Conditions for the origin, development and death of life on Earth

The paper describes a model of climate change on the Earth, built on the basis of the theory of global evolution of the Earth and the adiabatic theory of the greenhouse effect. It is shown that the main factors determining the temperature characteristics of the climate are solar radiation and atmospheric pressure. It is shown that the climate of the Earth was significantly influenced by the biota inhabiting it. Thus, the onset of ice ages at the end of the Proterozoic - in the Phanerozoic is explained by a gradual decrease in atmospheric pressure due to the activity of nitrogen-consuming bacteria, which gradually remove nitrogen from the air and preserve it in the sedimentary strata of the Earth. The warm period of the second half of the Mesozoic is explained by the formation at this time of the supercontinent Pangea and the increased generation of biogenic oxygen, which temporarily compensated for the decrease in the partial pressure of nitrogen. It is predicted that, despite the increase in the luminosity of the Sun, the next ice ages will be the most extensive of all previous ones,

In letters the model of the Earth climate changes, are based on the theories of the global evolution of the Earth and on the adiabatic theory of greenhouse effect is described. It is shown, that by the primary factors temperature determining characteristics of a climate, the sunlight and atmospheric pressure are. It is shown, that on climates of the Earth essential influence was rendered with a biota occupying it. So, approaching the glacial epoch at the end of the Proterozoic – and in the Phanerozoic speaks a gradual decrease in atmospheric pressure owing to vital activities azoth-fixation bacteria, a gradually excluding nitrogen from air and preserving it in sedimental strata of the Earth. Warm period of the second half of a Mesozoic speaks formation at this time a supercontinent Pangea and the strengthened generation of the biogenic oxygen temporarily compensating for the decrease in partial pressure of nitrogen. The prognosis is made, that despite the increase of the Sun lighting, the following glacial epoch will be most extensive of all previous,

1. The uniqueness of the Earth

When you consider the geological conditions that led to the emergence on Earth of exceptionally comfortable conditions for the origin and development of life, especially its higher forms, it becomes clear that the path of the Earth’s evolution was decisively predetermined by both the type of calm “yellow” star, which we call the Sun, its luminosity, and the place of the Earth in the solar system. If our Sun belonged to the type of variable stars, then on Earth it would alternately become either unbearably hot or unbearably cold. If the mass of the Sun were significantly greater, then within a few hundred million years after its formation it would explode and turn into a neutron star or even a black hole. We and all life on Earth are very lucky that the Sun is a quiet star with an average stellar mass, belongs to the dwarf stars of the spectral class G2 and is a stationary star, weakly changing its luminosity over many billions of years. This is especially important because over the past 4 billion years it has allowed earthly life to go through a long evolutionary path from the origins of simple and primitive life to its highest forms.

The distance of the Earth from the Sun also turned out to be optimal, since if they were closer to each other, it would be too hot on Earth and, like on Venus, an irreversible greenhouse effect could arise, and if they were more distant, the Earth would be frozen and it could turn into “white” ” a planet with sustainable global glaciation. We were also lucky with the Earth’s massive satellite – the Moon. Its appearance in a close near-Earth orbit significantly “pumped” tidal energy into the Earth, accelerated its tectonic development and spun the Earth in a forward direction, due to which the climate became more uniform. If our planet did not have a massive satellite, then the Earth, like Venus, would also be delayed in its tectonic development by 2.5 - 3 billion years. In this case, the conditions of the late Archean with a dense carbon dioxide atmosphere and high temperatures would now dominate on Earth, and instead of modern highly organized life, the Earth would be inhabited only by primitive bacteria - single-celled prokaryotes.

Considering the evolution of the Earth in close interaction with the Sun and Moon, one is amazed at how optimal and finely balanced this system is, which so successfully ensured the appearance on our planet of very comfortable climatic conditions for the emergence and development of highly organized life. In addition, the mass and chemical composition of the Earth itself is no less important. Upon closer examination of this system, attention is drawn to the optimal mass of the Earth, capable of maintaining a moderately dense atmosphere on its surface, as well as its exceptionally successful chemical composition. Indeed, even relatively small deviations from the initial concentrations in the earth’s matter of such elements and compounds as Fe, FeO, CO2, H2O, N2, etc., could lead to irreversible and catastrophic consequences for life. In particular, if there was less water in the primary earth's substance, then carbon dioxide would be absorbed with less intensity, and it would begin to accumulate in the earth's atmosphere. As a result, even in the Archean, an irreversible greenhouse effect could have arisen and our Earth would have turned into a “hot” planet like Venus. If there were noticeably more water or less free iron, then the Earth would turn into the planet “Ocean”. If there was less nitrogen in the Earth, then even in the early Proterozoic it would have turned into a “white” and cold planet completely covered with snow. With a larger amount of free (metallic) iron in the primary terrestrial matter in the modern atmosphere, as in the Proterozoic, free oxygen could not accumulate, and, consequently, the animal kingdom could not arise on Earth. On the contrary, with a lower initial concentration of iron, an abundant release of endogenous (abiogenic) oxygen should have begun now or even earlier, and all life on Earth would have already “burned out” in such an atmosphere by now. In addition, the process of degassing deep oxygen would lead to a strong greenhouse effect, after which the Earth would also turn into a hot planet like Venus. From here, in particular, it follows that the lifetime of highly organized and, especially, intelligent life on any of the planets in the star systems of the Universe is relatively limited.

Luckily for us, the Earth turned out to be a harmonious planet in all respects, with a comfortable nitrogen-oxygen atmosphere and wonderful terrestrial life. From here, in particular, it follows that such a “lucky” planet as our Earth is an extremely rare phenomenon in the Universe and it is unlikely that such planets inhabited by intelligent beings exist in our Galaxy. We can only be glad that we live on such a beautiful planet.

2. The origin of life on Earth

The primordial Earth, as follows from modern theories of planet formation, was formed due to the accretion of initial protoplanetary matter irradiated by hard cosmic radiation. That is why the young Earth had to be a completely lifeless planet. This is due to the fact that the very substance of the protoplanetary gas and dust cloud was formed due to supernova explosions and was completely sterilized by hard cosmic radiation long before the start of accretion of the planets of the Solar System. In addition, on Earth in those distant times there was neither a dense atmosphere nor a hydrosphere, i.e., the most favorable environment for the emergence, habitation and protection from the destruction of life. This is explained by the fact that terrestrial matter from the very beginning was sharply depleted in volatile compounds, and that insignificant part of them, which was nevertheless released during impacts and thermal explosions of planetesimals, was immediately sorbed by very porous soil (regolith) and quickly removed from the surface of the Earth, gradually being buried in its depths as more and more portions of protoplanetary matter fall out. In addition, during the formation of the Earth, its surface was subjected to extremely intense radiation from the young Sun - a powerful corpuscular flow that arose in the depths of stars when nuclear reactions were “ignited” in them. The Sun, like the stars of t-Taurus, was then at the very beginning of the main sequence of its development. This intense flow of corpuscles, mainly protons and helium nuclei, should have literally blown away all remaining gaseous components from the Earth's surface.

After the first active stage of development of the young Sun, its luminosity decreased sharply and about 4.6 billion years ago it became approximately 30–25% lower than its modern level. In the absence of an atmosphere, this led to the emergence of negative temperatures on the surface of the young Earth (see below) and therefore at that time it was a cold desert, moreover, subjected to constant and intense irradiation by streams of hard cosmic rays. Of course, under such conditions no life could have arisen on Earth.

Unfavorable conditions for the emergence of life on Earth continued until the process of degassing of the earth's matter began to operate. However, this could only happen after the temperature in the interior of the young Earth rose to the level of melting of the earth’s matter and the appearance of its asthenosphere, as well as after the emergence of convective movements in the mantle, i.e., after the start of the most powerful process of gravitational differentiation of the earth’s matter. At the same time, the formation of the asthenosphere and the process of zone melting of the earth's matter led to a sharp increase in the tidal interaction of the Earth with the Moon and to a significant overheating of the upper mantle in the equatorial belt of the Earth. And this, in turn, accelerated the processes of degassing of the Earth. But these events occurred approximately 600–700 million years after the formation of the Earth itself, i.e., about 4.0–3.9 billion years ago [Sorokhtin, 2007].

At the early stages of the tectonic development of the Earth, i.e. in the early Archean, it was still almost entirely composed of primary earthly matter, and most of the earth’s surface was covered by primordial regolith (see Fig. 4.16). With the beginning of degassing of the Earth, carbon dioxide and water began to actively enter the atmosphere. At the same time, the first acid rains fell on the porous regolith. In this case, most of the water falling on the earth’s surface was absorbed by the regolith of the pristine soil of the young Earth. Due to the high content of iron in the primary terrestrial matter, dissociation reactions of water and carbon dioxide began to occur in the porous regolith with the release of hydrogen and the formation of methane according to reactions (6.3), (6.4) and (6.16). As a result, the earth's atmosphere acquired a reducing character, which is the most favorable condition for the emergence of life [Galimov, 2001]. High porosity, the presence of transition metals in the regolith and its sorption ability with a fairly high capillary pressure, apparently, could provide the necessary conditions for the formation of complex organic compounds, and subsequently the origin of life. Therefore, most likely, life originated precisely in the small pores of the primordial regolith after they were filled with degassed and mineralized water from the mantle [Sorokhtin, Ushakov, 1991]. In addition to simple hydrocarbons, at the same time, in the nitrogen-carbon dioxide atmosphere, due to thunderstorm activity, nitrogen oxides, nitrates, nitrites, ammonia, as well as chlorides, ammonium sulfates and other numerous compounds of nitrogen and carbon should have appeared. Phosphorus compounds apparently entered solutions directly from the primordial regolith. The necessary conditions for the reactions of the formation of more complex organic molecules at elevated atmospheric temperatures already at the beginning of the Archean were provided by the capillary pressure of aqueous solutions in the pores of the regolith and the catalytic action of the free transition metals contained in it (Fe, Ni, Cr, Co, etc.). The formation of complex proto-organic molecules was also facilitated by the fact that only in small pores of the regolith, due to their high sorption activity and high capillary pressures, the concentration of organoelement compounds could reach the level necessary for the synthesis of more complex organic substances (in sea basins these compounds would be too diluted).

The development of the young Earth and its “unwinding” around its own axis of rotation significantly depended on the tidal interaction of our planet with the Moon. In addition, the Moon directly influenced the development of the Earth, heating its upper equatorial belt before the melting of the earth's substance began. As soon as the first melts appeared, the differentiation of terrestrial matter immediately began in the equatorial belt of the Earth at the level of the upper mantle with the separation of molten iron from the silicate matrix of the terrestrial matter. The differentiation of the Earth, naturally, was accompanied by its active degassing with the release of carbon dioxide, nitrogen and water. Moreover, as paradoxical as it may seem, the beginning of basaltic magmatism on the Moon clearly recorded the beginning of tectonomagmatic development and degassing of our planet [Sorokhtin, 1988], and, consequently, the moment of the emergence of the initial forms of terrestrial life on its surface. Therefore, in order to correctly understand the paths of development of the Earth and the life forms inhabiting it, we simultaneously had to consider a new model of the origin of the Moon. The model is based on the physical principle according to which the greatest contribution to the development of our planet was made by those energy processes that maximally reduced the potential energy of the planetary system consisting of the Earth and its massive satellite, the Moon. Their modern structure and composition, as well as the entire geological record of the development of the Earth and the Moon, were taken as the boundary conditions of the problem. The thermodynamic approach to solving the problem determines and ranks the main energy sources of the Earth's development, and also explains the irreversibility of the process of its evolution.

