphere from reducing to oxidizing. Yuri assumed that it was during the period of hydrogen volatilization, when the atmosphere was in an intermediate redox state, that complex organic matter could have formed on Earth in large quantities. According to his estimates, the ocean, apparently, was then a 1% solution of organic compounds. The result was life in its most primitive form.
Yuri's theory had one important consequence: it gave impetus to successful experimental research. However, before talking about experiments based on the hypothesis of a primitive atmosphere rich in hydrogen, one should find out how this hypothesis corresponds to geological data. This issue has been actively discussed in recent years. for many geologists now doubt that a highly reducing atmosphere ever existed on Earth. All these arguments, only slightly modified, apply to Mars; therefore, it is advisable to consider them briefly here.
Primitive Earth
It is believed that the solar system was formed from the protosolar nebula - a huge cloud of gas and dust. The age of the Earth, as established on the basis of a number of independent estimates, is close to 4.5 billion years. To find out the composition of the primordial nebula, it is most reasonable to investigate the relative abundances of various chemical elements in the modern solar system. Table 3 presents data on the nine most common elements (which account for 99.9% of the total mass of the solar system), obtained using spectroscopic studies of the sun; the relative content of some other elements has been determined by chemical analysis of meteorite matter. As can be seen from the table, the main elements - hydrogen and helium - together make up over 98% of the mass of the Sun (99.9% of its atomic composition) and, in fact, the solar system as a whole. Since the Sun is an ordinary star and many stars in other galaxies belong to this type, its composition generally characterizes the abundance of elements in outer space. Modern concepts of stellar evolution suggest that hydrogen and helium also prevailed in the "young" Sun, which it was 4.5 billion years ago.
Table 3 also shows data on the elemental composition of the Earth. Although the four main elements of the Earth are among the nine most common on the Sun, our planet is significantly different in composition from outer space as a whole. (The same can be said for Mercury, Venus, and Mars; however, Jupiter, Saturn, Uranus, and Neptune are not included in this list.) Earth is made up primarily of iron, oxygen, silicon, and magnesium. The deficiency of all biologically important light elements (with the exception of oxygen) is obvious and the "shortage" of the so-called rare, or noble, gases is striking. like helium and neon. In general, our planet looks very unpromising for the origin of any life.
Elemental composition (percent by mass) of the Solar System and Earth
| In decreasing order of contents | solar system | Land | ||
|---|---|---|---|---|
| Element | % | Element | % | |
| 1 | Hydrogen | 77 | Iron | 34.6 |
| 2 | Helium | 21 | Oxygen | 29,5 |
| 3 | Oxygen | 0,83 | Silicon | 15,2 |
| 4 | Carbon | 0,34 | Magnesium | 12,7 |
| 5 | Neon | 0,17 | Nickel | 2,4 |
| 6 | Nitrogen | 0,12 | Sulfur | 1,9 |
| 7 | Iron | 0,11 | Calcium | 1,1 |
| 8 | Silicon | 0,07 | Aluminum | 1,1 |
| 9 | Magnesium | 0,06 | Sodium | 0,57 |
| Total | 99,70 | Hydrogen + carbon + nitrogen | 0,05 | |
| Neon | 1-10^-3 | |||
| Total | 99,12 |
The main thesis of the Oparin - Yuri theory is that the atmosphere of the young Earth, which corresponded in its chemical composition to the protosolar nebula, had a pronounced reductive character. However, whatever it is, now the Earth's atmosphere is oxidative. It contains 77% nitrogen, 21% oxygen, on average 1% water vapor, about 1% argon and trace amounts (traces) of other gases. How could a reducing atmosphere arise? Probably, the main role here was played by the gases of the protosolar nebula: from the moment of its origin, the Earth was provided with hydrogen and other light elements, which, according to the Oparin-Urey theory, are necessary for the beginning of chemical evolution. Given the deficiency of light elements and especially noble gases, it is reasonable to assume that the Earth initially formed without an atmosphere at all. With the exception of helium, all noble gases - neon, argon, krypton, and xenon - have sufficient specific gravity to be held by gravity. Krypton and xenon, for example, are heavier than iron. Since these elements form very few compounds, they apparently existed in the primitive atmosphere of the Earth in the form of gases and could not escape when the planet finally reached its current size. But since there are millions of times less of them on Earth than on the Sun, it is natural to assume that our planet has never had an atmosphere similar in composition to that of the Sun. The earth was formed from solid materials that contained only a small amount of absorbed or adsorbed gas, so there was no atmosphere at first. The elements that make up the modern atmosphere, apparently, appeared on the primitive Earth in the form of solid chemical compounds; subsequently, under the influence of heat arising from radioactive decay or the release of gravitational energy accompanying the accretion of the Earth, these compounds decomposed with the formation of gases. In the course of volcanic activity, these gases escaped from the depths of the earth, forming a primitive atmosphere.
The high content of argon in the modern atmosphere (about 1%) does not contradict the assumption that noble gases were originally absent in the atmosphere. The isotope of argon, which is widespread in outer space, has an atomic mass of 36, while the atomic mass of argon, formed in the earth's crust during the radioactive decay of potassium, is 40. The abnormally high oxygen content on Earth (in comparison with other light elements) is explained by the fact that this an element is capable of combining with many other elements, forming such very stable solid compounds, such as silicates and carbonates, which are part of rocks.
Yuri's assumptions about the reductive nature of the primitive atmosphere were based on the high iron content on Earth (35% of the total mass). He believed that the iron, of which the Earth's core now consists, was originally distributed more or less evenly throughout its entire volume. When the Earth warmed up, iron melted and gathered in its center. However, before this happened, the iron contained in the layer of the planet now called the Earth's upper mantle interacted with water (it was present on primitive Earth in the form of hydrated minerals, similar to those found in some meteorites); as a result, huge amounts of hydrogen were released into the primitive atmosphere.
Research conducted since the early 1950s has called into question a number of aspects of the scenario described. Some planetary scientists have expressed doubts about the fact that iron, now concentrated in the earth's crust, could ever be evenly distributed throughout the entire volume of the planet. They are inclined to believe that accretion was uneven and iron condensed from the nebula earlier than other elements that now form the mantle and crust of the Earth. With an uneven accretion, the content of free hydrogen in the primitive atmosphere should have been lower than in the case of a uniform process. Other scientists have a preference for accretion, but in a way that should not lead to the formation of a reducing atmosphere. In short, in recent years various models of the formation of the Earth have been analyzed, some of which to a greater extent, others to a lesser extent, agree with the concept of the reductive nature of the early atmosphere.
Attempts to reconstruct the events that took place at the dawn of the formation of the solar system are inevitably associated with many uncertainties. The time interval between the emergence of the Earth and the formation of the most ancient rocks, amenable to geological dating, during which the chemical reactions that led to the emergence of life took place, is 700 million years. Laboratory experiments have shown that the synthesis of the components of the genetic system requires a regenerative environment; Therefore, we can say that once life arose on Earth, this can mean the following: either the primitive atmosphere had a reducing character, or the organic compounds necessary for the origin of life were brought to Earth from somewhere. Since even today meteorites bring a variety of organic matter to the Earth, the latter possibility does not look absolutely fantastic. However, meteorites, apparently, do not contain all the substances necessary for building a genetic system. Although meteorite-derived materials may have made a significant contribution to the general pool of organic compounds on primitive Earth, it currently seems most likely that conditions on Earth itself were reductive to the extent that the formation of organic matter became possible, leading to the emergence of life.
