The biological properties of molecular oxygen (O 2) are at least twofold. Oxygen is a powerful oxidizing agent with which you can obtain a lot of useful energy, and at the same time a strong poison that freely passes through cell membranes and destroys cells if not handled carefully. It is sometimes said that oxygen is a double-edged sword ( Current Biology, 2009, 19, 14, R567–R574). All organisms that deal with oxygen necessarily have special enzyme systems that dampen its chemical effects. Those who lack such enzyme systems are doomed to be strict anaerobes, surviving only in oxygen-free environments. On modern Earth these are some bacteria and archaea.
Almost all the oxygen on Earth is of biogenic origin, that is, released by living beings (of course, we are now talking about free oxygen, and not about oxygen atoms that are part of other molecules). The main source of O 2 is oxygenic photosynthesis; There are simply no other known reactions capable of producing it in comparable quantities. From a school biology course we know that photosynthesis is the synthesis of glucose C 6 H 12 O 6 from carbon dioxide CO 2 and water H 2 O, which occurs with the help of light energy. The main “actor” here is carbon dioxide, which is reduced by water; oxygen in this reaction is nothing more than a by-product, a waste. It is less widely known that photosynthesis may not lead to the release of oxygen if, instead of water, some other substance is used as a reducing agent - for example, hydrogen sulfide H 2 S, free hydrogen H 2 or some iron compounds; This type of photosynthesis is called oxygen-free photosynthesis, and there are several different variants of it.
It is almost certain that oxygen-free photosynthesis appeared much earlier than oxygen-based photosynthesis. Therefore, in the first billion years of life’s existence (and most likely longer), although photosynthesis occurred, it did not cause any saturation of the Earth’s atmosphere with oxygen. The oxygen content in the atmosphere in those days was no more than 0.001% of what it is today - simply put, this means that it wasn’t really there.
Everything changed when blue-green algae, or cyanobacteria, came onto the scene. Subsequently, these creatures became the ancestors of plastids, photosynthetic organelles of eukaryotic cells (remember that eukaryotes are organisms with cell nuclei, in contrast to prokaryotes, which have nuclear-free cells). Cyanobacteria are a very ancient evolutionary branch. By the standards of earthly history, they are remarkably unchanged. For example, the blue-green algae oscillatorium ( Oscillatoria) has fossil relatives that lived 800 million years ago, and they are practically indistinguishable from modern oscillators (Ecology of Cyanobacteria II. Their Diversity in Space and Time, Springer, 2012, 15–36). Thus, the oscillatorium is an impressive example of a living fossil. But the very first cyanobacteria appeared much earlier than this - this is confirmed by paleontological data.
At first, cyanobacteria were not numerous, because the oxygen photosynthesis they mastered did not provide any serious advantages compared to the oxygen-free photosynthesis that other groups of microbes possessed. But the chemical environment of these microbes gradually changed. There came a point when there simply was no longer enough “raw material” for oxygen-free photosynthesis. And then the hour of cyanobacteria struck.
Oxygen photosynthesis has one big advantage - a completely unlimited supply of the initial reducing reagent (water) and one big disadvantage - the high toxicity of the by-product (oxygen). Not surprisingly, this type of exchange was not “popular” at first. But at the slightest shortage of substrates other than water, those with oxygen photosynthesis should immediately gain a competitive advantage, which is what happened. After this, an era of approximately a billion years began, during which the appearance of the Earth was determined primarily by cyanobacteria. Recently, it was even proposed to unofficially call it a “cyanozoan” in their honor (M. Barbieri, Code Biology. A New Science of Life, Springer, 2015, 75–91).
It was because of cyanobacteria that the oxygen revolution began 2.4 billion years ago, also known as the oxygen catastrophe, or the Great Oxidation Event ( Great Oxidation Event, GOE). Strictly speaking, this event was neither instantaneous nor absolutely unique ( Nature, 2014, 506, 7488, 307–315). Short bursts of oxygen concentration, “oxygen puffs,” have happened before, this is paleontologically recorded. Yet something new happened 2.4 billion years ago. In a short time by the standards of earthly history (a few tens of millions of years), the oxygen concentration in the atmosphere increased approximately a thousand times and remained at this level; it never again fell to its former insignificant values. The biosphere has irreversibly become oxygenated.
For the vast majority of ancient prokaryotes, this level of oxygen was deadly. It is not surprising that the first result of the oxygen revolution was mass extinction. Those who survived were mainly those who managed to create oxygen-protecting enzymes, and sometimes even thick cell walls to boot (the cyanobacteria themselves also had to do this). There is reason to believe that in the first 100–200 million years of the “new oxygen world,” oxygen was only poison for living organisms and nothing more. But then the situation changed. The response of the biota to the oxygen challenge was the appearance of bacteria, which included oxygen in the chain of reactions that decompose glucose, and thus began to use it to produce energy.
It immediately turned out that oxygen oxidation of glucose (respiration) is much more energetically efficient than oxygen-free oxidation (fermentation). It provides several times more free energy per glucose molecule than any no matter how complicated version of oxygen-free metabolism. At the same time, the initial stages of glucose breakdown in users of respiration and fermentation remained common: oxygen oxidation served as just a superstructure over the already existing ancient biochemical mechanism, which itself did not require oxygen.
A group of microbes that have mastered the risky but effective production of energy using oxygen are called proteobacteria. According to the now generally accepted theory, it was from them that the respiratory organelles of eukaryotic cells - mitochondria - originated.
According to genetic data, the closest modern relative of mitochondria is the purple spiral alphaproteobacterium Rhodospirillum rubrum (Molecular Biology and Evolution, 2004, 21, 9, 1643–1660). Rhodospirillum has respiration, fermentation, and anoxic photosynthesis, which uses hydrogen sulfide instead of water, and can switch between these three types of metabolism depending on external conditions. Undoubtedly, such a symbiont - that is, in this case, an internal cohabitant - was very useful to the ancestor of eukaryotes.