Therefore, there is good reason to believe that life on Earth originated in primordial soil saturated with water and organoelement compounds at the beginning of the Early Archean, about 4.0–3.9 billion years ago at the time when a reducing nitrogen-carbon dioxide-methane atmosphere arose on Earth. Thus, the origin of life on Earth coincided with the first and most powerful tectonic and geochemical boundary in the history of its development - with the initial moment of separation of the earth’s core (with the beginning of chemical-density differentiation of the earth’s matter), which subsequently led to the formation of the hydrosphere, dense atmosphere and continental earth's crust. However, as we showed in section 2.9, the beginning of tectonomagmatic activity on the Earth coincides with the beginning of basaltic magmatism on the Moon. Consequently, the origin of life on Earth, paradoxical as it may sound, is also clearly marked by the formation of basalt “seas” on the Moon.

Let us recall that the classical experiments of S. Miller, A. Wilson, J. Oro, S. Fox and other researchers showed the possibility of synthesizing quite complex organic molecules from inorganic compounds when they are heated in the fields of electric discharges. In Russia, the direction of the autochthonous origin of life by synthesizing organic molecules from inorganic compounds was actively developed by the academician.

The work, devoted to the problems of the origin and evolution of life on Earth, shows that the origin of life had to be associated with the occurrence of energetic chemical reactions that reduce the entropy of the system. Such high-energy and low-entropy reactions can occur, for example, with the participation of adenosine triphosphate (ATP), and ATP synthesis could well have occurred in the early stages of the Earth's development. In order to form ATP, it is first necessary to synthesize the base adenine, a product of the polymerization of hydrocyanic acid HCN, and ribose, a product of the polymerization of formaldehyde HCOH. Thus, according to the synthesis of adenosine triphosphate, it seems to be a necessary prerequisite for the origin and development of the evolutionary process of the development of life on Earth.

But in our model, the formation of the initial products of ATP synthesis could occur in the most natural way. Indeed, at the very beginning of the Archean, as we showed in Chap. 4, most of the Earth’s surface was still composed of primordial finely porous regolith (see Fig. 4.16), which contained up to 13% free (metallic) and about 23% divalent iron (see Table 1.1). After the start of degassing of the Earth about 4 billion years ago, the first acidic rains (containing HCO31– ions) occurred, soaking this regolith with water with carbon dioxide dissolved in it. As a result, abundant methane generation occurred

4Fe + 2H2O + CO2 → 4FeO + CH4 + 41.8 kcal/mol. (10.1)

Methane passed into the atmosphere, as a result the young atmosphere became sharply reducing and nitrogen-carbon dioxide-methane in composition.

Formaldehyde also appeared in a similar way, although in smaller quantities.

2Fe + H2O + CO2 → 2FeO + HCOH + 3.05 kcal/mol. (10.2)

At the same time, formaldehyde remained dissolved in the water that permeated the regolith and was washed out of it by rainwater into the newly formed and still shallow sea basins, and methane entered the atmosphere, giving it a strictly reducing character. But in the methane-rich reducing atmosphere of the early Archean, the formation of hydrogen cyanide could already have occurred, for example, due to lightning discharges

N2 + 2CH4 + Q → 2HCN + 3H2, (10.3)

where Q is the part of the lightning discharge energy absorbed by this reaction.

Thus, at the very beginning of the Archean, conditions actually developed on Earth that were favorable for the emergence of initial chemical compositions suitable for the further synthesis of more complex organic substances and prebiological compounds. This was also facilitated by the presence of active catalysts in the regolith - transition metals Fe, Cr, Co, Ni, Pt, etc. The simplest associations of complex organic molecules or primitive, but already containing ribonucleic acids, formations that had arisen in the soil by this time could then move into the water young marine basins of the Early Archean.

As the Earth degassed and the atmosphere developed, its reduction potential gradually decreased due to the photodissociation of CH4

CH4 + CO2 + hn → 2HCOH, (10.4)

therefore, after some time, the atmosphere became almost purely carbon dioxide-nitrogen with only a small admixture of methane, constantly generated by reactions such as (6.4) or (10.1). However, this admixture of methane, apparently, could play a significant role in the nutrition of primitive Archaean microorganisms. Further improvement of life should have occurred thanks to high-energy but low-entropy reactions [Galimov, 2001] and according to the biological laws of the development of living matter, under the influence of directional pressure and “filtering” properties of the external environment, and then competition.

Thus, the emergence of life on Earth can only be explained by a favorable combination of the composition of the primordial terrestrial matter, the forms of its occurrence on the Earth’s surface (fine-porous regolith), as well as the composition of the atmosphere with a noticeable methane content and the relatively not very high temperature of young sea basins at the very beginning of the Early Archean . After the first, but probably not very long period of the emergence and existence of organic compounds of abiogenic origin, which served as the basis for the formation of the first ribonucleic molecules, viral formations appeared. Thanks to the biosymbiosis of such formations and the synthesis of the first amino acids, as well as semi-permeable membranes, the first cellular forms of bacteria probably arose. However, later, due to intensive degassing of the earth's interior, the pressure of carbon dioxide quickly increased, and the temperature of the atmosphere rose significantly, and the climate became hot, if not hot. Apparently, this is precisely why the rate of further improvement of life in the Archean decreased and during almost all of this time only the most primitive thermophilic and prokaryotic (non-nuclear) bacterial forms such as archaeabacteria with chalcophilic and siderophilic specialization dominated. Most likely, the energy sources of these primitive life forms then were chemogenic reactions of the type that are currently used by thermophilic bacteria in the hot hydrotherms (“black smokers”) of the mid-ocean ridges, as well as other anaerobic chemogenic reactions.

As a result, even in the early Archean, the most primitive viruses and single-celled organisms probably appeared - prokaryotes, already limited from the external environment by protective semi-permeable membranes, but not yet possessing a separate nucleus. Apparently, photosynthetic unicellular microorganisms (such as cyanobacteria) capable of oxidizing iron appeared somewhat later. This is evidenced, in particular, by the prevalence of iron ore formations composed of ferric iron oxides in Early Archean sediments about 3.75 billion years old (for example, the Isua formation in Western Greenland).

15.3. The influence of geological processes and oxygen accumulation on the development of life

The most important factor ensuring the very existence of life on Earth is, of course, the habitat of living organisms in the oceans and continents, and the conditions of such habitat are, first of all, determined by the Earth’s climate, i.e., the composition, state and temperature of the atmosphere, the origin and development of which was associated with the processes of degassing of the planet and ... the vital activity of microorganisms (see section 12.4). The degassing of the Earth that began at the boundary of the Katarchean and Archean, as we have repeatedly noted, led to the formation in the Archean of a relatively dense, essentially reducing carbon dioxide-nitrogen atmosphere. In the Archean, volcanoes and differentiated igneous rocks appeared, and the first isolated shallow sea basins arose, uniting by the end of the Archean into a single, but still shallow ocean. Due to high atmospheric pressure (from 2 to 6 atm.), the average temperatures of oceanic waters, as well as the surface layers of the troposphere, in the Archean rose to +60 ... +70 ° C (see Fig. 8.5), and due to the carbon dioxide composition of the atmosphere , ocean waters were characterized by an acidic reaction (pH ≈ 3–5) [Makkaveev, Sorokhtin, 2005].

However, despite the not very favorable conditions for the existence of life in the Archean, stromatolite deposits are already known starting from approximately 3.6–3.5 billion years ago. Thus, in the Onverwacht series of South Africa (3.5–3.3 Ga), stromatolites have a siliceous composition and compose layers of small thickness and extent that occur among chert beds in volcanic rocks of the greenstone belt [Semikhatov et al., 1999]. In the mid-Archaean, terrestrial life was already characterized by somewhat greater diversity, as evidenced, for example, by the numerous remains of bacterial life forms of the “Figure Three” formation in South Africa. It is possible that cyanobacteria, capable of generating oxygen in small quantities, also appeared in the Archaea. Therefore, it is likely that the gradual oxygenization of the atmosphere began in the Archean. This process was also facilitated by the fact that in the Archean convective mantle above the differentiation zones of earthly matter, the concentration of free iron was reduced (see Fig. 3.20), and then it could not absorb oxygen in noticeable quantities.

In the Late Archean, about 2.8 billion years ago, the mass of water in the Earth’s hydrosphere increased so much that individual sea basins began to merge with each other into a single World Ocean and its surface then overlapped the crests of the mid-ocean ridges (see Fig. 5.4 and 5.6) . Around the same time, thanks to the process of formation of the earth's core (see Fig. 3.3-d and e), free iron and iron-containing silicates (fayalite) began to enter the oceanic crust. As a result, the processes of abiogenic methane generation intensified (see Fig. 6.13), which, in turn, should have led at the end of the Archean to an increase in the mass of stromatolite deposits in greenstone belts of that time (see Fig. 6.12), although their share in volcanogenic formations of such belts, as before, remained insignificant [Semikhatov et al., 1999]. However, at the same time, the absorption of oxygen slightly increased and its partial pressure decreased slightly (see Fig. 6.9).

After binding atmospheric carbon dioxide in crustal rocks, in the Early Proterozoic the Earth's atmosphere about 2.5–2.4 billion years ago became almost purely nitrogen, with only small additions of other gases (see Fig. 6.15). At the same time, the temperature of the Earth's surface decreased significantly. But the sharp changes in living conditions at the turn of the Archean and Proterozoic could not but affect the biota of that time. The community of thermophilic prokaryotes had to give way to more cold-loving bacteria. Another revolutionary restructuring was associated with these events in the ocean biota. Already at the beginning of the Early Proterozoic, photosynthetic microorganisms - blue-green algae - became widespread, and there was a sharp increase in the abundance of stromatolites in geological history (see Fig. 6.12) [Semikhatov et al., 1999], which coincided with the era of mass deposition of iron ore formations and the generation of abiogenic methane (see Fig. 6.11 and 6.13)

At the beginning of the Early Proterozoic, due to a sharp drop in the Earth's tectonic activity, the depths of the ocean basins increased significantly and the standing level of the oceans decreased (see Fig. 5.6). As a result, the supply of iron to the hydrosphere decreased, which, in turn, led about 2.4 billion years ago to a noticeable increase in the partial pressure of oxygen (see Fig. 6.9), and this contributed to the emergence of new forms of eukaryotic unicellular organisms that already had a separate nucleus with deoxyribonucleic molecules, carriers of the cell genome. Eukaryotes probably arose through biosymbiosis of more primitive prokaryotic bacteria. At the same time, apparently, eukaryotic organisms arose that were capable of producing oxygen under the influence of the energy of sunlight, i.e., the first microalgae appeared and the formation of the plant kingdom began.