Experiments in prebiological chemistry: synthesis of monomers
Oparin, apparently, did not try to test his theory experimentally. Perhaps he realized that existing analytical methods were not suitable for characterizing complex mixtures of organic substances that could form as a result of a variety of reactions between hydrocarbons, ammonia and water. Or perhaps he was content with the logical elaboration of general principles, not considering it necessary to delve into numerous details. Be that as it may, Oparin's theory was never tested until Yuri approached her. And in 1957 his graduate student Stanley Miller staged his famous experiment, thanks to which the problem of the origin of life turned from purely speculative into scientific, into an independent branch of experimental chemistry.
Modeling the conditions on the primitive Earth, Miller poured some water on the bottom of the flask and filled it with a mixture of gases, which, according to Yuri, should have made up a primitive atmosphere: hydrogen, methane, ammonia. Then, an electric discharge was passed through the gas mixture. By the end of the week, while conducting a chemical analysis of products dissolved in water, the scientist found among them a significant number of biologically important compounds, including glycine, alanine, aspartic and glutamic acids - four amino acids that make up proteins. Subsequently, the experiment was repeated using more advanced analytical methods and a gas mixture, to a greater extent consistent with the currently accepted models of a primitive atmosphere. At the same time, ammonia (which was probably dissolved in the primary ocean) was mainly replaced by nitrogen, and hydrogen was completely excluded, since it is now assumed that, at best, its content in the primitive atmosphere was negligible. In this experiment, 12 amino acids that make up proteins were formed, as well as a number of other non-protein compounds, which was of no less interest for the reasons that we will discuss later.
The study of these unusual fusion reactions has shown that an electrical discharge causes the formation of certain primary products, which in turn participate in subsequent reactions until they are completely dissolved in water, forming the final products. Hydrogen cyanide (HCN), formaldehyde (HCHO), other aldehydes and cyanoacetylene (HCCCN) are among the most important primary products arising from the synthesis. Amino acids are formed from hydrogen cyanide in at least two ways: as a result of interaction in a solution of cyanide, aldehyde and ammonia and by converting HCN itself into amino acids - through a complex sequence of reactions that occur in an aqueous solution.
In all likelihood, the main source of energy on the primitive Earth, as at the present time, was the radiation of the Sun, and not electrical discharges. Therefore, various researchers have tried to use ultraviolet (UV) radiation as a source of energy necessary for the synthesis of amino acids. The experiment gave positive results. The maximum yield of amino acids was obtained when hydrogen sulfide (H 2 S) was included in the gas mixture proposed by Yuri, which absorbs the longer wavelength UV radiation that prevails on the Earth's surface. Amino acids were also formed in the case when shock waves were used as a source of energy, generating short "bursts" of high temperature and pressure. Energy sources of this type probably originated in the primary ocean under the influence of waves, and in the atmosphere they were created by thunderclaps, electric discharges and falling meteorites.
An important addition to Miller's experiments were the experiments of Juan Oro, Leslie Orgel and their collaborators. They showed that four RNA bases (three of them are also found in DNA) are formed in subsequent reactions, into which the primary products of reactions caused by a spark discharge enter. It is characteristic that in a series of reactions occurring in an aqueous solution, hydrogen cyanide self-condenses with the formation of the purine base of adenine; another type of reaction of this type produces another purine guanine. The pyrimidine bases cytosine and uracil are obtained in appreciable amounts from cyanoacetylene in reactions that may have also occurred on primitive Earth. However, until now there have been no reports of the receipt in such a "prebiological synthesis" of thymine, which enters the DNA molecule instead of uracil.
It has long been known that, under certain conditions, formaldehyde condenses in solution to form various sugars. One of the products of this reaction is ribose, the carbohydrate component of RNA. Thus, as we can see, most of the molecular components that form the genetic system can arise as a result of a number of reactions that are quite probable in the conditions of a primitive Earth.
Meteorites and clouds of interstellar dust
Recent discoveries concerning the chemical composition of meteorites and interstellar gas and dust clouds indicate that biologically important molecules are being synthesized on a large scale in our Galaxy, both before and now. The meteorites, which will be discussed, belong to the class of carbonaceous chondrites and make up about 5% of the total number of meteorites that annually fall on the Earth's surface. These interesting objects represent the "fragments" of the protosolar nebula that have not undergone significant changes. They are considered primary, since they were formed simultaneously with the solar system, i.e. 4.5 billion years ago. Meteorites are too small to have an atmosphere of their own, but carbonaceous chondrites are very similar to the Sun in terms of the relative content of nonvolatile elements. Their mineral composition indicates that they were formed at low temperatures and have never been exposed to high temperatures. They contain up to 20% water (bound in the form of mineral hydrates) and up to 10% organic matter. Since the last century, carbonaceous chondrites have attracted attention for their potential biological significance. The Swedish chemist Jacob Berzelius, having discovered organic substances in the Ale meteorite (which fell on the territory of France in 1806), posed the question whether their presence in the meteorite's substance indicates the existence of extraterrestrial life? He himself believed not. It is said that Pasteur had a specially designed probe to obtain uncontaminated samples from the interior of the Orgueil meteorite, another famous chondrite that also fell in France in 1864. After analyzing the samples for the content of microorganisms, Pasteur obtained negative results.
Until recently, the identification of organic compounds in carbonaceous chondrites was not given much importance, since it is rather difficult to identify the differences between the compounds that make up the meteorite itself and the pollution acquired when entering the Earth's atmosphere, hitting its surface, or subsequently introduced by humans when collecting samples. Now, with the development of highly sensitive analytical methods and careful precautions in sample collection, attitudes have changed dramatically. Two recently studied chondrites - meteorites that fell in 1969 in the Murchison region (Australia) and in 1950 in Murray (USA) - contained a number of endogenous amino acids.
There is strong evidence that most of the amino acids found are not contaminants. So, many of them belong to amino acids of an unusual type that are not part of terrestrial organisms. Another piece of evidence is that some of the widespread amino acids, which are usually caused by pollution, are not found in meteorites. And finally, amino acids in carbonaceous chondrites are found in the form of two optical isomers, that is, in different spatial forms, which are mirror images of each other - this is characteristic only of amino acids synthesized by a nonbiological way, but not those that are found in living organisms (see chapter 1). The set of amino acids found in meteorites resembles the amino acids that were obtained in experiments with spark discharges. These sets are not identical, but the similarity is so noticeable that it suggests that the synthesis mechanisms in both cases coincide. Another possible mechanism for the synthesis of amino acids in meteorites is the Fischer-Tropsch reaction, named after two German chemists who developed a catalytic process for producing gasoline and other hydrocarbons from carbon monoxide (CO) and hydrogen. Both of these gases are widespread throughout the universe, as are the catalysts needed for the reaction, such as iron or silicates. Trying to explain the relative content of organic matter in space on the basis of this reaction, Edward Anders and his colleagues from the University of Chicago found that when ammonia is added to the reaction mixture, amino acids, purines and pyrimidines are formed. In this reaction, the same intermediate products - hydrogen, cyanide, aldehydes, cyanoacetylene - are produced, which are obtained in reactions that occur under the influence of electric discharges. Apparently, the presence of hydrocarbons in meteorites, as well as purines and pyrimidines, is easier to explain by the Fischer-Tropsch synthesis reaction than by the reaction under the action of an electric discharge. Until now, however, no laboratory experiment has been able to accurately reproduce the set of substances found in meteorites.
