Development of the biosphere. Organisms that require little oxygen, but are able to survive for a long time at its concentrations below the Pasteur point, are called facultative anaerobes Pasteur point and its significance

But let’s return to the question of aerobic and anaerobic, oxygen and oxygen-free lifestyle. As we already know, microorganisms use many more ways to obtain energy than higher organisms. Let's look at photosynthesis and respiration first. Both of these processes are aerobic in the truest sense of the word: they are possible only in contact with the atmosphere. But photosynthesis can be both oxygen-free and oxygen-free, while respiration is necessarily an oxygen process.

However, microorganisms also know other ways to extract energy from the environment. The most famous of these methods is fermentation. The variety of fermentation options is amazing, but the result is always the same: an electron is gradually transferred by the body from an electron donor to an electron acceptor. The energy released during this transfer is used by the body in vital processes *.

* (In this case, the substance from which the electron is removed is oxidized, and the substance accepting the electron is reduced. Therefore, such reactions are called “exergonic redox reactions.” Because they do not use radiant energy sunlight, they are also called “dark”. In respiration, the final electron acceptor is free oxygen, which is reduced by accepting an electron and forms water with hydrogen. But microorganisms can use many other compounds, both organic and inorganic, as their final electron acceptor.)

According to Stanier et al. , the following main types of microbial metabolism can be distinguished:

1) using radiant energy (sunlight) - organic photosynthesis;

2) using redox reactions that provide energy (so-called “dark” reactions): a) respiration (the final electron acceptor is free oxygen); b) anaerobic respiration (ultimate electron acceptor - other inorganic substance); c) fermentation (the final electron acceptor is organic matter).

This diversity of metabolic processes characteristic of microorganisms leads to the “smearing” of the transition zone between oxygen and oxygen-free life. Instead of two clearly demarcated groups of organisms - those unable to live without oxygen (the so-called obligate aerobes) and those unable to live in the presence of oxygen (obligate anaerobes) - among microorganisms we find many forms capable of living in both oxygen and oxygen-free environments. They are called facultative anaerobes. About such forms of early life it would be better to say that they had the ability of facultative respiration.

Yeasts and the vast majority of bacteria belong to facultative anaerobes. When the oxygen content drops below a certain level, they switch to fermentation; above this level they exist due to respiration. The transition point is called the Pasteur point.

This transition in facultative anaerobes is quite clearly expressed. The Pasteur point corresponds to an oxygen content of about 0.01 of its content in the modern atmosphere (personal communication from L. Berkner and L. Marshall, 1966) *.

* (In the modern atmosphere, the partial pressure of free oxygen can be taken to be 159 mmHg. Art. Then the Pasteur point corresponds to a pressure of 1.59 mm Hg. Art.)

The Pasteur point should not be confused with the level of free oxygen below which strictly aerobic organisms cannot breathe. This level is designated P50. Its values ​​for different vertebrates are shown in Table. eleven.

At an O 2 pressure corresponding to the Pasteur point, organisms capable of facultative respiration receive significant advantages over those that are unable to live in the presence of oxygen. After all, the breathing process provides much more energy than fermentation. This is explained by the fact that the final product of fermentation is an organic substance that has accepted an electron, whereas in respiration free oxygen serves as an electron acceptor. organic matter, which is the final product of fermentation, is completely oxidized during respiration, providing an additional large amount of energy. As can be seen from the equations given here, the energy output of respiration is more than ten times the energy output of fermentation.

Physical processes, developing in the atmosphere, determine the weather conditions on earth's surface and are one of main reasons that shape the planet's climate. These include processes of changes in the composition and circulation of the atmosphere, affecting the absorption of solar radiation and the formation of long-wave radiation fluxes.

The spherical shape of the Earth contributes to different absorption of solar radiation by the earth's surface. The greatest amount of it is absorbed in low latitudes, where the air temperature at the earth's surface is much higher than in middle and high latitudes. Although this temperature difference existed throughout the entire geological history of the planet, it was at different time intervals in quantitatively she expressed herself in different ways.

The main planetary factors that affect the distribution of temperature are the variability of the composition of the atmosphere, the different areas of land and sea, and the location of the continents on the earth's sphere. The temperature difference between equatorial and polar latitudes was the main reason causing the movement of air masses in the atmosphere and water masses in the seas and oceans. When there is a significant difference between temperatures in the polar and equatorial latitudes, as, for example, in the modern era or during the largest glaciations, large horizontal and vertical movements of water and air masses arose.

Among the components of the atmosphere, the greatest influence on the temperature regime of the Earth is exerted by the so-called thermodynamically active impurities. They are carbon dioxide, aerosol and water vapor. Although the concentration of oxygen does not have an effective direct effect on the temperature regime, its influence on the development of the organic world and the creation of the ozone screen is very large. Atmospheric composition has a direct impact on the development of biological processes. Highest value

For the development of biological processes, as is known, oxygen, ozone and carbon dioxide are present.

Without touching on the peculiarities of the emergence of the atmosphere and the evolution of its composition in the Archean and Proterozoic, we will consider only the main trends in changes in the composition of the atmosphere during the Phanerozoic history of the Earth. According to L. Berkner and L. Marshall, in the Vendian era the so-called Pasteur point was reached, when the oxygen content in the atmosphere increased to 0.01% compared to modern levels. This point is a very important milestone for the development of organisms. At Pasteur's point, a number of microorganisms move to oxidative reactions during respiration and during metabolism, approximately 30-50 times more energy is released than during anaerobic (enzymatic) fermentation. L. Berkner and L. Marshall believe that when the Pasteur point was reached, the effectiveness of the ozone screen increased. The emergence of the ozone screen limited the supply of the shortest wavelengths to the earth's surface ultraviolet radiation. A small dose of ultraviolet radiation arriving at the earth's surface was no longer able to penetrate, as before, through the entire thickness of water. This opened up the possibility of populating the vast seas and oceans with organisms. Relatively quickly, already at the beginning of the Cambrian, many groups of invertebrates appeared and quickly settled.