Moreover, many modern scientists believe that the symbiosis of ancient archaea with proteobacteria - the ancestors of mitochondria - was the impetus for the very formation of the eukaryotic cell (Evgeniy Kunin. The logic of chance. M.: Tsentrpoligraf, 2014). This hypothesis is called the "early mitochondrial" hypothesis. She suggests that the division of the future eukaryotic cell into cytoplasm and nucleus occurred only after the introduction of a proteobacterial symbiont into it. The older “late mitochondrial” scenario, in which the proteobacterium was simply swallowed up by a ready-made eukaryotic cell (which spontaneously arose from an archaeal cell), now looks much less likely. In fact, both cells - archaeal and proteobacterial - were seriously “reassembled” during the process of unification, giving rise to a kind of chimera with new properties. This chimera became a eukaryotic cell; the molecular components of archaeal and proteobacterial origin were strongly mixed in it, dividing the functions among themselves (“Palaeontological Journal”, 2005, 4, 3–18). Without proteobacteria, eukaryotes would not have arisen. This means that their appearance was a direct consequence of the oxygen revolution.
In light of the above, the words of two modern prominent scientists, a paleontologist and a geologist, almost do not seem like an exaggeration: “Everyone agrees that the evolution of blue-green algae was the most significant biological event on our planet (even more significant than the development of eukaryotic cells and the emergence of multicellular organisms).” (Peter Ward, Joe Kirschvink. A new history of the origin of life on Earth. St. Petersburg: Publishing House "Peter", 2016). Indeed, the world of animals and plants we know today would not exist if it were not for cyanobacteria and the crisis they caused.
Epochs of life
The entire history of the Earth is divided into four huge periods called eons (this is higher than an era). The names of the eons are as follows: Katarchean, or Hadean (4.6–4.0 billion years ago), Archaean (4.0–2.5 billion years ago), Proterozoic (2.5–0.54 billion years ago) and Phanerozoic (started 0.54 billion years ago and continues now). This division will constantly help us, it is really convenient. Let us make a reservation that in almost all such cases it is worth remembering not the time boundaries, but the sequence of eras and events related to them: this is much more important. An exception can be made only for two or three fundamental dates, such as the age of the Earth.
The Katarchean is the so-called pre-geological era, from which no “normal” rocks arranged in layers remained. Classical geological and paleontological methods, based precisely on the comparison of successive layers, do not work there. The objects remaining from the catarchaean are mostly small zircon grains, the same ones in which supposedly biogenic carbon was recently found. Very little is known about Catarchaean life (if it existed).
In the archean, the Earth belongs to prokaryotes - bacteria and archaea (just don’t get confused, the coincidence of the roots in the name of the geological era “archaea” and the group of microbes “archaea” is actually accidental). The Archean-Proterozoic boundary occurs approximately at the time of one of the strong “oxygen blows” that preceded the oxygen revolution. The oxygen revolution itself occurred at the beginning of the Proterozoic.
Proterozoic is the era of oxygen and eukaryotes. There is an interesting paradox associated with dating the origin of eukaryotes. The fact is that more or less reliably identifiable multicellular eukaryotes appear in the fossil record noticeably earlier than equally reliably identifiable unicellular eukaryotes. filamentous algae Grypania spiralis, which is generally considered a eukaryote, appeared 2.1 billion years ago ( Australasian Journal of Palaeontology, 2016, doi: 10.1080/03115518.2016.1127725). To be fair, it must be said that the main argument for the eukaryotic nature of influenza is its large size - all other signs do not give confidence that this is not a giant cyanobacterium ( Palaeontology, 2015, 58, 1, 5–17). But the fact is that this find is not the only one. The oldest known eukaryote is now considered to be a fungus-like organism. Diskagma buttonii 2.2 billion years old ( Precambrian Research, 2013, 235, 71–87). And there are also mysterious large spiral-shaped creatures - most likely algae, the age of the remains of which is no less than 2.1 billion years, like that of flupania ( Nature, 2010, 466, 7302, 100–104). But the earliest single-celled organisms, unambiguously defined as eukaryotes, are only 1.6 billion years old ( , 2006, 361, 1470, 1023-1038). This, of course, does not mean that multicellular eukaryotes actually appeared earlier than unicellular eukaryotes - such an assumption contradicts all available molecular data. Single-celled organisms are simply less preserved, and they have fewer signs by which the organism can be identified.
Nevertheless, very important conclusions follow from such dating. Let us remember that the date of the oxygen revolution is 2.4 billion years ago. Therefore, we know that just 200 million years after it, not just eukaryotes, but multicellular eukaryotes appear in the fossil record. This means that the first stages of eukaryotic evolution were completed very quickly by the standards of global history. Of course, it took time for a eukaryotic cell to form a symbiosis with the ancestors of mitochondria, to create a nucleus, and to complicate the cytoskeleton - the intracellular system of supporting structures. But when these processes ended, the first multicellular organisms were created almost immediately. This did not require any additional devices at the cell level. Any eukaryotic cell already has a complete set of molecular elements necessary to build a multicellular body (at least a relatively simple one) from such cells. Of course, all these elements are no less useful for the life of a single cell, otherwise they simply would not have arisen. The common ancestor of eukaryotes was undoubtedly single-celled, and many of its descendants never benefited from multicellularity. We know examples of modern unicellular eukaryotes - amoebas, euglena, ciliates - thanks to school textbooks, but in fact there are much more of them.
The oxygen revolution had another important consequence, affecting the composition of the atmosphere. The Archean atmosphere had a lot of nitrogen (as it does now), as well as carbon dioxide and methane (much more than now). Carbon dioxide and methane absorb infrared radiation very well and thereby retain heat in the Earth's atmosphere, preventing it from escaping into space. This is called the greenhouse effect. Moreover, it is believed that the greenhouse effect from methane is at least 20–30 times stronger than from carbon dioxide. And in Archean times, there was about 1000 times more methane in the Earth's atmosphere than now, and this provided a fairly warm climate.
Astronomy also intervenes here. According to the generally accepted theory of stellar evolution, the luminosity of the Sun is slowly but continuously increasing. In the Archean it was only 70–80% of what it is today—it’s clear why the greenhouse effect was important for keeping the planet warm. But after the oxygen revolution, the atmosphere became oxidizing and almost all methane (CH 4) turned into carbon dioxide (CO 2), which is much less effective as a greenhouse gas. This caused the catastrophic Huronian glaciation, which lasted about 100 million years and at some points covered the entire Earth: traces of glaciers were found on land areas that were then only a few degrees of latitude from the equator ( , 2005, 102, 32, 11131–11136). The peak of the Huronian glaciation occurred 2.3 billion years ago. Fortunately, glaciation could not stop the tectonic activity of the earth's mantle; Volcanoes continued to release carbon dioxide into the atmosphere, and over time it accumulated enough to restore the greenhouse effect and melt the ice.