From that time on, the further development of life took place according to biological laws, under the pressure of environmental conditions and under the influence of gradual oxygenation of the atmosphere. At the same time, the oxygenization of the atmosphere had an exceptionally great influence on the development of terrestrial life (Fig. 10.1).

Rice. 15.1. Evolution of the partial pressure of oxygen in the earth's atmosphere (on a logarithmic scale). In the Precambrian, oxygen was generated only by oceanic biota, and in the Phanerozoic, oxygen from terrestrial plants was also added. In addition, it is assumed that the generation of oxygen by archaean prokaryotes (cyanobacteria) was an order of magnitude lower than that of eukaryotic microalgae of the Proterozoic

As can be seen from this figure, each epoch of mass deposition of ferruginous sediments in the Precambrian corresponds to minima of the partial pressure of oxygen, which is understandable, since iron during oxidation was an active absorber of this gas. In the same figure, the evolution of the partial pressure of oxygen is compared with the main stages of the development of terrestrial life, from which it can be seen that new forms of life on Earth always arose only after the next increase in the partial pressure of oxygen (i.e., after the end of the next era of massive deposition of iron ore sediments). This happened at the turn of the Archean and Proterozoic, although the replacement of thermophilic prokaryotic forms of microorganisms with more cold-loving eukaryotic forms probably also occurred due to a significant cooling of the climate in the early Proterozoic. The metabolism of eukaryotic microorganisms was already associated with the absorption of small amounts of oxygen, so they could spread widely only after increasing the partial pressure of oxygen to a level of the order of 10–3 from its modern value (Urey point). At the same time, the efficiency of oxygen generation by biota increased, which also contributed to an increase in the partial pressure of oxygen and the massive formation of new life forms (Fig. 15.2).

Rice. 15.2. Scheme of the distribution of remains of the main types of microfossils in the Archean and Early Proterozoic, according to and his colleagues. In Archaea, mainly single spherical and filamentous nanobacteria were distributed ( 1 ; 2 ), trichomes ( 3 ) and possibly filaments of cyanobacteria ( 4 ). The diversity of Early Proterozoic microfossils ranges from cyanobacteria ( 5–7 ), coccoid forms ( 8 ; 9 ), trichomes ( 10 ) to imprints of large morphologically complex ( 11–17 ) spiral ( 18 ), ribbon-shaped ( 19 ), round and spherical ( 20 ) forms

After the end of the main era of iron ore accumulation (see Fig. 9.10) and a new increase in the partial pressure of oxygen, about 1.9 billion years ago there was a new major leap in the development of terrestrial life. So, judging by the finds of microfacilities of the chain type, the first multicellular organisms arose about 1.9–1.8 billion years ago. At the same time, in the Middle Proterozoic, many species of unicellular bacteria and algae rapidly developed. Therefore, at the turn of the Middle and Late Proterozoic, the next deep restructuring of the trophic structure of the ocean began, associated with the rapid spread of eukaryotic organisms and phytoplankton. At approximately the same time, bacterial colonization of the land also occurred, as evidenced by the red-colored weathering crusts of earth rocks of the same age [Anatolyeva, 1978].

After the complete disappearance of metallic iron from the Precambrian mantle and its transition to the earth's core [Sorokhtin, 1974; 2002], biogenic oxygen began to accumulate more actively in the earth’s atmosphere (Fig. 10.1), which led to the most significant restructuring of the entire biota of the Earth - to the emergence of higher forms of life. Moreover, to an increase in the partial pressure of oxygen above 30 mbar, the Earth's biota responded with a literally explosion of its diversity, the emergence of its skeletal forms and the appearance of all the main types of currently existing life forms (Fig. 10.3). Let us note here that the emergence and flourishing of higher forms of life on Earth was apparently facilitated by a decrease in average surface temperatures on the planet below 20 °C (see Fig. 8.5).

The total biomass of phytoplankton generating oxygen in the ocean, as already noted, is determined by the amount of phosphorus compounds dissolved in its waters [Schopf, 1982], but its concentration in ocean waters has always remained in equilibrium with respect to the basalts of the ocean crust and close to modern ones. It follows that the mass of ocean phytoplankton increased over time approximately in proportion to the increase in the mass of water in the ocean (see Fig. 6.7). Consequently, about 2 billion years ago the biomass of oceanic organisms was already quite significant, about a third of its modern value.

The process of enhanced oxygen generation at the turn of the Proterozoic and Phanerozoic was also associated with the most radical restructuring of the biota, which occurred at the very end of the Proterozoic after the complete disappearance of metallic iron from the mantle (see Fig. 3.20), which by that time had completely transferred to the earth’s core. Immediately after this event, oxygen began to accumulate in noticeable quantities in the atmosphere (see Fig. 10.1). Along with the accumulation of oxygen, earthly life also improved. Thus, already in the Vendian, the first forms of macroorganisms appeared - animals similar to jellyfish, which left their imprints in many sediments of this age. In the Cambrian, skeletal forms of animals appeared and almost all modern types of plant and animal kingdoms arose (Fig. 15.3)

.

Rice. 15.3. “The Tree of Life” from the book “Life on Earth” by D. Attenborough. The development of life at the turn of the Proterozoic and Phanerozoic was in the nature of a biological explosion

Thus, and third sharp geological and biological boundary During the transition from the Proterozoic to the Phanerozoic, it was clearly reflected in the biological history of the Earth and radically changed the ecological situation on its surface - from now on, the earth’s atmosphere turned from neutral or weakly oxidizing to oxidizing. In this new situation, the most effective were those forms of life whose metabolism was based on the oxidation reactions of organic substances with oxygen synthesized by the plant kingdom.

It is obvious that the magnificent development of life on Earth occurred, among other things, thanks to the comfortable climatic conditions of a moderately warm climate in conditions of a weakly oxidizing atmosphere of not very low or high pressure. Indeed, after the hot climate of the Archean with a high pressure carbon dioxide atmosphere, in which only the most primitive and thermophilic forms of bacterial life could survive, the rest of the history of the Earth's development was dominated by moderate temperature conditions, the most favorable for the occurrence of biological processes. Even a noticeable cooling of the climate in the Pleistocene, with periodic glaciations of continents in the polar and boreal zones of the Earth characteristic of this time, apparently had a positive significance for the improvement of higher forms of life, including the formation of modern man.

The development of life on Earth in the distant future, however, is threatened with imminent death due to the degassing of abiogenic oxygen, which should begin to be released after the complete oxidation of mantle iron to magnetite stoichiometry due to the reactions of the ongoing release of the earth's core, which we discussed in section 3.11 and expressed by the formula ( 3.21). However, these events will not happen very, very soon - in about 600 million years.

15.4. The influence of continental drift and marine transgressions on the environmental conditions of the Phanerozoic

Restructurings of the biota also occurred in the Phanerozoic, but all of them already had the character of differentiation and evolutionary development of organisms within the framework of large taxa formed at the beginning of the Phanerozoic. Moreover, in addition to the main impact on the evolution of life in the Phanerozoic - a gradual increase in the partial pressure of oxygen, noticeable factors in the influence of the external environment on the evolutionary changes in life forms were continental drift, climate change, transgression and regression of the sea. All these factors changed the established ecological niches of biological communities and intensified their competition for survival.

As is known, during the Phanerozoic there were two major planetary transgressions. The first occurred from the Ordovician to the Devonian inclusive during the development of the Caledonian orogeny (from 500 to 350 million years ago), its amplitude reached 200–250 m (see Fig. 5.8). The second and largest global transgression, caused by the process of formation of the supercontinent Pangea, occurred in the Late Jurassic - Cretaceous and reached 350–400 m in amplitude. In addition, the conservation of water in continental glaciers during periods of ice sheets could lead to global regressions of the ocean with decreases in its level at 120–130 m (see section 5.4).

Transgressions of the ocean onto land and their inverse regressions associated with eustatic fluctuations in sea level should have significantly influenced global variations in the Earth's climate in the geological past. Due to the fact that the heat capacity of water is much greater than the heat capacity of the atmosphere, any significant increase in sea surface area due to a decrease in land area mitigated seasonal and latitudinal climate changes. With water flooding up to 30% or more of the continental surface area in the mid-Cretaceous, the moderating effect of transgression on global climate variations in temperate and high latitudes was quite significant. This was also facilitated by the expansion of epicontinental seas, which created sea corridors through which heat exchange could occur between low and high latitudes. During periods of regression, as the sea receded, the overall continentality of the Earth's climate increased and seasonal temperature contrasts increased.

However, the main processes that influenced the climate and its latitudinal zonation were still bacterial removal of nitrogen from the atmosphere and fluctuations in the Earth's precession angle depending on continental drift and high-latitude glaciations (see sections 8.3 and 8.4). In addition, the spatial location of continents and oceans also had a significant impact on climatic contrast. Thus, during the era of glaciations, land areas that fell into high-latitude regions as the ensemble of lithospheric plates moved, began to gradually be covered first by mountain glaciers, and then by sheet glaciations, which played the role of global refrigerators. Consequently, the sharpest latitudinal zonation on Earth was observed when, during glacial periods, continents covered with ice and snow or frozen oceanic waters were located in the polar regions. In addition, changes in the relative position of continental masses changed the nature of the circulation of ocean waters, which also greatly influenced the formation of the earth's climate. It is known that modern glaciation in Antarctica began due to a general cooling of the climate, but reached its maximum only after Australia broke away from it and moved to the north, and the Drake Passage opened south of Tierra del Fuego. After this, a southern circumpolar current arose around Antarctica, completely “cutting off” this continent from the warmer counter-trade currents of the three oceans washing it. This system of climate insulation in Antarctica is still in effect today.

In the light of the presented geohistorical interpretation of the processes of change in the global climate of the Earth, it is now possible to consider the nature of the major ecological boundaries of the Phanerozoic, i.e. those that took place over the last 550–600 million years of the development of life on our planet. A gradual increase in the partial pressure of oxygen made it possible for highly organized life to reach land around 400 million years ago. This event is a unique phenomenon associated with a radical restructuring of the metabolism of oceanic organisms and the appearance in the animal kingdom of forms with lungs - an organ ideally adapted to gas exchange in the air.

In addition to purely terrestrial factors, the Earth’s climate, as we saw above, was also influenced by the Lunar-Solar influence, which always reduced the value of the Earth’s precession angle, which, in turn, led to significant cooling. The gradual cooling of the climate in the Proterozoic and Phanerozoic was also caused by the removal of nitrogen from the atmosphere due to the activity of nitrogen-consuming bacteria. On the other hand, the processes of formation of supercontinents have always influenced in the opposite direction - towards climate warming. As a result of the interaction of these factors in the Proterozoic and Phanerozoic, there was a general cooling of the climate by approximately 10–15 °C and its fluctuations occurred with periods of about 800 million years and the amplitude of changes in average temperature up to 8–10 °C (see Fig. 8.23 ​​and 8.24).