Subsequently, according to Berkner and Marshall, approximately at the border between the Silurian and Devonian, i.e. about 400 million years ago, the oxygen content further enhanced the effectiveness of the ozone screen, which absorbed part of the ultraviolet solar radiation harmful to living organisms and the surface of the seas and sushi became safe for organisms. It was at this time that primitive plant forms emerged onto land, the appearance of phyto- and zooplankton, and the relatively rapid occupation of land expanses by vegetation and terrestrial organisms.

Current level of oxygen in the atmosphere was achieved approximately in the middle of the Devonian, as a result of rapid photosynthesis that occurred in primitive forests. According to a number of researchers, the current level of oxygen in the atmosphere was even surpassed at the beginning of the Carboniferous, when vegetation flourished rapidly. Massive death of vegetation at the end of the Early Carboniferous caused a reduction in oxygen concentrations. Not all researchers agree with the point of view of Berkner and Marshall. According to M. Rutten, the oxygen content in the atmosphere has reached Pasteur points not in the Vendian, but much earlier, in the middle Archean, and a 10% oxygen level in the Late Riphean.

Currently, the main reservoirs of oxygen are sedimentary rocks, which contain about 6-1022 g of oxygen, in ocean waters it contains 1.4-1024 g, in the atmosphere - 1.2-1021 g, and in the biosphere - about 1019 g of oxygen. All of these oxygen reservoirs are interconnected through the atmosphere. Moreover, the greatest supply of oxygen into the atmosphere occurs through the biosphere. The annual production of oxygen in the biosphere is approximately (1... 1.5) -1017 g. Such high productivity makes it possible to believe that all the oxygen in the atmosphere could have been created over several tens of millennia. In fact, during this period of time, the oxygen in the atmosphere is completely renewed. The oxygen produced by the biosphere is spent on respiration, on the oxidation of organic substances and volcanic gases, and is spent on weathering. In particular, during weathering, 3-10 g of oxygen are consumed annually. This means that in the complete absence of oxygen sources, all atmospheric oxygen would be spent on weathering and oxidation of minerals in just 4-106 years.

Based on modern reviews, the main source of oxygen in the atmosphere was the process of photosynthesis. M.I. Budyko and A.B. Ronov calculated that 7.3-1021 g accumulated in Phanerozoic sediments organic carbon and 19-1021 g of oxygen were formed. These data refer only to the continents, but the actual supply of oxygen must be large, since photosynthesis in the oceans was not taken into account in this case. According to the authors’ calculations, it turned out that over 90% of the total supply of oxygen in the Phanerozoic was spent on the oxidation of mineral compounds and weathering. The rate of this consumption during Phanerozoic history was uneven; it completely depended on the mass of atmospheric oxygen. Knowing the total amount of organic carbon at certain time intervals, the oxygen consumption for oxidation and the value of the calculated proportionality coefficient allowed Budyko and Ronov to calculate the change in the amount of oxygen in the atmosphere during the Phanerozoic (7.3).

The mass of atmospheric oxygen at the beginning of the Phanerozoic, according to this calculation, was 1/3 of the modern content. During the Phanerozoic, the amount of oxygen grew unevenly. The first sharp increase in the oxygen content in the atmosphere occurred in the Devonian and Carboniferous, when its total amount reached the modern level. At the end of the Paleozoic, the mass of oxygen decreased, reaching at the beginning of the Triassic or at the end of the Permian the values ​​characteristic of the early Paleozoic. In the middle of the Mesozoic, a new increase in the amount of oxygen occurred, which was later replaced by a slow decrease.

The calculated oxygen content in the atmosphere is in good agreement with a number of geological facts. Thus, the increase in oxygen in the Devonian and Carboniferous coincided with the highest flourishing of vegetation, its occupation of land expanses and a significant increase in phytoplankton productivity in the seas and oceans. The development of arid conditions in the Permian and Triassic led to a reduction in plant mass, and in connection with this, oxygen consumption for oxidation processes sharply increased. This in turn contributed to a decrease in the total amount of atmospheric oxygen.

The most important role in climate formation also belongs to another thermodynamic active impurity - carbon dioxide, which creates Greenhouse effect. During certain periods of the Earth’s geological history, the concentration carbon dioxide in the atmosphere significantly exceeded the modern one, but over time there was a gradual removal of carbon dioxide from the atmosphere.

Carbon dioxide was removed from the atmosphere and hydrosphere as a result of the formation of carbonates, both chemogenic and organogenic.

A. B. Ronov, based on the distribution and thickness of carbonate and carbon-containing rocks within modern continents, calculated the consumption of carbon dioxide during various periods of the Phanerozoic. These data show that the volumes of carbonates do not change monotonically, but experience significant fluctuations.

Using data on the amount of carbon dioxide concentrated in sedimentary rocks during the Cenozoic era, the authors found that 0.05-1021 g of CO2 accumulated over the last million years. Based on the current content of carbon dioxide in the atmosphere (0.03%), a proportionality coefficient was determined, thanks to which the concentration of carbon dioxide was obtained during various periods of the Phanerozoic in the Earth’s atmosphere (7.4).