The content of purine and pyrimidine bases in meteorites has been studied to a lesser extent than the presence of amino acids. Nevertheless, adenine, guanine and uracil have been identified in the Murchison meteorite. Adenine and guanine are found at a concentration of approximately 1-10 ppm, which is close to the relative amino acid content. The concentration of uracil is much lower.
Recently, radio astronomers have discovered organic molecules in interstellar space, which has certainly added to our knowledge of the organic chemistry of the universe. Organic molecules have been found in giant dust and gas clouds that are found in areas of space where new stars and planetary systems are believed to form. At the time of this writing, in addition to the expected hydrogen molecules present there, about 60 compounds have been discovered. The most common is carbon monoxide. Much less common are such equally interesting compounds as ammonia, hydrogen cyanide, formaldehyde, acetaldehyde (CH 3 CHO), cyanoacetylene and water, that is, molecules that are considered in laboratory experiments on chemical evolution as precursors of amino acids, purines, pyrimidines and carbohydrates.
These discoveries indicate that the synthesis of organic matter is taking place on a large scale throughout the Universe, and there are many biologically important compounds among its end products, including the main monomers of the genetic system and their precursors. It is even possible (as it was once assumed) that organic compounds - or, at any rate, part of them - that formed the basis of the first living organisms, were of extraterrestrial origin. These discoveries made it possible to realize the important fact that the synthesis of biological compounds is not some specific chemical process that is possible only under especially favorable conditions characteristic of our planet, but is a phenomenon of cosmic scale. This immediately suggests that, in any area of \u200b\u200bthe universe, life must be based on carbon chemistry, similar to that observed on Earth, although not necessarily identical to it.
Synthesis of polymers under prebiological conditions
The formation of basic monomers of proteins and nucleic acids from gases of the protosolar nebula is only the first step in the creation of a genetic system. To form the required polymers, the monomers must then link into chains. This is a difficult problem, and although it is receiving close attention, no reliable methods have yet been proposed for the formation of polymers carrying genetic information from monomers that probably existed on primitive Earth.
The synthesis of polymers both in living systems and in the laboratory includes the stage of attaching the next monomer to the end of the growing chain. At each such stage, energy is consumed and a water molecule is released. When proteins are synthesized from amino acids, the bond formed between the monomeric units of the polymer is called peptide. The figure shows a diagram of the formation of a peptide bond between two amino acid molecules.
R represents any of the 20 different side chains of protein amino acids. When a third amino acid molecule attaches to the end of the dipeptide, a tripeptide is formed, and so on, until a polypeptide is formed. Such reactions are reversible: for example, the dipeptide shown above can, by attaching a water molecule, again turn into amino acids: this process is accompanied by the release of energy. A protein molecule is a polypeptide chain with a specific sequence of amino acids, which gives it special properties and is a product of long evolution. Each chain consists of hundreds of amino acids linked into one sequence, and the molecules of some proteins include two or more such chains. As a result of the interaction between their constituent amino acids, polypeptides form a three-dimensional structure, which is the active form of a protein molecule.
Polymerization of nucleotides, repeating monomeric units of nucleic acids, leads to the formation of polynucleotides, or nucleic acids. The formation of a dinucleotide from two nucleotides is as follows:
Here, the letter B denotes any of the four bases of DNA or RNA; the chains of carbon atoms (C) correspond to a five-carbon sugar with an - OH group linked to the third carbon atom. (The true cyclic designations of the carbohydrate structure are shown earlier in Fig. 1.) Phosphoric acid is attached first to the fifth carbon atom, and then to carbon atoms 5 and 3.
For the synthesis of polymers - both proteins and nucleic acids - living cells produce energy-rich molecules that, using specific enzyme proteins, provide energy for each step of monomer attachment. In addition to the fact that enzymes catalyze the corresponding reactions, they create the conditions necessary for its normal course, eliminating all other interfering molecules. This is essential in the case when the molecules required for the reaction constitute only a small part of all those present in the reaction medium. For example, water molecules are removed, which invariably interfere with the dehydration reaction.
Biological polymers can be synthesized in the laboratory without the participation of enzymes. The synthesis of polypeptides and polynucleotides is now routine. Proteins identical to those synthesized by the cell can be made and produced in the laboratory. They use anhydrous solvents, purified monomers of high concentration, resort to various tricks to protect the reaction groups, and use reagents that provide reactions with energy, which in essence corresponds to the functions usually performed by enzymes.
Let's try to compare these two highly advanced methods of biopolymer synthesis - realized in the cell and in the laboratory - with the conditions that apparently existed on the primitive Earth. The only solvent at that time was water, the monomers necessary for the synthesis made up only a fraction of the total amount of dissolved organic and inorganic substances, the reagents available in sufficient quantities were probably quite simple, and, of course, enzymes were completely absent. It is still not clear how even short polymers could form under such unfavorable conditions. Apparently, the primitive broth consisted of a wide variety of organic compounds. For the synthesis of a polypeptide or polynucleotide to occur, a special group of compounds had to appear in the broth, which would concentrate and combine with each other. This first stage is perhaps especially difficult to imagine. A simple concentration of primary broth is clearly not enough here. Most likely, this broth was a complex mixture of many compounds that were supposed to interfere with the formation of polymers, attaching, for example, to the end of the growing chain and thereby stopping its growth.
Photo 1. Nebula in the constellation Orion. The gigantic masses of gas and dust that surround the central star in the cluster forming Orion's "sword" are another illustration of the prevalence of hydrogen in the universe. Radiation from several "hot" stars in this nebula causes the gases around them to glow at certain, characteristic frequencies. The red color in the photo corresponds to the glow of hydrogen, blue - oxygen and nitrogen, white - a mixture of gases. (© California Institute of Technology, 1959)
Photo 3. The Great Red Spot is a long-lived formation in the atmosphere of Jupiter surrounded by turbulent clouds. (Photo courtesy of Voyager Space Station; NASA and JPL.)
Photo 4. In a photograph of Saturn's northern hemisphere taken
Photo 8. Lake Don Juan in Antarctica. (Photo by Roy Cameron.)
A possible solution to this problem is related to the adsorption of the required molecules on the surface of clay minerals. This mechanism was emphasized by the late J. D. Bernal (1901–1971), a well-known English crystallographer. Compared to organic compounds, clay minerals have a high adsorption capacity. They also interact differently with the different types of compounds that they adsorb. Bernal himself was not sure of the correctness of his assumption; this was due to the fact that silicon, the main constituent of clays, plays almost no role in modern biochemistry. Nevertheless, adsorption is considered the most likely mechanism (although it is not proven) of prebiological separation and concentration processes.