Global tectonic processes also played an important role in the ecological evolution of the Phanerozoic, as in previous eras. Indeed, changes in the spatial location, size and shape of both continents and oceans in the Phanerozoic history of the Earth had a significant impact on the power and structure of ocean currents, and, consequently, on the distribution of biological productivity, i.e. on the formation of the most adapted communities of animals and plants to certain natural conditions and ecological systems.

As examples of the dependence of the development of life on the climatic zonation of the Earth, we can consider the influence that the latitude of the continents and oceans has on the species distribution of marine organisms. It is known that the vast majority (about 90%) of all species of marine animals live on continental shelves or shallow waters near underwater hills and islands at depths of less than 200 m. Therefore, we can assume that in the Phanerozoic the main development of marine fauna occurred at shallow depths. At the same time, the richest shallow-water marine fauna is now in the tropics, where it is represented by a large number of highly specialized species. The diversity and abundance of marine fauna decreases with increasing latitude, especially in oligotrophic ocean areas. However, in the circumpolar waters, due to the higher concentration of dissolved oxygen in their waters, there is again a significant increase in the bioproductivity of the oceans. The lowest productivity of the oceans is observed in mid-subtropical (oligotrophic) latitudes. The degree of diversity of modern shallow-water marine fauna correlates well with changes in the stability of food supplies, which depend on the seasonality of climate. In addition to this most important latitudinal factor, there are also longitudinal components, which also determine the overall diversity of modern marine fauna. In particular, at the same latitude, greater faunal diversity is observed where there is stability of food resources. Therefore, in each latitude zone, the greatest diversity of marine fauna is observed near the coasts and island archipelagos and large oceans.

Of particular note are the areas of upwelling, in which deep waters rich in phosphorus and organic compounds rise along the continental slope to the surface, providing abundant food for shallow-water organisms (see Fig. 9.20). Typically, upwellings occur on the eastern shores of oceans in their tropical zones. In these zones, unique oases of life arise, blooming luxuriantly among the relatively deserted waters of the adjacent oligotrophic water areas. Examples of such oases of life are the Peruvian and West African upwellings in the Pacific and Atlantic oceans.

Naturally, deep-sea ocean basins become a significant obstacle to the spread of shallow-water fauna. Volcanic arcs that arise above zones of subsidence of the oceanic lithosphere into the mantle and intraplate chains of volcanic islands often serve as good routes for the distribution of marine fauna, especially when such chains of volcanic islands have a sublatitudinal strike or are located, such as the islands of Polynesia and Micronesia, within a single climate belts Another mechanism for the dispersal of shallow-water fauna may be the migration of the larval forms of these animals. However, as a result of the rather divided modern position of each of the large continents, the marine shallow-water fauna inhabiting their shelves is currently developing in approximately 30 provinces and is characterized by a relatively small percentage of species common to all these provinces. Estimates show that shallow marine fauna now contains an order of magnitude more species than would be observed if only a single faunal shelf province existed on Earth, even at the highest species diversity.

The same patterns can be traced with the dispersal of some species of deep-sea fauna. For example, the biological communities of the hot springs of the “black smokers” of the Pacific Ocean are dominated by large tube worms – vestimentiferans and bivalves – calyptogenes, while in the Atlantic Ocean the same hydrothermal springs are completely occupied by small shrimps that have adapted to feeding on sulfur bacteria.

Using these patterns of fauna settlement, taking into account data on continental drift, information on eustatic changes in the level of the World Ocean, as well as the climatic consequences of these phenomena, one can try to explain the nature of changes in the number of taxa of shallow-water fauna in the Phanerozoic, for example, the mass death of many groups of animals at the boundary of the Paleozoic and Mesozoic. Indeed, the separate position of most continental masses in the Early Paleozoic and their predominant location from tropical to high latitudes, as well as the presence of shelf areas in each of them, led to a significant increase in the number of families of shallow-water fauna in the Ordovician. This increased number of families was preserved in the process of evolution of marine fauna throughout most of the Paleozoic. At the Permian-Triassic boundary, when many continental fragments began to unite into a single supercontinent - Pangea, and especially in the Early Jurassic, when Pangea was fully formed, the precession angle increased significantly to 30-34o, and the Earth's climate warmed significantly (see Fig. 8.23 ​​and 8.24). At the same time, the number of biological provinces and ecological niches on the Pangea shelf has decreased significantly. In addition, the regression of Permo-Triassic time (see Fig. 5.8) led to a sharp reduction in the areas of shallow seas. In such conditions, at the Permian-Triassic boundary, only those representatives of shallow-water fauna survived that could find food in the bottom layers. In other words, the faunal families that survived at the boundary of the Paleozoic and Mesozoic apparently developed in unstable environmental conditions, while the majority of Paleozoic populations, which developed in stable conditions similar to modern tropical ones, after the formation of Pangea turned out to be less adapted and were doomed to extinction . Therefore, it can be assumed that the rapid extinction of many species of marine fauna at the turn of the Paleozoic and Mesozoic was due to a reduction in the number of ecological niches surrounding the then formed supercontinent and a decrease in the potential for bioproductivity surrounding this single continent of shelf seas.

In the Early Cretaceous, the collapse of Pangea began under the influence of a powerful ascending mantle flow that arose under the center of the supercontinent, similar to that shown in Figure 4.21. This process was accompanied by a significant increase in the tectonic activity of the Earth (see Fig. 5.9), the centrifugal spreading of individual continents (Fig. 15.4) and the largest transgression of seas onto land in the Phanerozoic (see Fig. 5.8). The result was an increase in the diversity of the animal world, which increased significantly in the Cenozoic as the shelf provinces of various continents separated from each other and, especially, due to the emergence of a sharper contrast in the Earth's climatic zones.

Rice. 15.4. The initial stages of the collapse of Pangea about 100 million years ago (in the Lambert projection), according to work

Of course, such a general approach to the problem of the evolution of life requires significant development and detail. Thus, the Cretaceous transgression led to the flourishing of carbonate-consuming fauna and microflora on shelves and in epicontinental seas, especially foraminifera and colithophorid microflora, which formed unique strata of writing chalk. However, this same transgression also caused crisis phenomena in the life of biocenoses of coral atolls in the open ocean. One gets the impression that in the middle of the Cretaceous period a very powerful mechanism existed and was operating, leading, on the one hand, to a sharp weakening of oceanic sedimentation, and on the other, to an increased transfer of calcium carbonate and phosphorus from the waters of the open ocean to the shallow seas of flooded areas of the former land [Bogdanov et al. 1990; Sorokhtin, 1991].

In the Cretaceous era, the location of the continents on the Earth's surface was somewhat different than it is now (see Fig. 9.27) and most of the shallow epicontinental seas were then located in arid zones with a sharp predominance of evaporation over precipitation. Therefore, such seas served as natural pumps, pumping water out of the oceans, and with it carbonates and phosphorus (Fig. 9.28). The sharp reduction in the supply of calcium carbonate and phosphorus to the bioherm structures of atolls in the open ocean (see Fig. 15.5), which occurred in Aptian-Cenomanian times, led to the suppression and degradation of reef communities that built their calcareous skeletons and frameworks from calcium carbonate.

Rice. 15.5. The rate of accumulation of Cenozoic oceanic sediments in the Pacific Ocean according to T. Davis and his colleagues: 1 – total rate; 2 – rate of carbonate sedimentation

In conditions of severe carbonate starvation, corals, rudists and other skeletal organisms could no longer create durable limestone structures that could withstand the abrasive activity of ocean waves, especially in stormy weather. In such a situation, reef structures on oceanic islands - atolls and rudist banks of Aptian-Cenomanian age - no longer had time to build up and compensate with their growth for the smooth subsidence of their volcanic foundations below the ocean level (remember that the subsidence of the ocean floor occurs according to the law of the square root of its age ( 4.6')). At some point, the erosion of the former reefs completely destroyed the shallow-water fauna that lived on them, turning the former coral atolls and rudist banks into flat-topped seamounts - guyots (Fig. 15.6), so named by G. Hess in memory of the 19th-century French geographer A. Guyot ( Guyot).

In the Aptian-Cenomanian time, in this way, in the Pacific Ocean alone, about 300 previously thriving atolls perished, which then turned into flat-topped seamounts, the peaks of which are now located at depths of about 1500 m below sea level. The very origin of the volcanic mountains - the socles of the atolls - was associated with membrane tectonics and deformations of lithospheric plates as they moved along the Earth’s ellipsoid of rotation. An assessment of the ages of formation of 82 guyots, based on an analysis of the geomorphological conditions of their modern location on the ocean floor, showed that almost all of the analyzed guyots, like seamounts, arose in a relatively narrow age range from approximately 95 to 105 million years ago [Bogdanov and al., 1990]. From this it can be seen that almost all the numerous coral islands of the Pacific Ocean died almost simultaneously in Albian-Cenomanian times and from a common cause - carbonate starvation and, probably, lack of phosphorus [Sorokhtin, 1991].

.

Rice. 15.6. General scheme of guyot formation

Thanks to the constant mixing of water masses in the oceans that occurred during the ice ages, bottom waters at this time are always saturated with oxygen, and this led to the possibility of the existence of bottom deep-sea fauna on the ocean floor. A different situation developed during warm eras on Earth, when glaciations were completely absent. At this time, the mixing of ocean waters almost completely stopped and stagnant, oxygen-deprived, and sufficiently heated deep waters accumulated in deep-sea basins. Often under such conditions, water stagnation and hydrogen sulfide contamination occurred, accompanied by the accumulation of sapropel silts. Naturally, during such periods, all deep-sea and benthic fauna that existed on the ocean floor in past ice ages perished. With the onset of a new glacial era, the settlement of benthic and the formation of benthic communities actually occurred anew due to the migration of animals from the pelagic zones of the oceans to the depths.

In particular, according to deep-sea drilling data, sapropel silts are found in the Cretaceous sediments of the Pacific Ocean, which leads to the conclusion that all benthic animal communities of this ocean turn out to be very young - no older than the Middle Cenozoic [Kuznetsov, 1991], most likely no older than 55 million years [ Nesis, 2001], i.e., not older than the beginning of glaciation of Antarctica about 50 million years ago\.