Carbon dioxide content in the atmosphere throughout the Phanerozoic it varied unevenly from 0.4 to 0.03%. The first significant decrease in the total amount of carbon dioxide in the atmosphere occurred at the end of the Early Paleozoic. At the end of the Mesozoic era, a gradual decrease in carbon dioxide concentration began. The last significant decrease in CO2 concentration in the atmosphere occurred in the Pliocene. In the modern era, the content of carbon dioxide in the atmosphere has reached its lowest value in the entire history of the Earth.

Oscillations volcanic activity influence the amount of carbon dioxide in the atmosphere. The concentration of carbon dioxide in the atmosphere changed consistently with the change in the level of volcanic activity; the maximum activity was accompanied by an increase in CO2.

The content of water vapor in the modern atmosphere is 0.23% and its role in creating the greenhouse effect is very large. The saturation concentration of water vapor in the atmosphere increases with increasing temperature. The more water vapor in the atmosphere, the stronger the greenhouse effect, the higher the temperature. At present, it is not possible to obtain, at least indirectly, data on the amount of water vapor in certain eras of the Phanerozoic. To some extent, the relatively high content of water vapor may be evidenced by known periods of climate humidization.

However, there are organisms that do not need oxygen at all to live. They were first discovered by French biologist Louis Pasteur in 1861. They turned out to be bacteria from the genus Clostridium of the bacilli family, which carry out butyric acid fermentation. This caused a real sensation in science, since it was previously believed that life without oxygen was impossible. Pasteur called such organisms anaerobic.

Oxygen is a deadly poisonous gas for anaerobes. Pasteur established that they are able to live only in those environments where the oxygen content does not exceed 1% of its modern content in the atmosphere, i.e. less than 0.21% of the air volume ( Pasteur point). Such conditions occur in deep layers earth's crust, bottom sediments of reservoirs, in the earth's crust, in the internal cavities of organisms, etc.

Anaerobic organisms lack mitochondria, and energy production processes occur in the cytoplasm. From a biochemical point of view, it is more correct to call these processes not anaerobic respiration, but fermentation.

One form of fermentation is alcohol fermentation, or the breakdown of glucose into ethyl alcohol and carbon dioxide:

C 6 H 12 0 6 → 2 C 2 H 5 OH + 2 CO 2

Another type of fermentation is lactic acid fermentation, or the breakdown of a glucose molecule into two lactic acid molecules:

C 6 H 12 0 6 → 2 C 3 H 6 O 3

In both cases, the breakdown of one glucose molecule produces only 2 ATP molecules, instead of 38 with aerobic respiration.

Subsequently, anaerobic microorganisms in relation to oxygen, anaerobes were divided into two groups. Microorganisms that are able to exist at concentrations only at oxygen concentrations below the Pasteur point are called obligate anaerobes.

A very important indicator characterizing the oxygen conditions of the environment is redox potential(rH 2), which is the ratio between O 2 and H 2 content. It is determined electrometrically using a potentiometer. The unit of measurement for rH 2 is the volt. The rH 2 values ​​vary from 0 to 40 volts. The Pasteur point corresponds to rH 2 = 14 volts.

Aerobic microorganisms are able to exist at rH 2 from 14 to 40. Obligate anaerobes survive at rH 2 from 0 to 14. Facultative anaerobes exist at rH 2 from 7.4 to 20.

Currently, more than 2000 species of obligate anaerobes are known. The absolute majority are prokaryotic microorganisms from different taxonomic groups. One order of prokaryotes, Chladymia, is represented exclusively by obligate anaerobes.

At the very Lately anaerobic bacteria were discovered that live in oil fields at depths of up to 700 m, in conditions of complete absence of oxygen. They obtain energy through decomposition organic compounds oil. Another discovery of this kind is the discovery of bacteria living at depths of up to 3 km in the thickness of calcareous rocks. They are named lithotrophic bacteria.

There are many obligate anaerobes among chemoautotrophic organisms. Among photoautotrophic prokaryotes (cyanobacteria and a number of bacteria), anaerobic forms are unknown. There are no anaerobes among nitrogen-fixing bacteria.

All free-living protists are strictly aerobic forms.

All multicellular eukaryotic organisms are aerobic. However, many species of fungi and animals are, to one degree or another, capable of facultative anaerobiosis, since they are able to survive for a certain period of time in conditions of lack of oxygen, obtaining energy for their vital functions through fermentation.

Among fungi, an example of this is yeast. Under anaerobic conditions, such as in dough, yeast proceeds to fermentation, which is used in cooking.

Even individual tissues of multicellular animals differ in their ability to undergo anaerobiosis. Muscle tissue has a high capacity for anaerobiosis. During intensive tower work, animals often lack oxygen. Then the muscle protein glycogen is broken down into lactic acid, which has toxic properties. It accumulates in the muscles, causing a feeling of muscle fatigue. From here, after intense work, organisms breathe intensely in order to quickly oxidize lactic acid to carbon dioxide and water.

During daylight hours, plants are little sensitive to oxygen deficiency in the external environment, since they themselves produce it during photosynthesis. In the dark, on the contrary, they become very sensitive to a decrease in its concentration. Therefore, in reservoirs where the oxygen concentration is very low, plants, like other autotrophic organisms, are absent or in a depressed state.

On the other hand, the roots of terrestrial plants stop growing in soil oversaturated with water. However, it is not entirely clear whether this is caused by a lack of oxygen or an excess of methane and hydrogen sulfide released by anaerobic bacteria.