However, the main climate tests were still ahead.
The end of the "boring billion"
The turbulent events of the beginning of the Proterozoic were followed by the so-called “boring billion years” ( Boring Billion). At this time, there were no glaciations, no sharp changes in the composition of the atmosphere, no biosphere upheavals. Eukaryotic algae lived in the oceans, gradually releasing oxygen. Their world was diverse and complex in its own way. For example, multicellular red and yellow-green algae are known from the era of the “boring billion”, surprisingly similar to their modern relatives ( Philosophical Transactions of the Royal Society B, 2006, 361, 1470, 1023–1038). Mushrooms also appear at this time ( Paleobiology, 2005, 31, 1, 165–182). But multicellular animals are absent in the vastness of the “boring billion years”. Let's be careful: at the moment, no one can say with complete confidence that there were no multicellular animals then, but all the data on this topic is, at best, very controversial ( Precambrian Research, 2013, 235, 71–87).
What's the matter? This suggests that multicellularity as such is much more compatible with the lifestyle of a plant than an animal. Any plant cell is enclosed in a rigid cell wall, and there is no doubt that this greatly facilitates the regulation of the relative arrangement of cells in a complex body. On the contrary, animal cells lack a cell wall, their shape is unstable, and even constantly changes during acts of phagocytosis, that is, the absorption of food particles. Assembling a whole organism from such cells is a difficult task. If no multicellular animals had appeared at all, and representatives of plants or fungi had become biologists, they most likely, after studying this problem, would have come to the conclusion that the combination of multicellularity with the absence of a cell wall is simply impossible. In any case, this explains why multicellularity arose many times in different groups of algae, but only once in animals.
There is another idea. In 1959, Canadian zoologist John Ralph Nersell linked the sudden (as was then thought) appearance of animals in the fossil record with rising oxygen concentrations in the atmosphere ( Nature, 1959, 183, 4669, 1170–1172). Animals, as a rule, have active mobility, which requires so much energy that they cannot do without oxygen breathing. And you need a lot of oxygen. And in the era of the “boring billion,” the O2 content in the atmosphere almost certainly did not reach 10% of modern levels, the minimum often considered necessary to support animal life. True, this suspiciously round figure is most likely overestimated ( Proceedings of the National Academy of Sciences USA, 2014, 111, 11, 4168–4172). Such reservations, however, do not prevent us from recognizing that Nersell’s old idea at least does not contradict modern data: the supposed beginning of the evolution of multicellular animals is very approximately, but coincides in time with a new increase in the concentration of atmospheric oxygen at the end of the Proterozoic ( Annual Review of Ecology, Evolution, and Systematics, 2015, 46, 215–235). This simply could not help but become a factor that facilitated the appearance of animals: after all, the more oxygen, the better. Just don’t consider the oxygen factor strictly the only one. Let us remember that even at times when there was as much oxygen as possible, no repeated attempts to create multicellularity of the animal type were noted. This experiment was successful for nature only once.
The cozy era of the “boring billion years” could have lasted for a long time if geography had not intervened in biology. Dramatic events, the hero of which was the planet itself, attracted the attention of scientists for half a century, but only about 15 years ago information about them was able to be put together into a more or less coherent picture. Let's take a quick look at this picture, starting, as expected, from the beginning.
In 1964, English geologist Brian Harland published an article in which he stated that absolutely on all continents there are traces of ancient glaciation dating back to the same time - the Late Proterozoic. Just in the early 60s, geologists learned to determine the past position of continents using data on the magnetization of rocks. Harland collected this data and saw that they can be explained in only one way: by assuming that the Late Proterozoic glaciation immediately covered all latitudes of the Earth, that is, it was planetary. Any other hypotheses looked even less plausible (for example, it would be necessary to assume an incredibly fast movement of the poles so that all the lands would be covered in turn by the polar cap). As Sherlock Holmes said during his search for Jonathan Small, “throw away all the impossible, what remains is the answer, no matter how incredible it may seem.” That's exactly what Harland did. The detailed article written by him and his co-author does not pretend to be any sensation - it simply honestly states the facts and conclusions ( Scientific American, 1964, 211, 2, 28–36). And yet the hypothesis of planetary glaciation was too bold for most scientists.
Literally in the same years, the famous geophysicist, Leningrader Mikhail Ivanovich Budyko, took up the theory of glaciations. He drew attention to the fact that glaciation can develop on its own. Ice cover has a high reflectivity (albedo), so the larger the total area of glaciers, the greater the share of solar radiation is reflected back into space, taking heat with it. And the less heat the Earth receives, the colder it becomes, and the area of ice cover grows as a result, increasing the albedo even more. It turns out that glaciation is a process with positive feedback, that is, capable of reinforcing itself. And in this case, there must be some critical level of glaciation, after which it will increase until waves of ice from the North and South Poles collapse at the equator, completely enclosing the planet in an ice cover and lowering its temperature by several tens of degrees. Budyko showed mathematically that such a development of events is possible ( Tellus, 1969, 21, 5, 611–619). But he had no idea that it happened several times in the history of the Earth! Because at that time Budyko and Harland had not yet read each other.
Snowball Earth
Now the glaciation that Harland discovered is usually called the era of “Snowball Earth” ( Snowball Earth). Apparently, it really was planetary. And its main reason is considered to be a sharp weakening of the greenhouse effect due to a drop in the concentration of carbon dioxide (which became the main greenhouse gas after oxygen “ate up” almost all the methane). Photosynthesis and respiration most likely have nothing to do with it. If the Earth's biota organized the oxygen revolution for itself, now it has turned out to be a victim of an external factor, completely non-biological in nature.
The fact is that the circulation of carbon dioxide depends much less on living beings than the circulation of oxygen. The main source of atmospheric CO 2 on Earth is still volcanic eruptions, and the main sink is a process called chemical weathering. Carbon dioxide interacts with rocks, destroying them, and itself turns into carbonates (HCO 3 − or CO 3 2 − ions). The latter dissolve well in water, but are no longer part of the atmosphere. And it turns out to be an extremely simple dependence. If the intensity of volcanic activity exceeds the intensity of chemical weathering, the atmospheric concentration of CO 2 increases. If it's the other way around, it falls.