Despite Bernal's doubts, other scientists did not hesitate to assign clay minerals a central role in the origin of life. Indeed, A.G. Kearns-Smith, a chemist at the University of Glasgow, suggested that life began with crystals that form minerals. Possessing the ability to reproduce their own kind, inorganic crystals, as it were, demonstrate rudimentary genetic properties. They also exhibit a limited ability to mutate, which is manifested in the fact that defects can arise in the regular arrangement of atoms in a crystal. Layered minerals such as clays tend to copy defects from one layer in the structure of the next, which can be regarded as a kind of genetic memory. It is noticed that defects in the structure of crystal faces often turn out to be sites of chemical activity, including catalysis. Kearns-Smith suggested that such a simple organic compound as formaldehyde, the synthesis of which could be catalyzed by a mineral bearing such a defect, had the ability to speed up the process of reproduction of a defective crystal and increase copying accuracy, as a result of which the number of such crystals increased rapidly compared to other types. ... This was the beginning of the evolution of the protein-nucleic acid genetic system, which later separated from its mineral ancestor. However, this is a very speculative assumption that has almost no experimental confirmation.
For all the considerable difficulties associated with understanding the conditions for the emergence of the first biologically important polymers, some "mitigating circumstances" should be kept in mind. It is possible that building the first genetic system first required not the large, complexly organized molecules that we find in modern organisms, but only short polymers. The first organism did not have to be highly efficient. Since his life took place in the "paradise booths" in the absence of enemies and problems associated with obtaining food, it was enough for him just the ability to reproduce himself rather quickly in order to outstrip his own chemical degradation. In addition, the chemical processes that preceded the emergence of life took place widely both in space and in time. For hundreds of millions of years, the primitive Earth was a grandiose laboratory, where, due to the gigantic scale of what is happening, even such processes that seem unlikely to us could be realized.
Such considerations, of course, do not give us the right to claim that we understand how the first biopolymers were formed. However, they suggest that the problem does not appear to be as difficult as it is believed. Recent results obtained in the laboratory of Orgel have shown the possibility of the formation of polynucleotides on the original polynucleotide chain in a manner similar to natural gene duplication, but without the participation of an enzyme. This remarkable result was achieved due to the fact that a method was found for introducing energy into the reaction: despite the absence of enzymes, this method is similar to the natural mechanism by which the cell provides energy for the synthesis of polynucleotides. These data make more plausible the assumption that a similar process could play an important role in the early stages of the evolution of the genetic system. In addition, it has recently been shown that some types of RNA have catalytic properties that were usually attributed only to proteins. All these results suggest that a primitive genetic system could have been built without proteins - just one RNA. If this was indeed the case, then the mysteries related to the origin of life are greatly simplified.
The problems concerning the appearance of the first nucleic acid molecule, the genetic code and the entire mechanism of information transfer from nucleic acids to proteins still remain unresolved, however, some progress has been noticeable here, as far as the current level of knowledge allows. Therefore, finishing our brief review of modern ideas about the nature and origin of life on our planet, we do without pretentious arguments about the emergence of the "primary protoplasmic primitive atomic globule." There is no doubt that the movement forward towards solving the problem of the origin of life will continue. Meanwhile, the principles we have outlined are so general that they are quite applicable to the problems of the origin of life in any area of \u200b\u200bthe Universe. We now turn to a discussion of questions about life on other planets of the solar system - this subject is the content of the rest of the chapters of our book.
From the book The Human Genome: An Encyclopedia Written in Four Letters author Tarantul Vyacheslav ZalmanovichOrigin and evolution of great apes Approximately at the turn of the Oligocene and Miocene (23 million years ago), or a little earlier (see Fig. 2), there is a division of the hitherto single trunk of narrow-nosed monkeys into two branches: cercopithecoids, or dog-like (Cercopithecoidea) and hominoids,
From the book BRAND OF THE CREATOR. The hypothesis of the origin of life on Earth. author Filatov Felix PetrovichPART III. ORIGIN AND EVOLUTION OF THE HUMAN GENOME
From the book The theory of adequate nutrition and trophology [tables in text] authorChapter 211. Abiogenic (chemical) evolution (VIII) Hypotheses about the origin of life on Earth proceed mainly from two assumptions. This is either the hypothesis of panspermia (which does not suit many, since, as they believe, only pushes the event into the past and does not solve the problem), or
From the book The theory of adequate nutrition and trophology [tables with pictures] author Ugolev Alexander Mikhailovich From the book The prevalence of life and the uniqueness of the mind? author Mosevitsky Mark Isaakovich1.8. The origin and evolution of endotrophy and exotrophy Trophism and the origin of life In the light of modern knowledge, it is clear that the mechanisms of endotrophy and exotrophy are related, and not opposite, as it seemed earlier, when exotrophy was considered as heterotrophy, but
From the book Amazing Paleontology [History of the Earth and Life on It] author Eskov Kirill YurievichChapter IV. The first manifestations of life on Earth; Life is terrestrial or extraterrestrial
From the book The Logic of Chance [On the Nature and Origin of Biological Evolution] author Kunin Evgeny ViktorovichCHAPTER 4 Origin of life: abiogenesis and panspermia. Hypercycle. Geochemical Approach to the Problem Having considered the questions connected with the evolution of the Earth itself, we now proceed to study the evolution of life on it. I'll make a reservation right away: I'm not going to go deep into the wild
From the book The Ladder of Life [The Ten Greatest Inventions of Evolution] author Lane NickChapter 12 Origin of life. The emergence of translation, replication, metabolism, and membranes: biological, geochemical and cosmological approaches Per. A. The Unknown In the previous chapter, we discussed the possible scenarios for the emergence of cells and (hopefully) reached
From the book The Birth of Complexity [Evolutionary Biology Today: Unexpected Discoveries and New Questions] author Markov Alexander VladimirovichAppendix II The evolution of space and life: eternal inflation, the theory of the "world of many worlds", anthropic selection and a rough estimate of the likelihood of life. P. Averina A brief introduction to inflationary cosmology for non-specialists The theory of the "world of many worlds" (MMM),
From the book The Current State of the Biosphere and Environmental Policy author Kolesnik Yu.A.Chapter 1. The origin of life The planet revolved wildly. Day and night followed each other with dizzying speed: the day lasted only five or six hours. The heavy moon hung threateningly in the sky (much closer than today), which made it seem larger. The stars rarely looked out
From the book Anthropology and the Concepts of Biology author Kurchanov Nikolay AnatolievichChapter 1. The origin of life The question of the origin of life worries everyone, and it is a pity that it is still far from being solved. The main difficulty here is that the path from inorganic molecules to the first living cell was long and difficult. In one step, such transformations are not
From the book Brand of the Creator author Filatov Felix Petrovich2.3. Chemical evolution on Earth One of the modern hypotheses states that our planet has never been completely melted (Losev, 1985, pp. 40–41). The assumption that the Earth formed as a relatively cold solid and then gradually warmed up
From the author's bookThe origin and evolution of the Australopithecines At present, most anthropologists believe that the genus Homo originates from the Australopithecus group (although it should be said that some scientists deny this path). Australopithecus themselves evolved from Driopithecus
From the author's bookChapter 211. Abiogenic (chemical) evolution (VIII) Hypotheses about the origin of life on Earth proceed mainly from two assumptions. This is either the hypothesis of panspermia (which does not suit many, since, as they believe, it only pushes the event into the past and does not solve
Chemical evolution theory (prebiotic evolution, theory of abiogenesis) 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 development of self-organization processes inherent in all relatively complex systems, which undoubtedly are all carbon-containing molecules. 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.