Anaerobic organisms appeared on Earth much earlier than aerobic organisms and dominated it for a long time. However, with the advent of aerobic organisms, anaerobic organisms could not compete with them due to the very low efficiency of their energy processes. Therefore, they were preserved only in those places where there is no oxygen, in which aerobic organisms are not able to survive. Although not all types of anaerobes have yet been discovered, their total number is undoubtedly significantly lower than that of aerobic organisms.

In general, terrestrial animals very rarely experience a lack of oxygen for breathing. Therefore, terrestrial organisms living at low altitudes (up to 3 km) do not have special physiological adaptations to existence with a lack of oxygen.

With altitude, the partial pressure of oxygen in the atmosphere decreases, so organisms develop oxygen starvation" In high-altitude mammals, an increase in the content of erythrocytes in the blood was noted, and the erythrocytes themselves were larger. This increases the oxygen capacity of the blood.

The decrease in oxygen with altitude is a serious limiting factor that limits the spread of living organisms in the mountains. The highest of the major cities is the capital of Tibet, Lhasa, which is located at an altitude of about 4500 m.

A small lentil bird from the order Passeriformes builds nests in the Himalayas at altitudes of up to 6000 m. Climbers at altitudes above 7000 m cannot stay for more than several hours, so when climbing to high altitudes they must use oxygen devices.

However, the influence of oxygen levels on the distribution of invertebrates and plants in mountains is often very difficult to isolate against the background of other factors, in particular snow depth and temperature.

Summer deaths are explained by increased temperatures and increased concentrations of organic substances in water, since oxygen is spent on their oxidation. The most severe kills are observed in shallow stagnant lakes and ponds.

A decrease in the concentration of oxygen in water as a result of its consumption for the oxidation of organic substances located in the water column is called biochemical oxygen consumption (BOD).

Many species of aquatic invertebrates and fish, especially those living in the bottom layers of water and bottom sediments, where the lack of oxygen is most severe, have a number of adaptive mechanisms that allow them to exist for some time in the almost complete absence of oxygen. Their bodies have acquired higher resistance to increased levels of lactic acid. The lower the temperature, the longer these organisms can survive without oxygen.

In the waters of the seas and oceans, the oxygen content is on average lower than in continental waters, averaging 4 – 5 mg/l. As a rule, the oxygen concentration at depths is slightly lower than at the surface.

In general, in marine reservoirs, the water in which is constantly mixed as a result of horizontal and vertical currents, the oxygen content varies within much narrower limits than in continental ones. Therefore, in the seas and oceans, oxygen deficiency in water is very rare.

Light factor. The main source of light in the Earth's Biosphere is the Sun. The Moon and a number of nearby planets visible in the sky solar system shine with reflected light from the Sun. Electromagnetic radiation stars and artificial sources light in the Biosphere of great importance Dont Have. However, it is assumed that birds during their seasonal migrations can navigate by the constellations of the starry sky.

The following are important for an ecologist: quantitative characteristics Sveta:

– wavelength;

– luminous flux intensity(the amount of radiation energy received per unit time per unit area);

– photoperiod(the relationship between the light and dark phases of the day).

The human eye perceives electromagnetic waves (visible light) in a very narrow range - from 3900 Å (blue light) to 7600 Å (red). Radiation with a lower wavelength - UV, X-ray and gamma radiation, and a higher wavelength - infrared radiation, radio waves, etc., is not perceived by the human eye. However, some insects are able to see ultraviolet color, and many nocturnal animals are able to see infrared (thermal) radiation emanating from objects whose temperature is higher than the ambient temperature.

Green plants use waves in the range " photosynthetically active radiation"(PAR) from 3800 to 7100 Å.

Prokaryotes have photosynthetic pigments that use radiation energy outside the PAR range, namely waves with a length of 8000, 8500 and 8700 - 8900 Å. Overall, PAR accounts for about 44% of the solar radiant energy incident on the Earth's surface.

Maximum efficiency the use of PAR for photosynthesis is no more than 3 - 4.5%. It was observed in a seaweed culture under twilight lighting. In tropical forests this value is 1 - 3%, in temperate forests - 0.6 - 1.2%, in agricultural crops - no more than 0.6%.

Light intensity is important for the rate of photosynthesis, or the amount of organic matter produced per unit time. U different types In photosynthetic organisms, the maximum rate of photosynthesis is achieved when different sizes luminous flux intensity. For example, sun-loving grasses achieve maximum photosynthesis at a higher level of light intensity than aquatic diatoms. Based on this characteristic, plants are divided into light- And shade-loving.

However, in all species, in very light, the intensity of photosynthesis sharply decreases.

Sveta full moon in a cloudless sky it is quite enough for photosynthesis to occur in higher plants. The distribution of plants into the depths of the reservoir is determined by the depth of light penetration. The latter, in turn, depends on the content of dissolved and suspended substances in it. In the clean waters of the World Ocean, light penetrates to a depth of 200 m. In clean freshwater lakes, light can penetrate to a depth of 60-70 m (Baikal). In Lake Naroch, this figure is currently 6–8 m. In polluted reservoirs, light penetrates to a depth of several meters to several centimeters.

The depth of light penetration into water can be determined with probably the simplest of scientific instruments - Secchi disk. It is a white metal disk lowered onto a rope into the water. On the first scientific oceanographic vessel in history - English ship"Challenger" used a white porcelain plate for this purpose. Now the depth of light penetration and illumination at different depths can be measured with very high accuracy using lux meter.