At the end of the boring billion, 800 million years ago, almost all of the Earth's landmass was part of a single supercontinent called Rodinia. According to one famous geologist, giant supercontinents, like large empires in the social history of the Earth, have always turned out to be unstable (V. E. Khain, M. G. Lomize. Geotectonics with the fundamentals of geodynamics. M: Moscow State University Publishing House, 1995). Therefore, it is not surprising that Rodinia began to split. Along the edges of the faults, erupted basalt solidified, which immediately became the object of chemical weathering. There was no soil then, and weathering products were easily carried into the ocean. Rodinia eventually broke up into seven or eight small continents—about the size of Australia—that began to drift apart from each other. The consumption of CO 2 for weathering of basalt led to a drop in its level in the atmosphere.
Volcanism, which inevitably accompanied the collapse of the supercontinent, could compensate for this, if not for one random circumstance. Due to some quirks of continental drift, both Rodinia and its fragments were located near the equator, in a warm zone, where chemical weathering proceeded especially quickly. Mathematical models show that it is for this reason that the CO 2 concentration has dropped below the threshold beyond which glaciation begins ( Nature, 2004, 428, 6980, 303–306). And when it started, it was too late to slow down weathering.
It must be admitted that the position of the continents in the Late Proterozoic turned out to be as unfortunate (from the point of view of the inhabitants of the planet) as possible. Continental drift is controlled by flows of matter in the earth's mantle, the dynamics of which are essentially unknown. But we know that in this case these flows collected the entire earth's land into a single continent, located exactly at the equator and elongated in latitude. If it had ended up at one of the poles or had been stretched from north to south, the glaciation that had begun would have covered some of the rocks from weathering and thereby stopped the escape of carbon dioxide from the atmosphere - then the process could have slowed down. This is exactly the situation we are seeing now, with the ice sheets of Antarctica and Greenland ( Scientific American, 1999, 9, 38). And at the end of the Proterozoic, almost all large areas of land were close to the equator - and were exposed until the moment when the northern and southern ice sheets closed. The earth became an ice ball.
In fact, there were at least three episodes of Snowball Earth. The first of them dates back to the Huronian glaciation (which, as we remember, occurred not because of carbon dioxide, but because of methane). Then, for more than a billion years, there were no glaciations at all. And then two more planetary glaciations separated by a short break followed, one of which lasted about 60 million years, the other about 15 million years. They were discovered by Brian Harland. The geological period covering these glaciations is called the Cryogeny (it is part of the Proterozoic).
Little is known about the living nature of cryogenics. The climate then throughout the Earth was, by today's standards, Antarctic. Most of the world's oceans were covered by a kilometer layer of ice, so the rate of photosynthesis could not be high. Light, which suddenly became the most valuable resource, entered the ocean only in places, through cracks, holes or small areas of thin ice. It is surprising that some multicellular organisms managed to survive cryogeny without changing at all - for example, red algae. Even now they are adapted to use very weak light, penetrating to such a depth where no other photosynthetic creatures live (Yu. T. Dyakov. Introduction to algology and mycology. M.: Moscow State University Publishing House, 2000). Single-celled plankton has not gone anywhere either. The oxygen content of the cryogenian ocean dropped dramatically, so life on its floor was likely largely anaerobic, but the details of this are still hidden from us.
The endings of Snowball Land episodes are also dramatic in their own way. During planetary glaciations, all processes associated with the absorption of large volumes of carbon dioxide were literally frozen. Meanwhile, volcanoes (the operation of which no one stopped) emitted and emitted CO 2 into the atmosphere, gradually increasing its concentration to enormous values. At some point, the ice sheet could no longer resist the greenhouse effect, and then an avalanche-like process of heating the planet began. Literally in a few thousand years - that is, geologically in an instant - all the ice melted, the released water flooded a significant part of the land with shallow marginal seas, and the temperature of the earth's surface, judging by calculations, jumped to 50 ° C ( Engineering and Science, 2005, 4, 10–20). And only after this did the Earth begin to gradually return to its “normal” non-glacial state. During cryogeny, this entire cycle was completed at least twice.
Yastrebov S.A.
(“HiZh”, 2016, No. 9)
Double-edged sword O 2
The biological properties of molecular oxygen (O 2) are at least twofold. Oxygen is a powerful oxidizing agent, with which you can get a lot of useful energy, and at the same time, a strong poison that freely passes through cell membranes and destroys cells if not handled carefully. It is sometimes said that oxygen is a double-edged sword (Current Biology, 2009, 19, 14, R567-R574). All organisms that deal with oxygen necessarily have special enzyme systems that dampen its chemical effects. Those who lack such enzyme systems are doomed to be strict anaerobes, surviving only in oxygen-free environments. On modern Earth these are some bacteria and archaea.
The main source of O 2 is oxygenic photosynthesis; There are simply no other known reactions capable of producing it in comparable quantities. From a school biology course we know that photosynthesis is the synthesis of glucose C 6 H 12 O 6 from carbon dioxide CO 2 and water H 2 O, which occurs with the help of light energy. The main “actor” here is carbon dioxide, which is reduced by water; oxygen in this reaction is nothing more than a by-product, a waste. It is less widely known that photosynthesis may not lead to the release of oxygen if, instead of water, some other substance is used as a reducing agent - for example, hydrogen sulfide H 2 S, free hydrogen H 2 or some iron compounds; This type of photosynthesis is called oxygen-free photosynthesis, and there are several different variants of it.
It is almost certain that oxygen-free photosynthesis appeared much earlier than oxygen-based photosynthesis. Therefore, in the first billion years of life’s existence (and most likely longer), although photosynthesis occurred, it did not cause any saturation of the Earth’s atmosphere with oxygen. The oxygen content in the atmosphere in those days was no more than 0.001% of what it is today - simply put, this means that it wasn’t really there.
Everything changed when blue-green algae, or cyanobacteria, came onto the scene. Subsequently, these creatures became the ancestors of plastids, photosynthetic organelles of eukaryotic cells (remember that eukaryotes are organisms with cell nuclei, in contrast to prokaryotes, which have nuclear-free cells). Cyanobacteria are a very ancient evolutionary branch. By the standards of earthly history, they are remarkably unchanged. For example, the blue-green algae oscillatorium ( Oscillatoria) has fossil relatives that lived 800 million years ago, and they are practically indistinguishable from modern oscillators (“Ecology of Cyanobacteria II. Their Diversity in Space and Time,” Springer, 2012, 15-36). Thus, the oscillatorium is an impressive example of a living fossil. But the very first cyanobacteria appeared much earlier than this - this is confirmed by paleontological data.