Continental drift also had a significant impact on land fauna. Thus, it is well known that the Mesozoic, together with the Late Permian time of the Paleozoic, was the era of reptiles, while the Cenozoic was the era of mammals. Over 200 million years of development in the Permo-Mesozoic, only 20 orders of reptiles arose, while after the Cenozoic (over 65 million years) about 30 orders of mammals arose. This striking difference can be explained by comparing the developmental conditions of reptiles and mammals. Note that the initial period of rapid development of terrestrial reptiles coincided with the beginning of the formation of the fourth supercontinent - Pangea (see Fig. 4.20-e). The same period, about 250–200 million years ago, was characterized by significant ocean regression, but with a relatively mild global climate, which established at the end of the Permian. The heyday of the class of reptiles occurred in the second half of the Mesozoic, when, after an increase in the angle of precession of the Earth due to the formation of Pangea, the climate became warm. In the Cretaceous period, when the collapse of Pangea began, more or less stable ecological connections still existed between its fragments. Consequently, during the long period of reptile development on land, there was a single ecological province, or there were a very small number of semi-isolated provinces. It is clear that under such conditions a large variety of reptiles could not have arisen.

In the middle of the Cretaceous period, when there were probably still connections between the “scattering” fragments of Pangea (see Fig. 10.4), the settlement of primitive mammals occurred. In the late Cretaceous–early Cenozoic (i.e., during the beginning of the heyday of mammals), due to the significant distance from each other of individual continents and the still ongoing marine transgression (see Fig. 5.8), several large mainland ecological provinces were formed, significantly or completely isolated from each other . In particular, at the beginning of the Cenozoic, as is now known from the analysis of the anomalous magnetic field of the Polar Atlantic, land connections between America and Europe were still preserved (until the Eocene). But Eurasia in the Late Cretaceous and Early Cenozoic was divided by a large inland sea that stretched across Western Siberia from the Tethys to the Arctic Ocean. At the same time, land connections arose between Alaska and Chukotka, at least periodically, through the current Bering Strait. Thus, during the Late Cretaceous and the first half of the Cenozoic, three ecological provinces, not completely isolated from each other, were formed within the Laurasian continents, in which many orders of mammals arose.

The ecological division of the provinces within the Gondwanan continents probably began somewhat earlier than the Laurasian continents. In the middle of the Late Cretaceous, Africa was already quite distant from the rest of the continents of the Gondwana group (see Fig. 10.4); at the same time, most of this continent was flooded by shallow seas, which divided it into two or three land areas. South America, which separated from Gondwana in the Early Cretaceous but became a fairly independent ecological province only in the Late Cretaceous, was almost completely divided by a shallow sea in the basin of what is now the Amazon into two land areas. In addition, in Late Cretaceous time there were two more isolated land provinces, Indian and Australasian-Antarctic; the latter at the beginning of the Cenozoic (about 40 million years ago) was divided into two independent regions.

So, eight to ten land ecological provinces significantly isolated from each other, formed in the Early Cenozoic, are the main condition for the generic diversity of the class of mammals. Let us note that the combination of several continental fragments in the Late Cenozoic and the reduction in the number of ecological provinces to five - the most extensive, including Eurasia with Hindustan, Africa, North America, South America and Australia - led to the extinction of 13 orders of land mammals. At the same time, those orders survived that in the first half of the Cenozoic developed in not completely isolated ecological provinces. Mammals that developed before the connection of various continents in isolated conditions turned out, as a rule, to be less adapted and died.

A more general conclusion, which, based on the analysis of the Late Mesozoic and Cenozoic evolution of the ocean floor, can only be outlined: all the main boundaries of geological history (and, as a consequence, the division of the geochronological scale into eras, periods and epochs) are largely determined by events such as collisions and splits continents in the process of global movement of the ensemble of lithospheric plates.

Indeed, as reconstructions compiled by A. Smith and J. Bryden show, some of which are reproduced in Figures 4.20, 9.27 and 10.4, most of the boundaries of change in biological communities along which the Phanerozoic was divided into separate periods coincide with the main stages of the restructuring of the tectonic plan of the Earth . So, in the middle of the Mesozoic, all the continents were collected into a single supercontinent Pangea (perhaps only with the exception of the Chinese platform), and the Cretaceous period is the beginning of the split of Pangea, which continues to this day.

In the Triassic, the split of Europe and Asia began, but never took place, in the area of ​​the current West Siberian Lowland, and the separation of North America from Africa and Europe began, which led to the formation of a young oceanic basin of the North Atlantic in the mid-Jurassic. At the beginning of the Cretaceous, Africa broke away from South America and Antarctica, which were also divided among themselves. The beginning of the Cretaceous period - the splitting of Hindustan from Antarctica and Australia. At the end of the Cretaceous and the beginning of the Cenozoic, the modern New Zealand Plateau and the underwater Lord Howe Ridge break away from the united continent of Antarctica and Australia, these continents split, and Australia moves towards the equator. During the same period (at the boundary of the Cretaceous and Cenozoic), North America, Greenland and Europe separated in the Northern Hemisphere, as a result, the Polar Atlantic was formed in the Cenozoic. In addition, at the very beginning of the Cenozoic, the Arabian Plate broke away from the African Plate and the formation of the Red Sea and the Gulf of Aden began. In the middle of the Cenozoic, the Hindustan Plate collided with Asia and formed the currently largest mountain belt. At the boundary of the Oligocene and Miocene, the closure of the Tethys paleoocean continued, and in the mid-Miocene, the final closure of this paleoocean and the emergence of folded structures in the European part of the Alpine-Himalayan fold belt occurred. It is interesting to note that the process of closure of the Tethys paleoocean is clearly marked by a local increase in the temperature of the bottom waters of the World Ocean (see Fig. 8.3). In the late Miocene, the mountains of the Caucasus rise, at the same time the mountains of Central Asia and the Himalayas are formed. This process of formation of the giant Alpine-Himalayan fold belt, which separated Northern Eurasia from its southern regions, continues to this day.

Examples show that on the continents the evolution of plant and animal life forms was apparently also largely determined by continental drift and changes in climatic conditions on Earth. This should have been especially evident during the unification of previously separate continents or, conversely, during the splits of large continents and the separation of their fragments. The alternating eras of consolidation and fragmentation of continents with the formation of single and intercontinental oceans were, of course, accompanied by the emergence of new and closing of old ecological niches, i.e., radical changes in the ecological conditions for the existence of life on Earth. Therefore, such rearrangements could probably be one of the main reasons for both speciation and extinction of individual life forms, and their conservation. A striking example of this is the endemic life forms in Australia and South America.

Of course, all these events were reflected in the paleoclimate and the entire evolution of the ecological system of our planet in the geological past, which was reflected in the compilation of the geochronological scale and its division into eras, periods and, possibly, epochs. Naturally, when geohistorically analyzing the problem of the evolution of flora in the geological past, it is also necessary to use data on continental drift and take into account the ecological balance in each of the areas that were separated and united in the process of continental drift. Significant success along this path in our country was achieved, in particular, by examining in detail the history of the development of the flora of the Siberian Platform.

Let us note here once again that a characteristic feature of the Phanerozoic was a long-term cooling of the global climate, which occurred due to the activity of nitrogen-absorbing bacteria, which constantly reduced the partial pressure of nitrogen, and, consequently, the overall pressure of the earth’s atmosphere. In the Pleistocene, such a general cooling of the climate was superimposed by periods of ice ages associated with self-oscillating cycles of Earth's precession. Periodic glaciations in the Pleistocene, of course, should have affected the Earth's biota, especially the periodic migrations of fauna and flora across the Earth's latitudes. In addition, sharp climate fluctuations could apparently contribute to the exacerbation of evolutionary processes and speciation. Thus, the advance and degradation of glaciers had a significant impact on the distribution of flora and fauna of the northern continents. The problem of extinction of certain species of fauna is very interesting. For example, at the turn of the last phase of the Würm (Valdai) glaciation and the modern interglacial about 12 - 10 thousand years ago, i.e., with the onset of seemingly more favorable climatic conditions, most of the “mammoth” fauna, including the mammoths themselves, died out, tapirs, giant deer, woolly rhinoceroses, cave bears, saber-toothed tigers and many other forms of large and small animals. Humans certainly played a certain role in the extinction of the mammoth fauna, but it is difficult to explain such a radical change of faunas at the end of the last phase of the Würm glaciation by anthropogenic pressure on the composition of the animal world alone. According to opinion, during the development of the last period of glaciation in the regions adjacent to the edge of the ice sheets from the south, the conditions of dry steppes prevailed, and the winters had little snow and the herbivores of the mammoth fauna could easily obtain pasture for themselves. With the onset of the interglacial period about 12 - 10 thousand years ago, atmospheric humidity increased significantly and winters became snowy, making it difficult for herbivores to access pasture. As a result, herbivores died from starvation, and predators died from the lack of herbivores.

It is very likely that glacial conditions also affected the development of mankind. It is possible, for example, that Neanderthals died out about 30–27 thousand years ago, not only because of competition with Cro-Magnons, but also because they could not withstand the cooling of the last phases of the Würm Ice Age. Sharp fluctuations in the global climate at this time determined the moments of migration of peoples and the time of formation of the racial composition of people. Thus, the settlement of America through the Bering Strait most likely occurred along a dry route during a drop in sea level caused by the development of the last Würm glaciation, which lasted approximately from 60 to 12 thousand years ago. This glaciation reached its maximum extent about 20 thousand years ago [Imbri, Imbri, 1988], but approximately at the same time (20–12 thousand years ago), it is assumed that the settlement of the New World by people took place.

Thus, the entire biosphere of the Earth has been developing for over 3.5 billion years as a single whole system, but in close connection with the geological evolution of our planet. Therefore, only a comprehensive study of the biospheres of the past can lead to success based on an analysis of their relationships with the geological settings of ancient eras, taking into account the existing tectonogeochemical boundaries in the development of the Earth, the evolution of the Earth's climate in connection with changes in the angle of its precession, the influence of biota on the pressure and composition of the atmosphere, drift continents, the emergence of new oceans and the closure of old oceans, etc.

In addition to the direct influence of geological processes on the development of earthly life, of course, there is also a feedback relationship - the influence of life on the course of certain geological processes. We gave a striking example of this above in sections 6.3, 6.4 and 8.3, where we showed the decisive influence of nitrogen-absorbing bacteria and plant life forms on the formation of comfortable conditions of the modern climate, which allowed higher forms of life to appear and develop on Earth, including you and me. In addition, starting from the Mesozoic, life played a leading role in maintaining the equilibrium concentration of oxygen in the Earth’s atmosphere, which was not only a determining factor in climate formation on Earth, but also the main factor in the development of highly organized forms of life on Earth.

The large role of organic life in the sedimentogenesis of carbonates, phosphorites, coal-bearing formations, oil and gas deposits and pelagic sediments is well known; its role is also significant in the processes of weathering of earth rocks, and, consequently, in the processes of crustal matter circulation and the origin of minerals (see sections 9.3, 9.5 and 9.6, as well as the work [Sorokhtin et al., 2001]).