A significant part (up to half) of the organic matter created by plants during photosynthesis is immediately spent on their respiration. Therefore, a plant can only exist in light conditions under which the amount of organic matter created during photosynthesis will exceed, or at least be equal to, the amount used for respiration.

There are a number of autotrophic species of protists from the subphylum of plant flagellates capable of bioluminescence. An example of this is the usual overnight stay in the Black Sea. Noctiluca mirabilis. At night, its clusters produce enough light for their photosynthesis process.

Heterotrophic organisms with visual organs use visible light for orientation in space. Some nocturnal organisms are also able to perceive infrared radiation, and insects - ultraviolet radiation.

Some species, especially cave, underground and deep-sea species, do without light at all. However, most heterotrophic organisms require a certain amount of light, for example, to produce vitamins and other substances in the skin.

The annual pattern of changes in the ratio between the light (C) and dark (T) phases of the day ( photoperiod) is subject to strict laws, which is determined by the rotation of the Earth around the Sun.

At the equator, the photoperiod is strictly constant throughout the year and amounts to 12C: 12T. With promotion to more high latitudes in directions towards both poles, the photoperiod changes naturally.

In the Northern Hemisphere, up to a latitude of approximately 67°N. length of daylight hours is minimal on December 22 ( winter solstice), then it constantly increases. On March 22 (spring equinox), day and night are equal throughout the planet.

Day length reaches its maximum on June 22 (summer solstice). The higher the latitude, the longer the daylight hours. For example, in Minsk (54°N) on June 22 the photoperiod is approximately 17C:7T, and in St. Petersburg (60°N) – 22C:2T ( White Nights).

The higher the geographic latitude, the faster the daylight hours increase and the night decreases.

In polar latitudes (over 67°N), approximately between May 22 and August 22, the Sun does not set beyond the horizon at all; comes polar day, i.e. photoperiod 24C: 0T.

After June 22, the duration of daylight everywhere except the equator decreases and reaches a minimum on December 22. In Minsk on this day the photoperiod is approximately 7C:17T.

In polar latitudes, approximately between November 22 and January 22, the Sun does not rise above the horizon at all; comes polar night, i.e. photoperiod 0C: 24T.

The duration of daylight hours every day at a certain point on the globe is strictly constant, unlike other important environmental factors– temperature, amount of precipitation, etc. Therefore, for many organisms, especially birds, photoperiod is a signaling factor in many of the most important stages of their life cycle, for example, the beginning of reproduction, birds leaving for the winter, etc.

Availability of water and humidity. All organisms require water because it is the main component of the cytoplasm of their cells. Therefore, living organisms consist of 60–99% water.

Water is used for photosynthesis.

Water is one of the main habitats. A number of types of living organisms consist exclusively or almost exclusively of aquatic species (echinoderms, fish). Many other species are associated with water at certain stages of their life cycle (amphibians, semi-aquatics, insects).

Many organisms have adapted to living in conditions of water scarcity. Plants living in arid zones store water in their tissues ( succulents). Their most famous example are cacti.

Many animals, in case of lack of water, use metabolic water, obtained by oxidizing the fat reserves in their bodies. These include many insects that have fat reserves in fat body, as well as some desert-dwelling mammal species such as camels and mouse-like rodents.

In this case, the amount of water produced exceeds the amount of fat broken down, since almost all the oxygen in metabolic water is obtained from atmospheric oxygen.

The age of the Earth is about 4.6–4.7 billion years. The composition of the ancient atmosphere is considered close to the composition of gases released from modern volcanoes. Chemical analysis of gas bubbles in the oldest rocks of the Earth showed complete absence of free oxygen in them, about 60% CO 2 about 35% H 2 S, SO 2, NH 3, HCl and HF, some nitrogen and inert gases. Currently, there is already quite a lot of indisputable evidence that the early atmosphere of the Earth was oxygen-free. Life that arose on Earth gradually changed these conditions and transformed the chemistry of the upper shells of the planet.

The history of the Earth is divided into three large segments: archaea – the first approximately two billion years of its existence, Proterozoic – the next 2 billion years and phanerozoic, which began about 570 million years ago. Pre-Phanerozoic time is called cryptozoan, that is, an era of hidden life, since ancient rocks do not contain skeletal imprints of macrofossils.

Until recently, it was believed that the emergence of life on Earth was preceded by a very long (billions of years) chemical evolution, including spontaneous synthesis and polymerization of organic molecules, their integration into complex systems that precede cells, the gradual establishment of metabolism, etc. Possibility and ease of occurrence abiogenic synthesis of organic monomers under conditions simulating the atmosphere ancient earth, was convincingly proven back in the 50s in many laboratories around the world, starting with the famous experiments of S. Miller and G. Urey. However, the path from simple organic molecules to the simplest living cells with the ability to reproduce and the apparatus of heredity was considered very long. In addition, the ancient rocks seemed lifeless. With the development of sophisticated methods for studying organic molecules contained in Archean and Proterozoic rocks, as well as the remains of microscopic cellular structures, this opinion has changed. One of the most amazing paleontological discoveries of recent decades is the registration of traces of life even in the most ancient rocks of the earth's crust. Consequently, the evolution from organic compounds to living cells took place in a very short time, at the very beginning of the history of the Earth. Photosynthetic organisms also appeared very early. Rocks dating back 3.8 billion years already indicate the presence of cyanobacteria (blue-green algae) on Earth, and therefore the existence of photosynthesis and the biogenic release of molecular oxygen. At the border of the Archean and Proterozoic, cyanobacteria were already represented by a rich set of forms similar to modern ones. Along with the fossil remains of blue-green cells, traces of their large-scale geological activity were found in the Archean layers - rocks composed of stromatolites. These characteristic banded and columnar fossils arise from the functioning of cyanobacterial communities where photosynthetic blue-greens and a range of other species of bacteria, decomposers and chemosynthetics are closely spatially integrated. Each colony, therefore, represents a separate ecosystem in which the processes of synthesis and decay of organic matter are coupled. Modern stromatolites arise only in extremely extreme conditions - in oversaline or hot waters, where there is no more highly organized life.