At first, cyanobacteria were not numerous, because the oxygen photosynthesis they mastered did not provide any serious advantages compared to the oxygen-free photosynthesis that other groups of microbes possessed. But the chemical environment of these microbes gradually changed. There came a point when there simply was no longer enough “raw material” for oxygen-free photosynthesis. And then the hour of cyanobacteria struck.
Oxygen photosynthesis has one big advantage - a completely unlimited supply of the initial reducing reagent (water). and one big drawback is the high toxicity of the by-product (oxygen). Not surprisingly, this type of exchange was not “popular” at first. But at the slightest shortage of substrates other than water, those with oxygen photosynthesis should immediately gain a competitive advantage, which is what happened. After this, an era of approximately a billion years began, during which the appearance of the Earth was determined primarily by cyanobacteria. Recently, it was even proposed to unofficially call it a “cyanozoic” in their honor (M. Barbieri, “Code Biology. A New Science of Life,” Springer, 2015, 75-91).
It was because of cyanobacteria that the oxygen revolution began 2.4 billion years ago, also known as the oxygen catastrophe, or the Great Oxidation Event (GOE). Strictly speaking, this event was neither instantaneous nor absolutely unique (Nature, 2014, 506, 7488, 307-315). Short bursts of oxygen concentration, “oxygen puffs,” have happened before, this is paleontologically recorded. Yet something new happened 2.4 billion years ago. In a short time by the standards of earthly history (a few tens of millions of years), the oxygen concentration in the atmosphere increased approximately a thousand times and remained at this level; it never again fell to its former insignificant values. The biosphere has irreversibly become oxygenated.
For the vast majority of ancient prokaryotes, this level of oxygen was deadly. It is not surprising that the first result of the oxygen revolution was mass extinction. Those who survived were mainly those who managed to create oxygen-protecting enzymes, and sometimes even thick cell walls to boot (the cyanobacteria themselves also had to do this). There is reason to believe that in the first 100-200 million years of the “new oxygen world” oxygen was only poison for living organisms and nothing more. But then the situation changed. The response of the biota to the oxygen challenge was the appearance of bacteria, which included oxygen in the chain of reactions that decompose glucose, and thus began to use it to produce energy.
It immediately turned out that oxygen oxidation of glucose (respiration) is much more energetically efficient than oxygen-free oxidation (fermentation). It provides several times more free energy per glucose molecule than any no matter how complicated version of oxygen-free metabolism. At the same time, the initial stages of glucose breakdown in users of respiration and fermentation remained common: oxygen oxidation served as just a superstructure over the already existing ancient biochemical mechanism, which itself did not require oxygen.
A group of microbes that have mastered the risky but effective production of energy using oxygen are called proteobacteria. According to the now generally accepted theory, it was from them that the respiratory organelles of eukaryotic cells - mitochondria - originated.
According to genetic data, the closest modern relative of mitochondria is the purple spiral alphaproteobacterium Rhodospirillum rubrum(“Molecular Biology and Evolution”, 2004, 21, 9, 1643-1660). Rhodospirillum has respiration, fermentation, and anoxic photosynthesis, which uses hydrogen sulfide instead of water, and can switch between these three types of metabolism depending on external conditions. Undoubtedly, such a symbiont - that is, in this case, an internal cohabitant - was very useful to the ancestor of eukaryotes.
Moreover, many modern scientists believe that the symbiosis of ancient archaea with proteobacteria - the ancestors of mitochondria - was the impetus for the very formation of the eukaryotic cell (Evgeniy Kunin. The logic of chance. M.: Tsentrpoligraf, 2014). This hypothesis is called the "early mitochondrial" hypothesis. She suggests that the division of the future eukaryotic cell into cytoplasm and nucleus occurred only after the introduction of a proteobacterial symbiont into it. The older “late mitochondrial” scenario, in which the proteobacterium was simply swallowed up by a ready-made eukaryotic cell (which spontaneously arose from an archaeal cell), now looks much less likely. In fact, both cells - archaeal and proteobacterial - were seriously “reassembled” during the process of unification, giving rise to a kind of chimera with new properties. This chimera became a eukaryotic cell; the molecular components of archaeal and proteobacterial origin were greatly mixed in it, dividing the functions among themselves (“Palaeontological Journal”, 2005, 4, 3-18). Without proteobacteria, eukaryotes would not have arisen. This means that their appearance was a direct consequence of the oxygen revolution.
In light of the above, the words of two modern prominent scientists, a paleontologist and a geologist, almost do not seem like an exaggeration: “Everyone agrees that the evolution of blue-green algae was the most significant biological event on our planet (even more significant than the development of eukaryotic cells and the emergence of multicellular organisms).” (Peter Ward, Joe Kirschvink. A new history of the origin of life on Earth. St. Petersburg: Publishing House "Peter", 2016). Indeed, the world of animals and plants we know today would not exist if it were not for cyanobacteria and the crisis they caused.
Epochs of life
The entire history of the Earth is divided into four huge periods called eons (this is higher than an era). The names of the eons are as follows: Katarchean, or Hadean (4.6-4.0 billion years ago), Archean (4.0-2.5 billion years ago), Proterozoic (2.5-0.54 billion years ago) and Phanerozoic (started 0.54 billion years ago and continues now). This division will constantly help us, it is really convenient. Let us make a reservation that in almost all such cases it is worth remembering not the time boundaries, but the sequence of eras and events related to them: this is much more important. An exception can be made only for two or three fundamental dates, such as the age of the Earth.
The Katarchean is the so-called pre-geological era, from which no “normal” rocks arranged in layers remained. Classical geological and paleontological methods, based precisely on the comparison of successive layers, do not work there. The objects remaining from the catarchaean are mostly small zircon grains, the same ones in which supposedly biogenic carbon was recently found. Very little is known about Catarchaean life (if it existed).
In the archean, the Earth belongs to prokaryotes - bacteria and archaea (just don’t get confused, the coincidence of the roots in the name of the geological era “archaea” and the group of microbes “archaea” is actually accidental). The Archean-Proterozoic boundary occurs approximately at the time of one of the strong “oxygen blows” that preceded the oxygen revolution. The oxygen revolution itself occurred at the beginning of the Proterozoic.