15.5. The future development of life and the death of the biosphere

Climate cooling caused by bacterial absorption of atmospheric nitrogen will continue. Therefore, it is difficult to expect significant climate warming in the next 100–200 million years. Let us remember that modern climate warming, which has been talked about a lot lately, began back in the 17th century. (i.e., long before the industrial revolution), most likely it is temporary and associated with fluctuations in the magnetic activity of the Sun (see Fig. 8.26). This is evidenced, in particular, by measurements of paleotemperatures over the past 3000 years based on the remains of planktonic foraminifera of the Sargasso Sea (see Fig. 8.25), which clearly shows that the modern local increase in average temperatures is developing against the background of a general cooling of the climate. Geological data also speaks to this. Indeed, approximately 200–100 million years ago there were no ice sheets on Earth, and average temperatures reached almost +20 °C, while by now it has dropped to +15 °C (see Fig. 8.22). As a result of this seemingly insignificant cooling, the onset of a new glacial era occurred, and the emergence of sheet glaciation in Antarctica in the middle of the Cenozoic, and in Quaternary time - periodic glaciations on the continents of North America, Europe and Asia.

Despite the gradual increase in solar activity, the slow cooling of the climate will continue in the future, until a new equilibrium state of cool climate is reached. However, this new climate level, determined by the metabolism of nitrogen-absorbing microorganisms, may not be very favorable for higher forms of life to flourish on Earth.

According to our estimates, after 250 million years, the average temperature on Earth during the development of glaciations will drop to 0 ° C, although during interglacial stadials the average temperatures will remain positive - about +5 ... + 8 ° C (see Fig. 8.22). Due to the general weakening of the Earth’s tectonic activity, at the same time the level of the World Ocean will drop by approximately 200 m, after which all modern shelves will be exposed, although even in this situation at low and middle latitudes the conditions for the development of highly organized life will remain quite acceptable. Only in about 400 million years will average temperatures on the earth's surface drop to a temperature of about –5 ° C (see Fig. 8.22), and the ocean level compared to its current position at this time will drop by more than 400 m (see Fig. 5.6). In this case, all northern and southern continents, even at moderate latitudes, will be bound by ice sheets, and the elevated areas of the continents at the equator will also be covered with ice.

But the cold snap will not last forever. In about 40 million years, there will have to be an equilibrium between a decrease in temperature due to bacterial removal of nitrogen from the atmosphere and an increase in temperature due to an increase in the luminosity of the Sun. However, this equilibrium state, approximately 600 million years in the future, will have to be sharply disrupted by the degassing of abiogenic oxygen, released during the formation of “nuclear” matter in the mantle according to reaction (3.21). After this point, free oxygen will begin to be generated in the mantle at a rate of about 2.1·1016 g/year, or 21 billion tons/year. If all this oxygen were to enter the atmosphere, its partial pressure would increase at a rate of approximately 4 atm. for every million years. In fact, the rate of oxygen degassing will be much lower, but it can still reach a rate of about 0.02 atm/million years. This means that 200 million years after the start of degassing of oxygen from the mantle, the partial pressure of this gas will reach almost 4 atm, while the average temperature of the Earth due to the greenhouse effect will rise to almost 76 °C. In another 200 million years (a billion years in the future), oxygen pressure will exceed 14 atm, and surface temperatures will rise to 110 °C.

Under such conditions, all terrestrial life, soon after the start of degassing of endogenous oxygen, will literally “burn out” in such an atmosphere. Only in the oceans, due to the low solubility of oxygen in water, will higher forms of life be able to exist for some time, until they too are “cooked” in its hot waters. But after the boiling of the oceans in approximately another 0.8 billion years and the emergence of an irreversible greenhouse effect with temperatures of about 550 ° C, even the most primitive thermophilic prokaryotes will not be able to survive.

From the analysis of the Earth's climate, it is clear that geodynamic conditions favorable for life are not endless. Most likely, the total lifespan of life on Earth is approximately 4.6 billion years (from 4 billion years in the past to 0.6 billion years in the future). For the development of highly organized life, nature has allocated even less time - in the oceans a maximum of 1.1 billion years (from 600 million years in the past to 700 million years in the future), and on land - no more than 1 billion years (from -400 to +600 million years). The lifetime of humanity, if it does not destroy itself first, is the shortest of all other forms of life, theoretically about 600 million years, but in reality it is much less, but how much is unknown.

Our generation, however, need not worry about their future (unless, of course, some kind of nuclear cataclysm occurs), but a sober assessment of the prospects for the development of the organic world on Earth, in our opinion, is not only interesting, but also important in ideological terms. Therefore, we tried here to provide not only retrospective coverage of the development of life on our planet in the past, but also to show the possible, albeit sad, direction of its further development. In the meantime, we should only be glad that we live on our beautiful and unique planet.

The history of life and the history of the Earth are inseparable from each other, since it was in the processes of development of our planet as a cosmic body that certain physical and chemical conditions necessary for the emergence and development of life were laid down.

First of all, it should be noted that life (at least in the form in which it functions on Earth) can exist in a fairly narrow range of temperatures, pressures and radiation. Also, for the emergence of life on Earth, very specific material bases are needed - chemical organogenic elements and, first of all, carbon, since it is this that underlies life. This element has a number of properties that make it indispensable for the formation of living systems. Carbon is capable of forming a variety of organic compounds, the number of which reaches several tens of millions. Among them are structures saturated with water, mobile, low electrical conductivity, and twisted in chains. Compounds of carbon with hydrogen, oxygen, nitrogen, phosphorus, sulfur and iron have good catalytic, construction, energy, information and other properties.

Along with carbon, the “building blocks” of life include oxygen, hydrogen and nitrogen. After all, a living cell consists of 70% oxygen, carbon - 17%, hydrogen - 10%, nitrogen - 3%. Organogenic elements belong to the most stable and widespread chemical elements in the Universe. They easily connect with each other, react and have a low atomic weight. Their compounds are easily soluble in water. These elements apparently arrived on Earth along with cosmic dust, which became material for the “construction” of the planets of the solar system. Even at the stage of planet formation, hydrocarbons and nitrogen compounds arose; in the primary atmospheres of the planets there was a lot of methane, ammonia, water vapor and hydrogen. They, in turn, became the raw material for the production of complex organic substances that make up proteins and nucleic acids (amino acids and nucleotides).

Water plays a huge role in the emergence and functioning of living organisms, because they are 90% water. Therefore, water is not only a medium, but also an obligatory participant in all biochemical processes. Water provides cell metabolism and

thermoregulation of organisms. In addition, the aquatic environment, as a structure unique in its elastic properties, allows all molecules that determine life to realize their spatial organization. Therefore, life originated in water, but even when it came out of the sea onto land, it retained the oceanic environment inside the living cell.

Our planet is rich in water and is located at such a distance from the Sun that the bulk of water necessary for life is in liquid, and not in solid or gaseous form, as on other planets. The Earth maintains an optimal temperature for carbon-based life to exist.

What was ancient life like?

Our knowledge of previously living organisms is limited. After all, billions of individuals representing a wide variety of species disappeared without leaving any traces behind. According to some paleontologists, the remains of only 0.01% of all species of living organisms that inhabited the Earth have reached us in fossil form. Among them are only those organisms that could preserve the structure of their forms through replacement or as a result of the preservation of imprints. All other species simply have not reached us, and we will never be able to learn anything about them.

For a long time it was believed that the age of the oldest imprints of living organisms, which include trilobites and other highly organized aquatic organisms, is 570 million years. Later, traces of much more ancient organisms were found - mineralized filamentous and rounded microorganisms of about a dozen different species, reminiscent of protozoan bacteria and microalgae. The age of these remains, found in the siliceous beds of Western Australia, was estimated at 3.2-3.5 billion years. These organisms apparently had a complex internal structure; they contained chemical elements whose compounds were capable of participating in the process of photosynthesis. These organisms are infinitely complex compared to the most complex known organic compounds of abiogenic origin. There is no doubt that these are not the earliest forms of life and that there were more ancient predecessors.

Thus, the origins of life on Earth go back to that “dark” first billion years of our planet’s existence, which left no trace in its geological record. This point of view is also confirmed by the fact that the well-known biogeochemical carbon cycle associated with photosynthesis stabilized in the biosphere more than 3.8 billion years ago. This allows us to believe that the photoautotrophic biosphere existed on our planet for at least 4 billion years.

years ago. However, according to cytology and molecular biology, photoautotrophic organisms were secondary in the process of evolution of living matter. The autotrophic method of nutrition of living organisms should have been preceded by the heterotrophic method as a simpler one. Autotrophic organisms, which build their bodies using inorganic minerals, have a later origin. This is evidenced by the following facts:

All modern organisms have systems adapted to the use of ready-made organic substances as the initial building material for biosynthesis processes;

The predominant number of species of organisms in the modern biosphere of the Earth can exist only with a constant supply of ready-made organic substances;

In heterotrophic organisms there are no signs or rudimentary remains of those specific enzyme complexes and biochemical reactions that are characteristic of the autotrophic method of nutrition.

Thus, we can conclude about the primacy of the heterotrophic mode of nutrition. The earliest life probably existed as heterotrophic bacteria that received food and energy from organic material of abiogenic origin, formed even earlier, at the cosmic stage of the Earth's evolution. Consequently, the beginning of life as such is pushed back even further, beyond the rock record of the earth's crust, more than 4 billion years ago.

Speaking about the oldest organisms on Earth, it should also be noted that according to the type of their structure, they were prokaryotes, which arose soon after the appearance of the archecell. Unlike eukaryotes, they did not have a formed nucleus, and DNA was located freely in the cell, not separated from the cytoplasm by the nuclear membrane. The differences between prokaryotes and eukaryotes are much deeper than between higher plants and higher animals: both are eukaryotes. Representatives of prokaryotes still live today. These are bacteria and blue-green algae. Obviously, the first organisms that lived in the very harsh conditions of the original Earth were similar to them.

Scientists also have no doubt that the most ancient organisms that inhabited the Earth were anaerobes that received the energy they needed through yeast fermentation. Most modern organisms are aerobic and use oxygen respiration (oxidative processes) as a way to obtain energy.

Thus, V.I. Vernadsky was right when he suggested that life immediately arose in the form of a primitive biosphere. Only

the diversity of species of living organisms could ensure the fulfillment of all functions of living matter in the biosphere. After all, life is a powerful geological force, quite comparable both in energy costs and external effects with such geological processes as mountain building, volcanic eruptions, earthquakes, etc. Life not only exists in its environment, but actively shapes this environment, transforming it “for itself.” We should not forget that the entire face of the modern Earth, all its landscapes, sedimentary and metamorphic rocks (granites, gneisses formed from sedimentary rocks), mineral reserves, and the modern atmosphere are the result of the action of living matter.

These data allowed Vernadsky to argue that from the very beginning of the existence of the biosphere, the life entering it should have been a complex body, and not a homogeneous substance, since the biogeochemical functions of life, due to their diversity and complexity, cannot be associated only with any one form life. Thus, the primary biosphere was initially represented by rich functional diversity. Since organisms do not appear individually, but in a mass effect, the first appearance of life should have occurred not in the form of any one type of organism, but in their totality. In other words, primary biocenoses should have appeared immediately. They consisted of the simplest unicellular organisms, since all, without exception, the functions of living matter in the biosphere can be performed by them.