Thus, we can assume that already in the middle Archaea life on Earth was represented by various types of prokaryotes, beginning to influence it geological history. In a reducing environment, the oxygen released by cyanobacteria was first spent on the oxidation of various compounds and did not accumulate in free form in the atmosphere. In this case, ammonia was oxidized to molecular nitrogen, methane and carbon monoxide - to CO 2, sulfur and hydrogen sulfide - to SO 2 and SO 3. The composition of the atmosphere gradually changed.

The development of life took place against the background of the geological development of the planet. In the Archean, due to chemical and physical weathering and erosion of land, the formation of the first sedimentary rocks in the ocean began, their granitization occurred and the cores of future continental platforms were formed. According to some assumptions, at the beginning of the Proterozoic they formed a single continent, called Megagay, and were surrounded by a single ocean.

The tectonic activity of the Earth, as shown by the age of igneous rocks, is not constant over time. Short periods of increased activity alternate with longer periods of rest. This cycle takes up to 150–500 million years. In the history of the planet, geologists count 19 tectono-magmatic epochs, four of which occurred in the Phanerozoic and 15 in the Cryptozoic. As a result, there was an increase in the heterogeneity of the earth's crust. Increased volcanism, mountain-building processes, or, conversely, subsidence of platforms changed the area of ​​shallow waters and the conditions for the development of life. On Earth, climatic zonality either weakened or intensified. Traces of ancient glaciations have been known since the Archean era.

It is believed that early life first had a local distribution and could exist only at shallow depths in the ocean, from about 10 to 50 m. The upper layers, up to 10 m, were penetrated by destructive ultraviolet rays, and below 50 m there was not enough light for photosynthesis. The salts of the ancient ocean had a higher content of magnesium compared to calcium, in accordance with the composition of the rocks of the primary earth's crust. In this regard, one of the main sedimentary rocks of the Archean are magnesium-containing dolomites. Sulfate precipitation did not occur in the ocean, since there were no oxidized sulfur anions. Ancient rocks contain a lot of easily oxidized, but not completely oxidized substances - graphite, lapis lazuli, pyrite. In the Archean, as a result of the activity of anaerobic iron bacteria, significant layers of magnetite and hematite - ores containing under-oxidized divalent iron - were formed. At the same time, it has been established that the oxygen present in these rocks is of photosynthetic origin.

The gradually increasing scale of photosynthetic activity of cyanobacteria led to the appearance and accumulation of free oxygen in environment. The transition from a reducing atmosphere to an oxidizing one began Proterozoic, oh as evidenced by changes in the chemical composition of the earth's rocks.

Several threshold values ​​are significant in the history of atmospheric oxygen. On Earth, devoid of photosynthesis, oxygen is formed in the atmosphere due to the photodissociation of water molecules. Its content, according to G. Yuri’s calculations, cannot exceed 0.001 of the modern (Yuri point) and automatically stays at this level. At this oxygen content, only anaerobic life can exist. The emergence of molecular oxygen through photosynthesis made it possible for living cells to process respiration, which is a much more efficient way of releasing energy than anaerobic fermentation. From this point of view, the value of 0.01 oxygen content from the modern level is important - the so-called Pasteur's point. There are a number of microorganisms that are capable of switching their energy metabolism from respiration to fermentation and back when oxygen fluctuates below or above the Pasteur point. At the same time, life was able to spread almost to the surface of water bodies, since ultraviolet rays, due to weak ozone screen could now penetrate to depths of no more than a meter.

Third threshold content of O 2 (Berkner–Marshall point) corresponds to 10% of the modern one. It determines the formation of the ozone screen in such a way that streams of hard ultraviolet solar rays no longer reach the earth’s surface and do not interfere with the development of life. By modern research, the transition to the Pasteur point could have occurred as early as 2.5 billion years ago, and the 10% oxygen content (Berkner-Marshall point) was reached already in the period 1.8–2.0 billion years from modern times.

Thus, for more than two billion years, the biosphere was formed exclusively by the activity of prokaryotes. They completely changed the geochemical situation on Earth: they formed an oxygen atmosphere, cleared it of toxic volcanic gases, bound and transferred a huge amount of CO 2 into carbonate rocks, changed the salt composition of the ocean and formed huge deposits of iron ores, phosphorites and other minerals.

The formation of an oxidizing atmosphere entailed the rapid development of eukaryotic life, the energy of which is based on the process of respiration. It is obvious that eukaryotic life is closely related to the aerobic environment prepared for it by prokaryotes. The first aerobic organisms could have arisen quite early as part of cyanobacterial communities, which, as paleontologists put it, were a kind of “oxygen oases” in an anaerobic environment.

In general, the oxygen released by early photosynthetic organisms was toxic and lethal to anaerobic life forms. After its accumulation in water and atmosphere, anaerobic prokaryotic communities were pushed deeper into the soil, to the bottom of reservoirs, i.e., into local habitats with a lack of O 2.