Proterozoic is the era of oxygen and eukaryotes. There is an interesting paradox associated with dating the origin of eukaryotes. The fact is that more or less reliably identifiable multicellular eukaryotes appear in the fossil record noticeably earlier than equally reliably identifiable unicellular eukaryotes. filamentous algae Grypania spiralis, which is generally considered a eukaryote, appeared 2.1 billion years ago (Australasian Journal of Palaeontology, 2016, doi: 10.1080/03115518.2016.1127725). To be fair, it must be said that the main argument for the eukaryotic nature of influenza is its large size - all other signs do not give confidence that this is not a giant cyanobacterium (“Palaeontology”, 2015, 58, 1, 5-17). But the fact is that this find is not the only one. The oldest known eukaryote is now considered to be a fungus-like organism. Diskagma buttonii 2.2 billion years old (Precambrian Research, 2013, 235, 71-87). And then there are mysterious large spiral-shaped creatures - most likely algae, the age of the remains of which is at least 2.1 billion years old, like that of flupania (Nature, 2010, 466, 7302, 100-104). But the earliest single-celled organisms, unambiguously defined as eukaryotes, are only 1.6 billion years old (“Philosophical Transactions of the Royal Society B”, 2006, 361, 1470, 1023-1038). This, of course, does not mean that multicellular eukaryotes actually appeared earlier than unicellular eukaryotes - such an assumption contradicts all available molecular data. Single-celled organisms are simply less preserved, and they have fewer signs by which the organism can be identified.
Nevertheless, very important conclusions follow from such dating. Let us remember that the date of the oxygen revolution is 2.4 billion years ago. Therefore, we know that just 200 million years after it, not just eukaryotes, but multicellular eukaryotes appear in the fossil record. This means that the first stages of eukaryotic evolution were completed very quickly by the standards of global history. Of course, it took time for a eukaryotic cell to form a symbiosis with the ancestors of mitochondria, to create a nucleus, and to complicate the cytoskeleton - the intracellular system of supporting structures. But when these processes ended, the first multicellular organisms were created almost immediately. This did not require any additional devices at the cell level. Any eukaryotic cell already has a complete set of molecular elements necessary to build a multicellular body (at least a relatively simple one) from such cells. Of course, all these elements are no less useful for the life of a single cell, otherwise they simply would not have arisen. The common ancestor of eukaryotes was undoubtedly single-celled, and many of its descendants never benefited from multicellularity. We know examples of modern unicellular eukaryotes - amoebas, euglena, ciliates - thanks to school textbooks, but in fact there are much more of them.
The oxygen revolution had another important consequence, affecting the composition of the atmosphere. The Archean atmosphere had a lot of nitrogen (as it does now), as well as carbon dioxide and methane (much more than now). Carbon dioxide and methane absorb infrared radiation very well and thereby retain heat in the Earth's atmosphere, preventing it from escaping into space. This is called the greenhouse effect. Moreover, it is believed that the greenhouse effect from methane is at least 20-30 times stronger than from carbon dioxide. And in Archean times, there was about 1000 times more methane in the Earth's atmosphere than now, and this provided a fairly warm climate.
Astronomy also intervenes here. According to the generally accepted theory of stellar evolution, the luminosity of the Sun is slowly but continuously increasing. In the Archean it was only 70-80% of what it is today - it’s clear why the greenhouse effect was important for keeping the planet warm. But after the oxygen revolution, the atmosphere became oxidizing and almost all methane (CH 4) turned into carbon dioxide (CO 2), which is much less effective as a greenhouse gas. This caused the catastrophic Huronian glaciation, which lasted about 100 million years and at some points covered the entire Earth: traces of glaciers were found on land areas that were then only a few degrees of latitude from the equator (“Proceedings of the National Academy of Sciences USA”, 2005 , 102, 32, 11131-11136). The peak of the Huronian glaciation occurred 2.3 billion years ago. Fortunately, glaciation could not stop the tectonic activity of the earth's mantle; Volcanoes continued to release carbon dioxide into the atmosphere, and over time it accumulated enough to restore the greenhouse effect and melt the ice.
However, the main climate tests were still ahead.
The end of the "boring billion"
The turbulent events of the early Proterozoic were followed by the so-called Boring Billion. At this time, there were no glaciations, no sharp changes in the composition of the atmosphere, no biosphere upheavals. Eukaryotic algae lived in the oceans, gradually releasing oxygen. Their world was diverse and complex in its own way. For example, multicellular red and yellow-green algae are known from the era of the “boring billion”, surprisingly similar to their modern relatives (“Philosophical Transactions of the Royal Society B”, 2006, 361, 1470, 1023-1038). Mushrooms also appear at this time (“Paleobiology”, 2005, 31, 1, 165-182). But multicellular animals are absent in the vastness of the “boring billion years”. Let’s be careful: at the moment, no one can say with complete confidence that there were no multicellular animals then, but all the data on this topic is at best very controversial (“Precambrian Research”, 2013, 235, 71-87).
What's the matter? This suggests that multicellularity as such is much more compatible with the lifestyle of a plant than an animal. Any plant cell is enclosed in a rigid cell wall, and there is no doubt that this greatly facilitates the regulation of the relative arrangement of cells in a complex body. On the contrary, animal cells lack a cell wall, their shape is unstable, and even constantly changes during acts of phagocytosis, that is, the absorption of food particles. Assembling a whole organism from such cells is a difficult task. If no multicellular animals had appeared at all, and representatives of plants or fungi had become biologists, they most likely, after studying this problem, would have come to the conclusion that the combination of multicellularity with the absence of a cell wall is simply impossible. In any case, this explains why multicellularity arose many times in different groups of algae, but only once in animals.
There is another idea. In 1959, Canadian zoologist John Ralph Nursel linked the sudden (as was then believed) appearance of animals in the fossil record to rising oxygen concentrations in the atmosphere (Nature, 1959, 183, 4669, 1170-1172). Animals, as a rule, have active mobility, which requires so much energy that they cannot do without oxygen breathing. And you need a lot of oxygen. And in the era of the “boring billion,” the O2 content in the atmosphere almost certainly did not reach 10% of modern levels, the minimum often considered necessary to support animal life. True, this suspiciously round figure is most likely overestimated (“Proceedings of the National Academy of Sciences USA”, 2014, 111, 11, 4168-4172). Such reservations, however, do not prevent us from recognizing that Nursell’s old idea at least does not contradict modern data: the supposed beginning of the evolution of multicellular animals is very approximately, but coincides in time with a new increase in the concentration of atmospheric oxygen at the end of the Proterozoic (“Annual Review of Ecology, Evolution, and Systematics", 2015, 46, 215-235). This simply could not help but become a factor that facilitated the appearance of animals: after all, the more oxygen, the better. Just don’t consider the oxygen factor strictly the only one. Let us remember that even at times when there was as much oxygen as possible, no repeated attempts to create multicellularity of the animal type were noted. This experiment was successful for nature only once.