Life in our Universe is presented in the only possible way: as a "way of existence of protein bodies", which is realized due to the unique combination of polymerization properties of carbon and depolarizing properties of a liquid-phase aqueous medium, as jointly necessary and sufficient conditions for the emergence and development of all forms of life known to us ... This implies that, at least within one formed biosphere, there can be only one heredity code common to all living beings of a given biota, but the question remains open whether other biospheres exist outside the Earth and whether other variants of the genetic apparatus are possible.
Research
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, apart from geological data, astronomical data are also involved. So, the conditions on Venus and Mars are considered as close to those that were on Earth at various stages of its evolution. Basic data on chemical evolution were obtained as a result of model experiments, during which complex organic molecules were obtained by simulating various chemical compositions of the atmosphere, hydrosphere and lithosphere and climatic conditions. Based on the available data, a number of hypotheses have been put forward about specific mechanisms and direct driving forces of chemical evolution.
Abiogenesis
In a broad sense abiogenesis - the emergence of the living from the inanimate, that is, the initial hypothesis of the modern theory of the origin of life. In the 20s of the XX century, academician Alexander Oparin suggested that in solutions of high-molecular compounds zones of increased concentration can spontaneously form, which are relatively separated from the external environment and can support exchange with it. He called them coacervate drops, or simply coacervates.
In 1953, Stanley Miller experimentally carried out the abiogenic synthesis of amino acids and other organic substances under conditions that reproduce the conditions of the primitive Earth. 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 early stages of life. They managed to create a "protocell" with a membrane of simple lipids and fatty acids, capable of drawing in activated nucleotides from the environment - the "building blocks" necessary for DNA synthesis.
Aspects of
Chemical evolution hypotheses should explain various aspects:
1.
The non-biological origin of biomolecules, that is, their development from non-living and, accordingly, inorganic predecessors.
2.
The emergence of chemical information systems capable of self-replication and self-modification, that is, the emergence of a cell.
3.
The appearance of the mutual dependence of function (enzymes) and information (RNA, DNA).
4.
Conditions of the Earth's environment in the period from 4.5 to 3.5 billion years ago.
A unified model of chemical evolution has not yet been developed, possibly because the basic principles have not yet been discovered.
Reasoning
Biomolecules
The prebiotic synthesis of complex molecular compounds can be divided into three sequential stages:
1.
The emergence of simple organic compounds (alcohols, acids, heterocyclic compounds: purines, pyrimidines and pyrroles) from inorganic materials.
2.
Synthesis of more complex organic compounds - "biomolecules" - representatives of the most common classes of metabolites, including monomers - structural units of biopolymers (monosaccharides, amino acids, fatty acids, nucleotides) from simple organic compounds.
3.
The emergence of complex biopolymers (polysaccharides, proteins, nucleic acids) from the basic structural units - monomers.
Development of an ancient atmosphere
The evolution of the earth's atmosphere is part of chemical evolution and an important element in climate history as well. Today it is divided into four important stages of development.
In the beginning, the formation of chemical elements in space and the emergence of earth from them took place - approximately 4.56 billion years ago. Presumably, our planet already had an atmosphere of hydrogen and helium quite early, which, however, gradually flowed into space. Astronomers also assume that, due to relatively high temperatures and the effects of the solar wind, small amounts of light chemical elements (including carbon, nitrogen and oxygen) could remain on the Earth and on other planets close to the Sun. All these elements, which make up the main part of the biosphere today, were brought in by comet impacts from the outer parts of the solar system only after a long period of time, when the protoplanets cooled down a little. During the first few million years after the emergence of the solar system, collisions with celestial bodies were constantly repeated, and the collisions caused by them destroyed the living systems formed at that time. Therefore, the emergence of life could begin only after the accumulation of water for a long time, at least in the deepest depressions.
With the slow cooling of the earth, volcanic activity (the release of gases from the bowels of the earth) and the global distribution of materials from the fallen comets, a second earth's atmosphere arose. Most likely, it consisted of water vapor (H2O up to 80%), carbon dioxide (CO2 up to 20%), hydrogen sulfide (up to 7%), ammonia and methane. The high percentage of water vapor is explained by the fact that the surface of the earth at that time was still too hot for the formation of seas. First of all, small organic molecules (acids, alcohols, amino acids) could be formed from water, methane and ammonia under the conditions of a young earth, later - organic polymers (polysaccharides, fats, polypeptides), which were unstable in an acidic atmosphere.
After cooling the atmosphere to a temperature below the boil of water, a very long rain came, which formed the oceans. The saturation of other gases in the atmosphere relative to water vapor has increased. High UV exposure caused the photochemical breakdown of water, methane and ammonia, resulting in the accumulation of carbon dioxide and nitrogen. Light gases - hydrogen and helium - were carried away into space, carbon dioxide dissolved in large quantities in the ocean, oxidizing the water. The pH dropped to 4. The inert and poorly soluble nitrogen N2 accumulated over time and formed about 3.4 billion years ago the main constituent of the atmosphere.
The precipitation of dissolved carbon dioxide (carbonates), which reacted with metal ions, and the further development of living things that assimilated carbon dioxide, led to a decrease in CO2 concentration and an increase in the pH value in water bodies.
Oxygen O2 played a crucial role in the further development of the atmosphere. It was formed with the appearance of living creatures capable of photosynthesis, presumably cyanobacteria (blue-green algae) or similar prokaryotes. Their assimilation of carbon dioxide led to a further decrease in acidity, but the saturation of the atmosphere with oxygen remained still rather low. The reason for this is the immediate use of oxygen dissolved in the ocean to oxidize bivalent iron ions and other oxidizable compounds. About two billion years ago, this process ended, and oxygen began to gradually accumulate in the atmosphere.
Highly reactive oxygen readily oxidizes susceptible organic biomolecules and thus becomes a selection factor for early organisms. Only a few anaerobic organisms were able to move into oxygen-free living spaces, the other part developed enzymes that make oxygen not dangerous.
A billion years ago, the oxygen content in the atmosphere exceeded the level of one percent, and after several million years the ozone layer was formed. The current oxygen content of 21% was reached only 350 million years ago and has remained stable since then.
The importance of water for the emergence of life
H2O is a chemical compound present under normal conditions in all three states of aggregation.
Life as we know (or define it) requires water as a universal solvent. Water has a complex of qualities that make life possible. There is no evidence that life can arise and exist independently of water and it is generally accepted that only the presence of water in the liquid phase (in a certain area or on a certain planet) makes life likely to arise there.
Illustrations
Figure: 1-2. Ground and deep-sea volcanoes - probable preconditions for the emergence of life on Earth
Earlier it was said that the use of computers made it possible to construct and calculate the formation and development of the solar system and the Earth in particular on various models. Chemical Evolution of the Earth During the evolution of the Earth, certain proportions of various elements were formed. The Earth, the most massive of the inner planets, has gone through the most difficult path of chemical evolution. It should be emphasized that the geological history of the Earth ...
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Concepts of modern natural science
Lecture 16. Chemical evolution of the Earth
Earlier it was said that the use of computers made it possible to construct and calculate the formation and development of the solar system and the Earth in particular on various models.