In the second half of the Proterozoic, different groups unicellular algae and protozoa. Eukaryotic phytoplankton increased the rate of photosynthesis. In turn, cyanobacteria left huge deposits of stromatolites at this time, which indicates their high photosynthetic activity. At the end of the Proterozoic, so many biological products were already created in the seas that ancient oil and gas deposits arose on their basis.

The last stage of the Proterozoic, taking about 100 million years (Vendian), demonstrates an explosion of multicellular diversity. It is possible that multicellularity appeared earlier, since there is still no clarity regarding a number of controversial paleontological finds, but only in the Vendian did a huge variety of aquatic animals and plants of a fairly high organization appear. Large locations of Vendian biota have been found in different regions of the world: Australia, South Africa, Canada, Siberia, on the coast White Sea. Among the animals, coelenterates and worms predominated; there were forms resembling arthropods, but in general, most of them were distinguished by their peculiar appearance and were not found in later layers. Among the bottom algae there were many ribbon-like thallus forms. A distinctive feature of the entire Vendian biota is its lack of skeletons. The animals had already reached large sizes, some up to a meter, but had jelly-like gelatinous bodies that left imprints on soft soils. The good and widespread preservation of prints indirectly indicates the absence of carnivores and large predators in Vendian biocenoses.

Organic matter of biogenic origin became a constant and indispensable component of sedimentary rocks from the second half of the Proterozoic.

A new stage in the development of the organic world is the massive appearance of various external and internal skeletons in multicellular organisms. From this time on, it dates back to the Phanerozoic era - the “era of obvious life”, since the preservation of skeletal remains in the earth’s layers allows us to reconstruct in more detail the course of biological evolution. IN Phanerozoic The impact of living organisms on the geochemistry of the ocean, atmosphere and sediments is sharply increasing. The very possibility of the appearance of skeletons was prepared by the development of life. Due to photosynthesis, the World Ocean lost CO 2 and became enriched in O 2, which changed the mobility of a number of ions. Mineral components began to be deposited in the bodies of organisms as a skeletal basis.

By extracting a number of substances from the aquatic environment and accumulating them in their bodies, organisms no longer become indirect, but direct creators of many sedimentary rocks, being buried at the bottom of reservoirs. Carbonate accumulation has become predominantly biogenic and calcareous, since CaCO 3 is used more intensively for skeletal formation than MgCO 3 . Many species acquire the ability to extract calcium from water. At the beginning of the Phanerozoic, large deposits of phosphorites also appeared, created by fossils with a phosphate skeleton. Chemical precipitation of SiO 2 also becomes biogenic.

Within the Phanerozoic there are three eras: Paleozoic ,Mesozoic And Cenozoic , which, in turn, are divided into periods. The first period of the Paleozoic - Cambrian – characterized by such an explosion of biological diversity that it was called the Cambrian Revolution. Cambrian rocks are rich in numerous organisms. During this period, almost all types of currently existing animals and a number of others that have not reached our time arose. Archaeocyaths and sponges, brachiopods, the famous trilobites, various groups of mollusks, barnacle crustaceans, echinoderms and many others appeared. Among the protozoa, radiolarians and foraminifera arose. Plants are represented by a variety of algae. The role of cyanobacteria has diminished as stromatolites have become smaller and scarcer.

During Ordovician And Silurian The diversity of organisms in the ocean increased and their geochemical functions became more diverse. The ancestors of vertebrates appeared. The reef-forming role shifted from stromatolites to coral polyps. The main event of the Paleozoic was the conquest of land by plants and animals.

It is possible that the surface of the continents was populated by prokaryotes back in Precambrian times, given the tolerance of some forms of modern bacteria to hard radiation. However complex shapes Life was able to develop land only with the formation of a full-fledged ozone screen. This process apparently began in Silurian time, but the main period of its development was the Devonian. The first land plants - a collective group of psilophytes - are already characterized by a number of primitive anatomical and morphological adaptations to living in the air: conductive elements, integumentary tissues, stomata, etc. appear. In other features of their structure, psilophytes are still very similar to algae. Terrestrial vegetation evolved so quickly that by the end of the Devonian, forests of club mosses, horsetails, and pteridophytes arose in wet and near-water habitats. Even earlier, mosses appeared on land. This spore vegetation could only exist in moist, semi-flooded biotopes and, being buried under anaerobic conditions, left deposits of a new type of fossil - hard coal.

In the seas Devonian, Along with jawless fish, various forms of fish already dominated. One of the groups, lobe-finned animals, which acquired a number of adaptations for living in shallow water bodies littered with dying plants, gave rise to the first primitive amphibians. The first terrestrial arthropods have been known since the Silurian. In the Devonian, small soil arthropods already existed, apparently consuming rotting organic matter. However, the destruction process on land was not yet efficient enough, and the biological cycle was not closed. Mass grave plant organic matter and its exit from the biological cycle system entailed an accelerated accumulation of O 2 in the air. The atmospheric oxygen content at the beginning of the Phanerozoic was about a third of what it is today. In the Devonian, and especially in the next period - carbon, it has reached the modern and even surpassed it. Carbon forests are the peak of development of spore vegetation. They consisted of tree-like lycophytes - lepidodendrons and sigillaria, giant horsetails - calamites, powerful and diverse ferns. High plant production was stimulated and sufficiently high content CO 2 in the atmosphere, which was about 10 times higher than today. Carboniferous coals contain a large amount of carbon removed from air reserves of CO 2 during that period.