The cozy era of the “boring billion years” could have lasted for a long time if geography had not intervened in biology. Dramatic events, the hero of which was the planet itself, attracted the attention of scientists for half a century, but only about 15 years ago information about them was able to be put together into a more or less coherent picture. Let's take a quick look at this picture, starting, as expected, from the beginning.
In 1964, English geologist Brian Harland published an article in which he stated that absolutely on all continents there are traces of ancient glaciation dating back to the same time - the Late Proterozoic. Just in the early 60s, geologists learned to determine the past position of continents using data on the magnetization of rocks. Harland collected this data and saw that they can be explained in only one way: by assuming that the Late Proterozoic glaciation immediately covered all latitudes of the Earth, that is, it was planetary. Any other hypotheses looked even less plausible (for example, it would be necessary to assume an incredibly fast movement of the poles so that all the lands would be covered in turn by the polar cap). As Sherlock Holmes said during his search for Jonathan Small, “throw away all the impossible, what remains is the answer, no matter how incredible it may seem.” That's exactly what Harland did. The detailed article he and his co-author wrote does not pretend to be any sensation - it simply honestly presents the facts and conclusions (Scientific American, 1964, 211, 2, 28-36). And yet the hypothesis of planetary glaciation was too bold for most scientists.
Literally in the same years, the famous geophysicist, Leningrader Mikhail Ivanovich Budyko, took up the theory of glaciations. He drew attention to the fact that glaciation can develop on its own. Ice cover has a high reflectivity (albedo), so the larger the total area of glaciers, the greater the share of solar radiation is reflected back into space, taking heat with it. And the less heat the Earth receives, the colder it becomes, and the area of ice cover grows as a result, increasing the albedo even more. It turns out that glaciation is a process with positive feedback, that is, capable of reinforcing itself. And in this case, there must be some critical level of glaciation, after which it will increase until waves of ice from the North and South Poles collapse at the equator, completely enclosing the planet in an ice cover and lowering its temperature by several tens of degrees. Budyko showed mathematically that such a development of events is possible (“Tellus”, 1969, 21, 5, 611-619). But he had no idea that it happened several times in the history of the Earth! Because at that time Budyko and Harland had not yet read each other.
Snowball Earth
Nowadays, the glaciation that Harland discovered is commonly called the era of Snowball Earth. Apparently, it really was planetary. And its main reason is considered to be a sharp weakening of the greenhouse effect due to a drop in the concentration of carbon dioxide (which became the main greenhouse gas after oxygen “ate up” almost all the methane). Photosynthesis and respiration most likely have nothing to do with it. If the Earth's biota organized the oxygen revolution for itself, now it has turned out to be a victim of an external factor, completely non-biological in nature.
The fact is that the circulation of carbon dioxide depends much less on living beings than the circulation of oxygen. The main source of atmospheric CO 2 on Earth is still volcanic eruptions, and the main sink is a process called chemical weathering. Carbon dioxide interacts with rocks, destroying them, and itself turns into carbonates (HCO 3 - or CO 3 2- ions). The latter dissolve well in water, but are no longer part of the atmosphere. And it turns out to be an extremely simple dependence. If the intensity of volcanic activity exceeds the intensity of chemical weathering, the atmospheric concentration of CO 2 increases. If it's the other way around, it falls.
At the end of the boring billion, 800 million years ago, almost all of the Earth's landmass was part of a single supercontinent called Rodinia. According to one famous geologist, giant supercontinents, like large empires in the social history of the Earth, have always turned out to be unstable (V.E. Khain, M.G. Lomise. Geotectonics with the fundamentals of geodynamics. M: Moscow State University Publishing House, 1995). Therefore, it is not surprising that Rodinia began to split. Along the edges of the faults, erupted basalt solidified, which immediately became the object of chemical weathering. There was no soil then, and weathering products were easily carried into the ocean. Rodinia eventually broke up into seven or eight small continents—about the size of Australia—that began to drift apart from each other. The consumption of CO 2 for weathering of basalt led to a drop in its level in the atmosphere.
Volcanism, which inevitably accompanied the collapse of the supercontinent, could compensate for this, if not for one random circumstance. Due to some quirks of continental drift, both Rodinia and its fragments were located near the equator, in a warm zone, where chemical weathering proceeded especially quickly. Mathematical models show that it is for this reason that CO 2 concentrations have dropped below the threshold at which glaciation begins (Nature, 2004, 428, 6980, 303-306). And when it started, it was too late to slow down weathering.
It must be admitted that the position of the continents in the Late Proterozoic turned out to be as unfortunate (from the point of view of the inhabitants of the planet) as possible. Continental drift is controlled by flows of matter in the earth's mantle, the dynamics of which are essentially unknown. But we know that in this case these flows collected the entire earth's land into a single continent, located exactly at the equator and elongated in latitude. If it had ended up at one of the poles or had been stretched from north to south, the glaciation that had begun would have covered some of the rocks from weathering and thereby stopped the escape of carbon dioxide from the atmosphere - then the process could have slowed down. This is exactly the situation we are seeing now, with the ice sheets of Antarctica and Greenland (“Scientific American”, 1999, 9, 38). And at the end of the Proterozoic, almost all large areas of land were close to the equator - and were exposed until the moment when the northern and southern ice sheets closed. The earth became an ice ball.
In fact, there were at least three episodes of Snowball Earth. The first of them dates back to the Huronian glaciation (which, as we remember, occurred not because of carbon dioxide, but because of methane). Then, for more than a billion years, there were no glaciations at all. And then two more planetary glaciations separated by a short break followed, one of which lasted about 60 million years, the other about 15 million years. They were discovered by Brian Harland. The geological period covering these glaciations is called the Cryogeny (it is part of the Proterozoic).