The most convincing is the model of the formation of the Sun and planets from a single rotating gas and dust complex, i.e. in accordance with
rotary hypotheses. Recall that according to these hypotheses, a protostar, the Sun, was formed at the center of the rotating gas nebula. Centrifugal forces in the equatorial region led to the emergence of unstable flows of gas and dust. Subsequently, this part of the matter was torn away from the Sun, taking with it the excess angular momentum. This is how a gas and dust disk (ring) was formed in the equatorial plane of the Sun.The sun heated the inner part of this ring, causing evaporation and "expelling" the lighter elements to the far parts of the ring with the solar wind. This process took about 100 million years. Depending on the distance to the Sun, different parts of the nebula cooled at different rates, which led to differences in the course of chemical processes. The chemical evolution of the planets and the Earth in particular also proceeded in different ways: first, the most refractory elements condensed, and then the volatile ones. The further history of the development of chemical compounds is considered by us in the context of the development of the Earth.
To the beginning of the document
1. Chemical evolution of the Earth
In the course of the evolution of the Earth, certain proportions of various elements were formed. All elements of the periodic table are present in the matter of planets, comets, meteorites, the Sun, which proves their common origin, but the quantitative ratios are different. The number of atoms of any chemical element in various natural systems is usually expressed in relation to silicon, since silicon belongs to the abundant and hardly volatile compounds.
With an increase in the serial number, the prevalence of elements decreases, but not uniformly. It is noteworthy that
even numbered items, especially items with a mass multiple of 4, are more common... These include, in particular, He, CO, Ne, Mg, Si, S, Ar, Ca. The point is that stable nuclei correspond to these mass numbers. American cosmochemists G. Urey and G. Suess wrote the following about this: "... the abundance of chemical elements and their isotopes is determined by nuclear properties, and the substance around us looks like the ash of a cosmic nuclear fire from which it was created."Radioactivity is one of the most important properties of the Earth, which determines its origin and chemical evolution. All primary planets were highly radioactive. Heating up due to the energy of radioactive decay, they underwent chemical differentiation, which resulted in the formation of internal metallic nuclei in the terrestrial planets.
Lithophilic elements, i.e. the elements that form the hard shells of the planets (Si, O, Al, Fe, Ca, Mg, Na, K) went up, the release of gases from the molten material of the mantle during the melting of low-melting fractions led to basaltic melts, which also poured out onto the surface of the planets. The gaseous components escaping along with them gave rise to primary atmospheres, which could only be held by relatively large planets, to which the Earth belonged. The scheme of the formation of the structure of the Earth is shown in Fig. 1.
The Earth is the most massive of the inner planets, has passed the most difficult path of chemical evolution. Complex organic compounds were also assimilated, found also in meteorite matter. These substances were formed at the last stages of cooling of the protoplanetary cloud. Subsequently, they led to the emergence of life on Earth.
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Geochronology. Russian geochemist A.E. Fersman (1883-1945) divided the lifetime of the Earth's atoms into three eras:
The era of stellar conditions of existence,
- the era of the beginning of the formation of planets,
- the era of geological development.
To designate the times and sequence of formation of the Earth's rocks in the era of its geological development, the term
geochronology.In 1881 in Bologna at the International Geological Congress the terms era, epoch, period, century, time were introduced and the geochronological scale was adopted.
It should be emphasized that the geological history of the Earth is inseparable from its biological evolution; it took place in close connection and under the influence of developing life. These connections are reflected in geochronology as well.
According to the degree of study of the geological and biological history of the Earth, all the time of its existence is divided into two unequal parts:
1. Cryptose (criptos - secret), this part covers a huge time interval (from 570 to 3800 million years ago). This is a period with the latent development of organic life, including the Archean and Proterozoic eras.
2. Phanerozoic (Greek phaneros "explicit" + zoe "life"), a later component of 570 million years and including the Paleozoic, Mesozoic and Cenozoic eras;
The turning point in the history of the biological evolution of the Earth was the Cambrian period of the Paleozoic era. If the Precambrian era was the time of the sole dominance of unicellular organisms, then the post-Cambrian era became the era of multicellular forms. In the Cambrian period, for the first time in the history of evolution, multicellular organisms of the modern type arose, all the main characteristics of those bodily "plans" according to which these organisms are built up to now were formed, the prerequisites for the future release of these organisms from the seas to land and their conquest of the entire surface of the Earth were laid.
Until now, it seems mysterious that the emergence of new forms did not stretch over the entire Cambrian era, or at least a significant part of it, but occurred almost simultaneously, over some three to five million years. On a geological scale of time, this is an absolutely insignificant period - it is only one thousandth of the total duration of evolution. This evolutionary leap has been called the Cambrian Explosion.
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2. The concept of self-organization in chemistry.
The question of the origin of organic life still remains one of the most interesting and complex questions of modern natural science. To answer this question means to explain how nature created the most complex macromolecules from a minimum of chemical elements and compounds, and then a highly organized complex of biosystems?
The answer to this question is currently being sought in a special chemical science - Evolutionary chemistry. It is sometimes also called
prebiology - the science of self-organization of chemical systems.Self-organization is understood as a spontaneous increase in the ordering of the levels of complexity of material dynamic, i.e. qualitatively changing systems.
Substrate and functional approaches to the problem of self-organization of prebiological systems.
Within the framework of evolutionary chemistry, there are two approaches to the problem of self-organization: substrate and functional. The functional approach focuses on the study of the processes of self-organization of material systems themselves, on the identification of the laws that govern these processes. Here evolutionary processes are often viewed from the perspective of cybernetics. An extreme point of view in this approach is the statement about complete indifference to the material of evolving systems.Substratum approach
consists in the study of the material basis of biological systems, i.e. elements-organs and a certain structure of chemical compounds entering a living organism. The result of the substrate approach to the problem of biogenesis (i.e. the origin of life) is to obtain information about the selection of chemical elements and structures.Indeed, there is a certain selection of chemical elements for the creation of evolving systems. Currently, more than 100 chemical elements are known, however, the basis of living systems is only 6 elements, called organogens:
C, H, O, N, P, S , the total weight fraction of which is97,4 % ... They are followed by 12 more elements that take part in the construction of many physiologically important components of biosystems: Na, K, Ca, Mg, Mn, Fe, Si, Al, Cl, Cu, Zn, Co. Their weight fraction in organisms is 1.6%.The general chemical picture of the world also testifies to the selection. Currently, about 8 million chemical compounds are known. The overwhelming majority of them (about 96%) are organic compounds, the main building material of which is still the same 6 + 12 elements. It is interesting that from the rest of the chemical elements, Nature has created only about 300 thousand inorganic compounds.
It is important to note that from such a narrow circle of organic substances selected by nature, the entire hard-to-see world of the living was formed.
What are the principles of selection of chemical compounds - a kind of "chemical preparation" for the formation of the most complex biological systems?
It turned out that the decisive role here belongs to catalysts, i.e. substances that activate reagent molecules and increase the rate of chemical reactions. However, catalysts do not remain unchanged in the course of chemical reactions: their activity either decreases or increases.
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3. General theory of chemical evolution and biogenesis
In the 60s of the 20th century, it was established experimentally that in the course of chemical evolution those chemical structures were selected that contributed to a sharp increase in the activity and selectivity of catalysts. This allowed the professor of Moscow State University A.P. Rudenko in 1964, the theory of self-development of open catalytic systems, which can rightfully be considered the general theory of chemo- and biogenesis. The essence of this theory is that chemical evolution is the self-development of catalytic systems, and, therefore, catalysts are the evolving substance.