And finally, it should be said that primary organisms and the biosphere could only exist in water. We have already said above that all organisms on our planet are closely connected with water. It is bound water, which does not lose its basic properties, that is their most important component and makes up 60-99.7% of the weight.

It was in the waters of the primordial ocean that the “primary broth” was formed. After all, sea water itself is a natural solution containing all known chemical elements. It formed first simple and then complex organic compounds, among which were amino acids and nucleotides. It was in this “primordial soup” that the leap took place, giving rise to life on Earth. Of no small importance for the emergence and further development of life was the radioactivity of water, which was then 20-30 times greater than now. Although primordial organisms were much more resistant to radiation than modern ones, mutations in those days occurred much more often, so natural selection was more intense than today.

In addition, we should not forget that the primary atmosphere of the Earth did not contain free oxygen, so it did not have an ozone shield that protects our planet from ultraviolet radiation from the Sun and hard cosmic radiation. For these reasons, life simply could not arise on land; life arose in the primordial ocean, the waters of which served as a sufficient obstacle to these rays.

So, to summarize, it should be noted that the primary organisms that arose on Earth more than 4 billion years ago had the following properties:

They were heterotrophic organisms, i.e. ate ready-made organic compounds accumulated during the cosmic evolution of the Earth;

They were prokaryotes - organisms lacking a formed nucleus;

They were anaerobic organisms using yeast fermentation as an energy source;

They appeared in the form of a primary biosphere, consisting of biocenoses, including various types of single-celled organisms;

They appeared and existed for a long time only in the waters of the primary ocean.

Although life is based on cellular structure, genetic information and its replication and evolution over time, these are not sufficient for life to exist.

Structures and functions create a viable unit only in an environment that can support it. For all its processes, life needs energy. Almost the only source of energy for life on Earth it is the Sun. His energy is even used animals, feeding on plants that need sunlight to grow. However, some bacteria and archaea live off energy, extracted chemically from minerals, but these energy sources are very limited and cannot support the existence of a significant biosphere. Life also needs nutrients - building materials to maintain and reproduce its structure. These are organic compounds, as well as mineral ones, existing in the environment and circulating between organic and inorganic compounds. And life also needs a solvent to dissolve and transport all these chemicals. Here on Earth, the solvent is water, which is also an important component of living organisms.

Without a doubt, water is the most suitable solvent for all biochemical reactions. A water molecule consists of one oxygen atom and two hydrogen atoms connected by covalent bonds; this means that the shared electron pair moves around the oxygen atom and each of the hydrogen atoms

The oxygen atom attracts electrons more strongly than hydrogen, so they are located closer to the oxygen. This causes the oxygen end of the molecule to have a slight negative charge and the hydrogen end to have a slight positive charge: the water molecule is an electric dipole (polar molecule). This feature greatly affects the chemical properties of water. The electrical polarity of water molecules causes a weak electrostatic interaction—hydrogen bonding—between neighboring molecules (see Figure 28.12); this causes the water to behave like an interconnected, loosely connected network. Hydrogen bonds cause molecules to attract each other, turning the liquid slightly sticky or viscous. Because of this “stickiness,” a fairly high temperature and a lot of thermal energy are needed to evaporate water and convert it into gas form. Therefore, water remains liquid over a wide temperature range. This “stickiness” also prevents the water temperature from increasing (increasing the thermal movement of molecules), so this requires a lot of thermal energy. On the other hand, just as much energy is released when water cools; this makes water a very good thermostat, both in the environment and inside the cell.

Fig. Water molecules in the liquid state (left) and in ice (right). Hydrogen bonds are shown by lines

Water readily interacts with other charged molecules; this makes it a very good solvent for all ionic compounds of positively or negatively charged atoms. Water also dissolves polar compounds, when positive and negative charges are in the same molecule, but separately (like water). On the other hand, water cannot dissolve non-polar molecules such as long, uncharged hydrocarbon chains.

This property is very important in biology because it means that these molecules are “hydrophobic,” meaning that in an aqueous solution they tend to associate with each other rather than with water molecules.

A very important type of molecule is lipids (fats). A polar or charged group is attached to one end of such a molecule, making this end hydrophilic, that is, soluble in water. And a non-polar group (for example, a hydrocarbon chain) is attached to the other end, making this end hydrophobic. Such molecules with dual properties are amphiphilic: they assemble in aqueous solution and form bilayer membranes. Hydrophilic and hydrophobic interactions greatly influence the formation of the three-dimensional structure and all other molecules, including proteins, and help them assume the correct functional form.

Due to the attraction of hydrogen bonds and under the influence of surface tension and evaporation, water behaves very well in its environment. Thanks to the capillary effect, it can move against gravity, for example in the vascular system of plants, through which it rises to the crown of tall trees. Water also moves through the capillary channels of the soil, independently rising from the groundwater level to the root system of plants. Hydrogen bonds also affect the density of water at different temperatures in a very special way. As the temperature decreases, hydrogen bonds become stronger and shorter, so that at a temperature of +4 °C water molecules are located closest to each other; At this temperature, water is most dense. With a further decrease in temperature, the molecular configuration begins to change towards weaker hexagonal hydrogen bonds, typical of crystals ice, so the volume of water begins to increase. Low-density ice forms on the surface of water at a temperature of about °C, and denser water with a temperature of +4 °C remains at the bottom of the reservoir. Thus, if the reservoir is deep enough or the frost is not too severe, water with a temperature of +4 ° C can remain in liquid form under the ice crust even during the cold period, which allows you to survive in deep water and not freeze under the ice. This is a very important and very rare property. For example, ammonia, which might be a suitable alternative solvent for life, is heavier in solid form than in liquid form.

This means that an ammonia pond would freeze to the bottom and could remain frozen all the time. Due to the lack of hydrogen bonds, ammonia exists in a liquid state only in a very narrow temperature range and at much lower temperatures than water (between 78 °C and 33 °C at sea level). At these temperatures, all biochemical reactions would proceed very slowly. In addition, ammonia is easily destroyed by ultraviolet light, and its light component, hydrogen, easily flies into space. The sun's ultraviolet rays also destroy water, but this reaction occurs more slowly and produces oxygen (Oa) and ozone (Oe), which blocks ultraviolet radiation and prevents further destruction of water. This is why water exists in large quantities in the atmospheres of Earth-like planets, but ammonia does not.

INTRODUCTION

The origin of life on Earth was the third significant stage in the origin of our universe and the origin of the Earth.

There were many theories and hypotheses about the origin of life on Earth. Among them are the myth about the “act of creation of the world by God,” described in the Bible, the hypotheses of Aristotle, Epicurus and Democritus.

Louis Pasteur's research in the 19th century finally confirmed the fallacy of the idea of ​​the origin of life as a spontaneous spontaneous generation. True, they did not give definitive conclusions about the origin of life.

And only on May 3, 1924, at a meeting of the Russian Botanical Society, scientist A.I. Oparin examined the problem of the origin of life from a new point of view. His report “On the Origin of Life” became the starting point of a new look at the eternal problem of our appearance on Earth. It must be emphasized that, independently of Oparin, the English scientist J. Haldane came to the same conclusions.

What was common in the views of Oparin and Haldane was the explanation of the origin of life as a result of chemical evolution. Both of them emphasized the enormous role of the primordial ocean as a huge chemical laboratory in which the “primordial soup” was formed.

Alexander Ivanovich Oparin

John Burdon Sanderson Haldane

CONDITIONS FOR THE APPEARANCE OF LIFE

The origin of life did not happen on its own, but was accomplished thanks to certain external conditions that had developed by that time. The main condition for the emergence of life is related to the mass and size of our planet. It has been proven that if the mass of a planet is more than 1/20 the mass of the Sun, intense nuclear reactions begin on it.

The next important condition for the emergence of life was the presence of water ( Fig.1). The importance of water for life is exceptional. This is due to its specific thermal properties: huge heat capacity, low thermal conductivity, expansion upon freezing, good properties as a solvent, etc.

Figure 1. Structure of a water molecule and its appearance in an electron microscope

The third element was carbon, which was present on Earth in the form of graphite and carbides. Hydrocarbons were formed from carbides during their interaction with water ( Fig.2).

Figure 2. Hydrocarbon structure

The fourth necessary condition was external energy. Such energy was available on the earth's surface in several forms: radiant energy from the Sun, in particular ultraviolet light, electrical discharges in the atmosphere, and energy from the atomic decay of natural radioactive substances ( Fig.3).

Figure 3. Solar radiation and decay of radioactive matter using the example of Uranium-238

APPEARANCE OF LIVING CREATURES

After its origin, the earth was in such a hot state for a long time that no chemical compounds could exist on it. The first compounds that appeared after the globe cooled were hydrocarbons and ammonia. As a result of chemical transformations of the derivatives of these substances and their interaction with each other in the aquatic environment, carbohydrates, amino acids, fat-like substances and other complex organic compounds were formed. Further interactions between these compounds led to the emergence and association of large molecules into drop-shaped formations, separated from the surrounding aqueous solution, from which they could absorb various substances. In some cases, the absorption of various substances by drops could lead to the disintegration of these drops, in others - to their increase. There remained drops in which a certain relationship was established between the processes of synthesis and decomposition of their constituent substances, the course of which was influenced by various catalysts. Subsequently, the most advanced catalytic substances of a protein nature appeared - enzymes that help accelerate chemical processes and enhance their specificity.

Such systems, which were prototypes of very simple living beings, had a greater chance of surviving than those systems in which processes proceeded more slowly and their interaction with substances in the aqueous solution was less active. Thus, with a certain right, it can be argued that processes reminiscent of those that occur during natural selection played a role in the emergence of the first organisms.

The first organisms were heterotrophs and mainly fed on organic substances that arose without the participation of organisms. The scope of their synthetic processes was still insignificant; They could not create organic substances from inorganic compounds due to the lack of the necessary apparatus for this. There was no free oxygen in the atmosphere surrounding the organisms in question, since it was absorbed during the cooling of the Earth in the process of various reactions. Consequently, in the first organisms, dissimilation occurred according to the type of fermentation with the release of a relatively small amount of energy. This limited their activity and reduced the possibility of synthesizing organic substances formed with the absorption of energy. Their structure was simple; there were probably no special parts that performed specific functions.

When substances similar to proteins arose on Earth, a new stage in the development of matter began - the transition from organic compounds to living beings. Initially, organic substances were found in the seas and oceans in the form of solutions. There was no structure, no structure. But when similar organic compounds were mixed with each other, special semi-liquid, gelatinous formations were released from the solutions - coacervates ( Fig.4). All protein substances in solution were concentrated in them.

Figure 4. Coacervate drop under a microscope

Although the coacervate droplets were liquid, they had a certain internal structure. The particles of matter in them were not arranged randomly, as in a solution, but with a certain pattern. With the formation of coacervates, the rudiments of an organization arose, however, it was still very primitive and unstable ( Fig.5). For the droplet itself, this organization was of great importance. Any coacervate droplet was capable of capturing certain substances from the solution in which it floats. They chemically joined the substances of the droplet itself. Thus, a process of creation and growth took place within her. But in every drop, along with creation, there was also decay. One or another of these processes, depending on the composition and internal structure of the droplet, began to predominate.