Already in the Carboniferous, plants and animals arose that were capable of conquering low-water areas of land: the first gymnosperms - cordaites and the first reptiles. The first flying insects mastered the air environment. Cartilaginous and bony fish, cephalopods, corals, ostracods and brachiopods flourished in the seas. The end of the Paleozoic, the Permian period, was characterized by abrupt change climatic conditions. Intense volcanism and mountain-building processes (the end of the Hercynian tectonic era) led to regression of the sea and high standing of the continents: the southern supercontinent Gondwana and northern– Laurasia. Geographic zoning has sharply increased. Traces of extensive glaciation have been discovered in Gondwana. In Laurasia, in the arid climate zone, large areas of evaporation sediments appear - gypsum, rock and potassium salt (Solikamsk deposits), anhydrites, dolomites. In tropical areas, however, the accumulation of hard coal continues (Kuzbass, Pechora, China). Spore vegetation is in sharp decline. The mass of oxygen in the atmosphere is reduced to values ​​characteristic of the beginning of the Paleozoic.

At the border of the Paleozoic and Mesozoic eras, at the end Perm and the beginning Triassic There was, against the backdrop of a change in flora, a profound renewal of marine and terrestrial fauna. Among the plants, gymnosperms dominate - cycads, ginkgos and conifers. Many groups of amphibians and early reptiles are dying out, and trilobites are disappearing from the seas.

In the Mesozoic, Gondwana began to break up into separate continents and move away from each other. Mid Mesozoic (Yura) characterized again by the expansion of shallow waters, an even warm climate and a weakening of geographical zonality. Jurassic forests were much more diverse in composition than Carboniferous forests, less moisture-loving, and grew not only in swamps and along the edges of reservoirs, but also inside continents. They also left deposits of coal along the valleys and floodplains of rivers. Among vertebrates on land, reptiles dominate, having also mastered the air and secondary aquatic environment. Various groups of dinosaurs, pterosaurs, ichthyosaurs and many other forms emerge.

In the Mesozoic, the deposition of carbonate rocks sharply decreases; one of the reasons for this is considered to be a further decrease in CO 2 in the atmosphere and ocean due to consumption for photosynthesis. The very nature of carbonate deposits is also changing - they are represented mainly by biogenic chalk and marls with a high calcium content. At the beginning of the Mesozoic, a new group of unicellular algae appeared - diatoms with silicon shells, and due to them, thin silicon silts and new rocks - diatomites - began to form. Their thicknesses in the World Ocean in some places reach 1600 m with an accumulation rate of 7–30 cm per 1000 years. The intensity of photosynthesis and the scale of burial of organic matter are very high, the consumption of oxygen for the oxidation of rocks during the intertectonic period is insignificant, therefore, by the middle of the Mesozoic there is a sharp increase in the mass of oxygen in the air, which exceeds the modern one.

The development of vegetation led to the emergence of a new progressive group - angiosperms. This happened in chalky a period, by the end of which they, quickly spreading across all continents, significantly displaced the gymnosperm flora. In parallel with flowering plants, various groups of insect pollinators and consumers of angiosperm tissue are rapidly evolving. Flowering plants are characterized by accelerated rates of growth and development and a variety of synthesized compounds. Being independent of water in the processes of fertilization, they are nevertheless characterized by a higher consumption of moisture for transpiration processes and a more intense involvement of ash nutrition elements and especially nitrogen in the cycle. With the advent of angiosperm vegetation, 80–90% of the water cycle on the planet began to be determined by their activity. Under their influence, soils close to modern ones began to form with surface aerobic decomposition of plant residues. The processes of coal accumulation have slowed down significantly.

Throughout the Cretaceous period, reptiles dominated, many of which reached gigantic sizes. There were also toothed birds, and placental mammals arose, descending from primitive Triassic ancestors. By the end of the period, birds close to modern ones spread. Bony fish, ammonites and belemnites, and foraminifera flourished in the seas.

The end of the Cretaceous period was characterized by the beginning of a new tectonic era and global cooling. The change in floras entailed a change in faunas, which intensified as a result of the influence of global tectonic and climatic processes. At the border of the Mesozoic and Cenozoic eras, one of the most ambitious extinctions occurred. Dinosaurs and most other reptiles disappeared from the face of the Earth. Ammonites and belemnites, rudists, a number of planktonic unicellular organisms and many other groups became extinct in the seas. Intensive adaptive radiation of the most progressive groups of vertebrates - mammals and birds - began. Insects have begun to play a major role in terrestrial ecosystems.

The onset of the Cenozoic era was characterized by an increase in aerobic conditions in the biosphere not due to an increase in the mass of oxygen, but due to changes in soil regimes. The completeness of biological cycles has increased. Wet forests The Paleogene still left significant accumulations of hard and brown coals. At the same time, the flourishing of active angiosperm vegetation lowered the CO 2 content in the atmosphere to modern levels, resulting in a decrease in the overall efficiency of photosynthesis. IN Neogene the growing aerobiosis of soils and water bodies stopped the processes of formation of coal and oil. In the modern era, only peat formation occurs in swampy soils.

During the Cenozoic, sharp climate changes occurred. As a result of the evolution of angiosperms during periods of drying in the middle of the era, herbaceous plant formations and new types of landscapes emerged - open steppes and prairies. At the end, climatic zonality intensified and the ice age began with the spread of ice over large parts of the Northern and Southern Hemispheres. Last wave The glaciers retreated only about 12 thousand years ago.

Development of the organic world