A.P. Rudenko also formulated the basic law of chemical evolution:
with the greatest speed and probability those paths of evolutionary changes in the catalyst are formed, on which the maximum increase in its absolute activity occurs.Self-development, self-organization of systems can occur only due to a constant flow of energy, the source of which is the main one, i.e. basic reaction.
It follows from this that the maximum evolutionary advantages are obtained by catalytic systems developing on the basis ofexothermic reactions.Time period of chemical evolution.
In the early stages of the chemical evolution of the world, there was no catalysis. The first manifestations of catalysis begin with a decrease in temperature to 5000 ° K and below and the formation of primary solids. It is also believed that when the period of chemical preparation, i.e. the period of intense and varied chemical transformations was replaced by a period of biological evolution, chemical evolution seemed to freeze.Applied value of evolutionary chemistry.
Evolutionary chemistry not only helps to reveal the mechanism of biogenesis, but also allows the development of a new control of chemical processes, involving the application of the principles of synthesis of similar molecules and the creation of new powerful catalysts, including biocatalysts - enzymes, and this, in turn, is the key to solving problems of creating low-waste, waste-free and energy-saving industrial processes.To the beginning of the document
Theories of the origin of life
The most famous theories of the origin of life on Earth to date are the following.
Creationism ... According to this theory, life was created by a supernatural being - God at a certain time. This view is held by the followers of almost all religious teachings. However, even among them there is no single point of view on this issue, in particular, on the interpretation of the traditional Christian-Jewish concept of the creation of the world (Book of Genesis). Some literally understand the Bible and believe that the world and all living organisms inhabiting it were created in six days lasting 24 hours (in 1650, Archbishop Asher, adding up the ages of all the people mentioned in the biblical genealogy, calculated that God began to create the world in October 4004 BC. and finished his work in December 23 October at 9 am, creating a man. At the same time, however, it turns out that Adam was created at a time when a well-developed urban civilization already existed in the Middle East.). Others do not regard the Bible as a scientific book and believe that the main thing in it is the divine revelation about the creation of the world by the omnipotent Creator in a form understandable to people of the ancient world. In other words, the Bible does not answer the "how?" and "when?", and answers the question "why?" In a broad sense, creationism thus allows both the creation of the world in its complete form and the creation of a world that evolves according to the laws set by the Creator.
The process of divine creation of the world is thought of as taking place only once and therefore inaccessible for observation. However, for the believer, theological (divine) truth is absolute and does not require proof. At the same time, for a real scientist, scientific truth is not absolute, it always contains an element of hypothesis. Thus, the concept of creationism is automatically taken out of the scientific - research, since science deals only with those phenomena that are observable, can be confirmed or rejected in the course of research (the principle of falsifiability of scientific theories). In other words, science can never prove or disprove creationism.
Spontaneous generation
... According to this theory, life arose and arises repeatedly from non-living matter. This theory was spread in Ancient China, Babylon, Egypt. Aristotle, who is often called the founder of biology, developing the earlier statements of Empedocles about the evolution of the living, adhered to the theory of the spontaneous origin of life. He believed that "... living things can arise not only by mating animals, but also by decomposition of the soil." With the spread of Christianity, this theory found itself in the same church-damned "cage" with occultism, magic, astrology, although it continued to exist somewhere in the background until it was experimentally refuted in 1688 by the Italian biologist and physician Francesco Redi. The principle “Living things arise only from living things” is called the Redi principle in science. This is how the concept of biogenesis took shape, according to which life can arise only from a previous life. In the middle of the 19th century, L. Pasteur finally refuted the theory of spontaneous generation and proved the validity of the theory of biogenesis.Panspermia theory
... According to this theory, life was brought to the Earth from the outside, therefore, in essence, it cannot be considered a theory of the origin of life as such. It offers no mechanism to explain the primary origin of life, but simplycarries the problem of the origin of life to some other place in the universe.Biochemical evolution theory
... Life arose in the specific conditions of the ancient Earth as a result of processes obeying physical and chemical laws.The latter theory reflects modern natural science views and therefore will be considered in more detail.
According to modern science, the age of the Earth is approximately 4.5 - 5 billion years. In the distant past, conditions on Earth were radically different from modern ones, which led to a certain course of chemical evolution, which was a prerequisite for the emergence of life. In other words, biological evolution itself was preceded by
prebiotic evolution associated with the transition from inorganic matter to organic, and then to elementary forms of life. This was possible under certain conditions that took place on Earth at that time, namely:High temperature, about 4000
ABOUT FROM,An atmosphere consisting of water vapor, CO2, CH 3, NH 3 ,
Presence of sulfur compounds (volcanic activity),
High electrical activity of the atmosphere,
· Ultraviolet radiation from the Sun, which freely reached the lower layers of the atmosphere and the surface of the Earth, since the ozone layer has not yet formed.
It should be emphasized one of the most important differences between the theory of biochemical evolution and the theory of spontaneous (spontaneous) generation, namely: according to this theory
life arose in conditions that are unsuitable for modern biota!To the beginning of the document
Oparin-Haldane hypothesis
... In 1923, Oparin's famous hypothesis appeared, which boiled down to the following: the first complex hydrocarbons could arise in the ocean from simpler compounds, gradually accumulate and lead to the emergence of a "primary soup". This hypothesis quickly gained theoretical weight. It must be said that subsequent experimental studies testified to the validity of such assumptions. So in 1953 S. Miller, simulating the assumed conditions of the ancient Earth (high temperature, ultraviolet radiation, electrical discharges) synthesized in laboratory conditions 15 amino acids that make up the living, some simple sugars (ribose). Later, simple nucleic acids were synthesized (Orgel). Currently, all 20 amino acids, which make up the basis of life, have been synthesized.Oparin assumed that
the decisive role in the transformation of non-living into living belongs to proteins... Proteins are capable of forming hydrophilic complexes: water molecules form a shell around them. These complexes can separate from the aqueous phase and form the so-called coacervates (<лат. сгусток, куча) с липидной оболочкой, из которой затем могли образоваться примитивные клетки. Существенный недостаток этой гипотезы она не опирается на современную молекулярную биологию. Это вполне объяснимо, поскольку механизм передачи наследственных признаков и роль ДНК стали известны сравнительно недавно.(The English scientist Haldane (University of Cambridge) published his hypothesis in 1929, according to which life also appeared on Earth as a result of chemical processes in the Earth's atmosphere rich in carbon dioxide, and the first living things were, perhaps, “huge molecules.” He is not mentioned neither hydrophilic complexes nor coacervates, but his name is often mentioned next to Oparin's name, and the hypothesis was called the Oparin-Haldane hypothesis.)