Figure 5. Organization of a coacervate droplet

As a result, in some place in the primary ocean, solutions of protein-like substances mixed and coacervate droplets formed. They swam not in pure water, but in a solution of various substances. The droplets captured these substances and grew due to them. The growth rate of individual droplets was different. It depended on the internal structure of each of them.

If decomposition processes prevailed in the droplet, then it disintegrated. The substances that made it went into solution and were absorbed by other droplets. Only those droplets existed for more or less a long time in which the processes of creation prevailed over the processes of decay.

Thus, all randomly arising forms of organization naturally fell out of the process of further evolution of matter.

Each individual droplet could not grow indefinitely as one continuous mass - it disintegrated into daughter droplets. But each droplet was at the same time somehow different from the others and, having separated, grew and changed independently. In the new generation, all unsuccessfully organized droplets died, and the most perfect ones participated in the further evolution of matter. Thus, during the process of the emergence of life, a natural selection of coacervate droplets occurred. The growth of coacervates gradually accelerated. Moreover, scientific data confirm that life did not originate in the open ocean, but in the shelf zone of the sea or in lagoons, where there were the most favorable conditions for the concentration of organic molecules and the formation of complex macromolecular systems.

Ultimately, the improvement of coacervates led to a new form of existence of matter - to the emergence on Earth of the simplest living creatures ( Fig.6). In general, the exceptional diversity of life occurs on a uniform biochemical basis: nucleic acids, proteins, carbohydrates, fats and a few rarer compounds such as phosphates.

Figure 6. The first living creatures. These remains of ancient bacteria are 3.5 billion years old

The basic chemical elements from which life is built are carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus. Obviously, organisms use the simplest and most common elements in the Universe for their structure, which is due to the very nature of these elements. For example, hydrogen, carbon, oxygen and nitrogen atoms are small in size and form stable compounds with double and triple bonds, which increases their reactivity. And the formation of complex polymers, without which the emergence and development of life is generally impossible, is associated with the specific chemical properties of carbon.

Sulfur and phosphorus are present in relatively small quantities, but their role in life is particularly important. The chemical properties of these elements make it possible to form multiple chemical bonds. Sulfur is a component of proteins, and phosphorus is a component of nucleic acids.

For the emergence and development of life on the planet, a number of very general conditions must be met. It is quite obvious that life cannot arise on every planet. A good example is the Moon, which is practically devoid of an atmosphere and completely devoid of a water shell - the hydrosphere. Of course, under such conditions it is impossible to talk about any kind of life on the Moon.

The life activity of any organism is, first of all, a set of various complex chemical processes coordinated with each other. Life can only arise when the planet already has sufficiently complex molecular compounds. The very formation of such compounds, the chemical reactions between them, which ultimately gave rise to living matter, and the vital activity of the organisms formed on the planet require, in particular, suitable temperature conditions. Too high and too low temperatures exclude the possibility of the emergence and development of life. Very sharp temperature fluctuations are equally destructive for the emergence and development of life.

We can imagine around each star that has a planetary system, a region or zone where the temperature conditions on the planets do not exclude the emergence and development of life. It is clear that in sufficient proximity to the star, the temperatures of the planets will be too high for the emergence of life. A good illustration of this is Mercury, the temperature of the part facing the Sun is higher than the melting point of lead. At a sufficiently large distance from the star, the temperature of the planets will be too low. It is not easy to imagine, for example, life on Uranus and Neptune, whose surface temperatures are -200 °C. However, one cannot underestimate the enormous adaptability (“adaptation”) of living organisms to unfavorable environmental conditions. It should also be noted that very high temperatures are much more “dangerous” for the life of organisms than low ones, since the simplest types of viruses and bacteria can, as is known, be in a state of suspended animation at temperatures close to absolute zero.

The temperature of a planet is determined primarily by the amount of radiation from a star falling per unit area of ​​its surface per unit time. For this reason, the sizes of “habitable zones” are different for different stars. They are larger the higher the luminosity of the star, i.e., the “earlier” its spectral class is.

For red dwarfs of spectral class M, as well as late subclasses K, the outer radius of the “habitable zone” becomes very small, less, for example, than the radius of the orbit of “our” Mercury. Therefore, the likelihood that at least one of the planets orbiting such dwarf red stars is within the “habitable zone” is, as one might think, small. It should be noted, however, that the planetary systems surrounding stars may differ significantly in their characteristics from the only planetary system we currently know - our Solar System. In particular, it is possible that planets may orbit red dwarf stars in relatively small orbits.

If we make a very “optimistic” assumption that all main sequence stars with spectral types “later” than F5 and “earlier” than K5 have planets on which life is possible, then it turns out that only 1- -2% of all stars in the Galaxy may be “habitable”. Considering that the number of all stars in our stellar system is about 150 billion, we come to a rather “comforting” conclusion: at least a billion stars in our Galaxy may have planetary systems on which life is, in principle, possible.

However, one more circumstance must be taken into account. As is known, about half of all stars are part of multiple systems. Let's imagine a planet in a double star system. Generally speaking, its orbit will be a rather complex, open curve. Calculating the characteristics of such an orbit is a rather difficult mathematical problem. This is the so-called “restricted” three-body problem. Compared to the general problem of the motion of three bodies mutually attracting according to Newton’s law, the “limited” problem is simpler, since the mass of the planet is negligible compared to the stars and does not affect the motion of the stars.

Moving along its complex orbit, the planet can at times approach one of the stars at short distances, and at times move very far away from the stars. In accordance with this, the temperature of the planet's surface will change within limits unacceptable for the emergence and development of life. Therefore, at first it was believed that there could not be habitable planets near multiple stars. But over 30 years ago, Su Shuhuang reconsidered this issue and showed that in some cases there may be such a movement of planets in periodic orbits, in which the temperature of their surfaces changes within acceptable limits for the development of life. To do this, it is necessary that the relative orbits of the stars be close to circular.

Periodic orbits of planets that allow the development of life lie either inside the surface passing through L1 or outside the surface passing through L2. If the masses of both stars are the same, then inside the surface passing through L1, orbits suitable for the development of life will exist provided that the distance between the stars a > 2l1/2 (a expressed in astronomical units), where l is the luminosity of each stars (in units of solar luminosity). When a becomes greater than 13l1/2, each of the components of the binary system can be considered for the problem of interest to us as a single star.

Note that for many binary systems the distance between the components exceeds this “critical” value. Consequently, in principle, around components of a binary system that are sufficiently distant from each other and moving in an almost circular orbit, the presence of habitable planets is possible. In the case where the components of the binary system are sufficiently close to each other, suitable periodic orbits may be outside the surface passing through L2. As Su Shuhuang's calculations show, with equal masses, the components of a binary orbital system suitable for the emergence and development of life can be provided that a< 0,4l1/2. Таким образом, в области значений 2l1/2 >a > 0.4l1/2 excludes the possibility of the existence of habitable planets.

Similar results can be obtained by calculation for the more general case when the masses of the components of the binary system are unequal. Thus, we must conclude that in multiple star systems, in principle, there may be planets whose temperature conditions do not exclude the possibility of the emergence and development of life. It should be noted, however, that the probability of the existence of such planets near single stars is much higher. However, it is possible that the formation of multiple stars and planets are processes that are mutually exclusive.

To estimate the number of stars in the Galaxy around which habitable planets can be assumed to orbit, taking into account multiple stars is, of course, not of serious importance, since we can hardly roughly estimate only the order of this quantity. In such calculations, the coefficient 1.5 - 2 does not play a role. It's a different matter when it comes to the probability of the existence of habitable planets in some completely specific multiple system, which for one reason or another is of interest to us. For example, one of the nearest stars, Centauri, is a multiple system. Naturally, the question of the possible presence of habitable planets in this system is of particular interest to us.

Centauri is a triple system. The relative orbit of the two most massive components of this system is an ellipse with a semimajor axis equal to 23.4 astronomical units and a fairly significant eccentricity: 0.52. Thus, the distance between the two main components is large enough for suitable planetary periodic orbits to exist around each (see above). However, the large magnitude of the eccentricity of stellar orbits requires special consideration for this case (recall that the results of Su Shuhuang's calculations refer to the case of circular orbits of the components of a binary system). It should be noted, however, that the a Centauri system is apparently relatively young. The stars included in it may not have yet “sat down” on the main sequence. Therefore, it is unlikely that there could be planets there even with primitive life forms.

As has already been emphasized, for the development of life on any planet, it is necessary that the temperature of the latter be within certain acceptable limits. This requirement determines the size and the very presence of “habitable zones.” In addition, it is necessary that the radiation of the star remains approximately constant over many hundreds of millions and even billions of years. For example, a large class of variable stars whose luminosities vary greatly with time (often periodically) should be excluded from consideration.

However, the vast majority of main sequence stars radiate with surprising consistency. For example, according to geological data, the luminosity of our Sun has remained constant over the past few billion years with an accuracy of several tens of percent. Apparently, such constancy of luminosity is a general property of most main sequence stars. Thus, the important condition for the constancy of the luminosity of a star - the center of a planetary system - is satisfied in almost all cases, at least when we are talking about stars with a mass close to the Sun.

We have examined in some detail the temperature conditions under which the emergence and development of life on a particular planet is possible, but these conditions, of course, are not the only ones. The mass of the planet formed in any way and the chemical composition of its atmosphere are very important for the problem we are considering. Apparently, these two initial characteristics of the planet are not independent. Let us first consider the case when the mass of the formed planet is small. Molecules and atoms in the upper layers of the atomosphere, where its density is low, move at different speeds. Some of them have a speed exceeding the “second cosmic velocity” (astronomers call this speed “parabolic”), and will freely go beyond the planet. This process, which somewhat resembles evaporation, is called “dissipation.” Obviously, effective dissipation can occur where the density of the atmosphere is so low that the “escaping” atoms no longer experience collisions with other atoms. If such collisions took place, they could change the magnitude and direction of the speed of the escaping atoms, which would prevent dissipation.

The dissipation of planetary atmospheres occurs continuously, since there is always a certain number of molecules (atoms) that, at a given atmospheric temperature, have velocities directed “upwards” and exceeding the parabolic one. However, for different gases the proportion of dissipating particles will be different. Most of all it is for light gases - hydrogen and helium. It goes without saying that the number of dissipating particles depends, and, moreover, very sensitively, on the temperature of the atmosphere at those altitudes where dissipation occurs.

Thus, in order for life to arise and develop on a planet, its mass should not be too small. On the other hand, too much mass of the planet is also an unfavorable factor. Planets whose masses are large enough (for example, close to the masses of the giant planets Jupiter and Saturn) completely retain their original atmosphere. This “primordial” atmosphere must have been very rich in hydrogen, since the original environment from which the planets formed had approximately the same chemical composition as stars, which are mainly composed of hydrogen and helium.