The decisive role in the origin of life was later attributed to the emergence of the replication mechanism of the DNA molecule. Indeed, any arbitrarily complex combination of amino acids and other complex organic compounds is not yet life. After all, the most important property of life is its ability to reproduce itself. The problem here is that DNA itself is "helpless", it can function
only in the presence of enzyme proteins (for example, a DNA polymerase molecule that "unwinds" a DNA molecule, preparing it for replication). The question remains how such complex "machines" as primordial DNA and the complex complex of protein-enzymes necessary for its functioning could spontaneously arise.Recently, an idea has been developed
rNA-based life emergence, i.e. the first organisms could be RNA, which, as experiments show, can evolve even in a test tube. Conditions for the evolution of such organisms are observedduring clay crystallization... These assumptions are based, in particular, on the fact that during the crystallization of clays, each new layer of crystals is built in accordance with the characteristics of the previous one, as if receiving information about the structure from it. This resembles the replication mechanism of RNA and DNA. Thus, it turns out that chemical evolution began with inorganic compounds, and the first biopolymers could be the result of autocatalytic reactionssmall molecules clay aluminosilicates.To the beginning of the document
Hypercycles and the origin of life
... The concept of self-organization can contribute to a better understanding of the processes of the origin and evolution of life, based on the theory of chemical evolution by Rudenko, considered earlier and the hypothesis of the German physicist and chemist M. Eigen. According to the latter, the process of the emergence of living cells is closely related to the interactionnucleotides ( nucleotides - elements of nucleic acids - cytosine, guanine, thymine, adenine), which are material carriers of informationand proteins (polypeptides[ 1] ) serving as catalysts chemical reactions. In the process of interaction, nucleotides under the influence of proteins reproduce themselves and transmit information to the next protein, so that there isclosed autocatalytic circuit, which M. Eigen calledhypercycle ... In the course of further evolution, the first living cells arise from them, first non-nuclear (prokaryotes), and then with nuclei - eukaryotes.Here, as we can see, there is a logical connection between the theory of catalyst evolution and the concept of a closed autocatalytic chain. In the course of evolution, the principle of autocatalysis is supplemented by the principle of self-reproduction of a whole cyclically organized process in hypercycles, proposed by M. Eigen. Reproduction of the components of hypercycles, as well as their unification into new hypercycles, is accompanied by an increase in metabolism associated with the synthesis of high-energy molecules and the elimination of energy-poor molecules as "waste". (
It is interesting to note here the features of viruses as an intermediate form between life and non-life:they are deprived of the ability to metabolize and, penetrating into cells, begin to use their metabolic system). So, according to Eigen, there is a competition of hypercycles, or cycles of chemical reactions, which lead to the formation of protein molecules. Cycles that run faster and more efficiently than others “win” the competition.Thus, the concept of self-organization makes it possible to establish a connection between the living and the inanimate in the course of evolution, so that the emergence of life seems by no means a purely random and extremely unlikely combination of conditions and prerequisites for its emergence.
In addition, life itself prepares the conditions for its further evolution.To the beginning of the document
test questions
1. List the main stages of planet formation in accordance with the rotational model.
2. What are the common features of the planets of the solar system that indicate a single origin of the planets?
3. Explain the abundance of chemical elements in the solar system.
4. How did the differentiation of the Earth's matter take place? Explain the structure of the Earth.
5. What is geochronology?
6. What parts (according to the degree of study) is the history of the Earth divided into?
7. What elements are called organogens and why?
8. What elements form the chemical composition of living systems?
9. What is self-organization?
10. What is the essence of the substrate and functional approaches to the problem of self-organization of chemical systems?
11. What is evolutionary chemistry?
12. What can be said about the natural selection of chemical elements and their compounds in the course of chemical evolution?
13. What does self-development of catalytic systems mean?
14. What is the applied value of evolutionary chemistry?
15. List the main theories of the origin of life.
16. What is creationism? Can creationism be refuted? Explain your answer.
17. What is the weak point of the panspermia theory?
18. What is the difference between the theory of biochemical evolution and the theory of spontaneous (spontaneous) origin of life?
19. What conditions are considered necessary for the emergence of life as a result of biochemical evolution?
20. What is prebiotic evolution?
21. What is the Oparin-Haldane hypothesis?
22. What is the main problem in explaining the transition from “non-living” to “living”?
23. What is a hypercycle?
Literature
1. Dubnischeva T.Ya. Concepts of modern natural science. - Novosibirsk: YUKEA, 1997.
2. Kuznetsov V.N., Idlis G.M., Gutina V.N. Natural science. - M .: Agar, 1996.
3. Gryadovoy D.N. Concepts of modern natural science. Structural course in the basics of natural science. - M .: Uchped, 1999.
4. Concepts of modern natural science / ed. S.I. Samygin. - Rostov n / a: Phoenix, 1997.
5. Yablokov A.V., Yusufov A.G. Evolutionary doctrine. - M .: Higher school, 1998.
6. Ruzavin G.I. Concepts of modern natural science. - M .: "Culture and Sport", UNITI, 1997.
7. Solopov E.F. Concepts of modern natural science. - M .: Vlados, 1998.
8. Nudelman R. Cambrian paradox. - "Knowledge is Power", August, September-October 1988.
[ 1] polypeptides - long chain amino acids
To the beginning of the document
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Updated 02.19.2002.
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Theory abiogenic molecular evolution of life from inorganic substances was created by the Russian scientist A.I. Oparin (1924) and the English scientist J. Haldane (1929). According to natural scientists, the Earth appeared about 4.5-7 billion years ago. In the beginning, the Earth was a dusty cloud, the temperature of which fluctuated between 4000-8000 ° C. Gradually, in the process of cooling, heavy elements began to be located in the center of our planet, and lighter ones - along the periphery.
It is assumed that the simplest living organisms on Earth appeared 3.5 billion years ago. Life is the result first chemical, and then biological evolution.
Protobionts are also not yet a complete life form. It is assumed that compounds similar to enzymes (coenzymes, actually enzymes) and ATP appeared in them in an abiogenic way.
Cell emergence (matrix synthesis)
In the transformation of protobionts into real cells, the emergence of matrix synthesis as a result of mutual adaptation and fusion of the functions of proteins and nucleic acids played an important role.
Matrix synthesisIs a biological synthesis of protein molecules based on information contained in nucleic acids.
With the emergence of the process of matrix synthesis, chemical evolution gave way to biological. The development of life now continued through biological evolution.
AI Oparin was the first to put forward the idea of \u200b\u200bexperimental study of the origin of life. Indeed, S. Miller (1953) created an experimental model of the primary conditions of the Earth. By acting on heated methane, ammonia, hydrogen and water vapor with an electric discharge, he synthesized such amino acids as asparagine, glycine, glutamine (in such a system, gases imitated the atmosphere, an electric discharge - lightning; Fig. 57).
D. Oro, heating hydrogen cyanide, ammonia and water, carried out the synthesis of adenine. By exposing methane, ammonia and water to ionizing radiation, ribose and deoxyribose were synthesized. The results of such experiments have been confirmed by numerous studies. In the course of evolution, monomers gradually turned into biological polymers (polypeptides, polynucleotides), which was also confirmed empirically. Thus, in S. Fox's experiments, proteinoids (proteinaceous substances) were synthesized by heating a mixture of amino acids. Subsequently, nucleotide polymers were synthesized in experiments.
Compounds similar to coacervates were synthesized experimentally and thoroughly studied by A.I. Oparin and his students. Material from the site
However, it was not known what in the course of the biochemical evolution of life arose before: proteins or nucleic acids. According to the theory of A.I. Oparin, protein molecules were the first to appear. Proponents of the genetic hypothesis, on the other hand, believed that nucleic acids first appeared. This assumption was put forward in 1929 by G. Miller. Laboratory studies have proven the possibility of nucleic acid replication even without the effects of enzymes. According to scientists, the primary ribosomes consisted only of RNA, and the property of synthesizing protein in them could appear later. Later, new data were obtained confirming this assumption. Replication of ribonucleic acid without the participation of enzymes, reverse transcription, that is, the possibility of DNA synthesis based on RNA - all this is evidence of a genetic hypothesis.