Little is known about the living nature of cryogenics. The climate then throughout the Earth was, by today's standards, Antarctic. Most of the world's oceans were covered by a kilometer layer of ice, so the rate of photosynthesis could not be high. Light, which suddenly became the most valuable resource, entered the ocean only in places, through cracks, holes or small areas of thin ice. It is surprising that some multicellular organisms managed to survive cryogeny without changing at all - for example, red algae. Even now they are adapted to use very weak light, penetrating to such a depth where no other photosynthetic creatures live (Yu.T. Dyakov. Introduction to algology and mycology. M.: Moscow State University Publishing House, 2000). Single-celled plankton has not gone anywhere either. The oxygen content of the cryogenian ocean dropped dramatically, so life on its floor was likely largely anaerobic, but the details of this are still hidden from us.
The endings of Snowball Land episodes are also dramatic in their own way. During planetary glaciations, all processes associated with the absorption of large volumes of carbon dioxide were literally frozen. Meanwhile, volcanoes (the operation of which no one stopped) emitted and emitted CO 2 into the atmosphere, gradually increasing its concentration to enormous values. At some point, the ice sheet could no longer resist the greenhouse effect, and then an avalanche-like process of heating the planet began. Literally in a few thousand years - that is, geologically in an instant - all the ice melted, the released water flooded a significant part of the land with shallow marginal seas, and the temperature of the earth's surface, judging by calculations, jumped to 50 o C (“Engineering and Science”, 2005, 4 , 10-20). And only after this did the Earth begin to gradually return to its “normal” non-glacial state. During cryogeny, this entire cycle was completed at least twice.
British scientists have found that the Earth 720-640 thousand years ago was not a frozen “snowball”, as geologists believe today, but was similar to Europe and Enceladus, the moons of Jupiter and Saturn with their subglacial oceans and volcanoes, according to an article published in the journal Nature Geoscience.
The “white Earth” or “snowball Earth” hypothesis suggests that during one period of the Neoproterozoic era, approximately 625-850 million years ago, the planet “frozen” right up to the equator. There are different versions of this hypothesis - from the “slush” hypothesis, according to which the ocean near the equator thawed for at least several months a year, to the “ice” hypothesis, when absolutely the entire earth’s surface was covered with ice.
Until now, scientists believed that the Earth was unlikely to freeze completely, since in this case even massive emissions of CO 2 and other greenhouse gases should not be enough to melt all the ice. This is supported by the fact that in some regions of the Earth one can find typically “aqueous” deposits of alkaline rocks formed at this time. However, the mechanism itself for maintaining the oceans in liquid form remained unclear.
Tom Gernon from the University of Southampton (UK) and his colleagues found that in fact the Earth did not look like a “snowball”, but was a kind of analogue of the “water” moons of Jupiter and Saturn, recreating on a computer one of the key events of this era - the Rodinia Fault , the first supercontinent in the history of our planet.
"When volcanic rocks are ejected onto the surface of the ocean floor, they undergo a cycle of extremely rapid and violent chemical changes that greatly alter the biogeochemistry of ocean waters. We found that many of the geological and geochemical phenomena associated with the Snowball Earth fit well with the idea abundant eruptions of underwater volcanoes on the edges of mid-ocean ridges,” the scientist said.
Gernon's team tested this idea by creating a computer model of the breakup of Rodinia and the associated volcanic eruptions. These calculations showed that volcanoes released enormous amounts of heat and a range of chemicals that transformed the appearance of the Earth's subglacial ocean.
The interaction of ejected rocks and water led to the sedimentation and formation of huge amounts of so-called hyaloclastites - rocks containing large amounts of phosphorus, calcium ions and a number of other alkali metals. Hyaloclastites are unstable by their chemical nature. They quickly turn into a kind of “glass” from which all ions are washed out, which makes the surrounding water more alkaline.
Paradoxically, these ions prevented volcanoes from melting the Earth, since they served as a kind of “buffer” that absorbed most of the carbon dioxide that was emitted from the bowels of the planet and turned them into carbonate deposits on the ocean floor. Due to this, the share of CO 2 in the atmosphere grew slowly, and the Earth spent over 200 million years in the “ice age”.
This property - warm and very alkaline water - made the ancient Earth very similar to what Enceladus, a moon of Saturn, whose subglacial ocean has similar properties, looks like today. This in principle allows the use of fossil data from Earth and modern observational data to assess the habitability of such oceans and the conditions within them.
Somewhere 600–700 million years ago, something difficult to imagine happened on Earth: it froze. The land, which at that time was located entirely in the equatorial and tropical regions, bears clear imprints of glaciers. It seems clear that it was unlikely to be too hot at the poles either.
When such a theory was first expressed, it was met with severe criticism. Objections boiled down to two main theses: the Earth could not get into such a state and could not get out of such a state. Let us explain: glaciations on Earth occurred frequently, but not on a planetary scale. Our planet has effective feedback systems that prevent these types of events from happening. For example, when the temperature of the ocean drops, the solubility of gases in it increases, so that carbon of organic origin should quickly bind into carbon dioxide and saturate the atmosphere, up to a sharp increase in the greenhouse effect. The latter, even before the end of the planet being frozen in ice, would have sharply leveled the temperature, preventing global glaciation.
Finally, if this suddenly happened, opponents of the theory noted, unfreezing would be extremely difficult, and the total loss of life would be inevitable. Without open water, there would be almost no clouds in the atmosphere, and the high reflectivity of ice would lead to the Earth losing the energy it receives from sunlight. What mechanism could heat it when even volcanic activity under the ice sheet is difficult (as in today's Antarctica) - and therefore secondary saturation with carbon dioxide is also complicated? In addition, if the temperature at the equator was close to Antarctic, dry ice could theoretically fall at the poles, further removing carbon dioxide from the atmosphere.
Geologist Huiming Bao from Louisiana State University (USA) was one of the opponents of the snowball Earth theory. But studies of samples from the mentioned period led the scientist to the conclusion that the hypothesis was correct. However, it was important to answer the second question: how and when could the completely frozen Earth thaw, if the ice cover and the absence of clouds, on the contrary, should have cooled it to the limit?
To solve this puzzle, scientists studied barites BaSO 4 from that distant era. As it turned out after analyzing samples from Southern China, it was in barites close to the period of global glaciation that there was a strong deficiency of oxygen-17 and an excess of oxygen-18 in comparison with normal terrestrial concentrations. At first, such a strange isotopic state was attributed to the effect of severe erosion characteristic of the period after the retreat of glaciation. However, there were many glaciations on Earth, but layers depleted in oxygen-17 have not yet been noticed.
As Mr. Bao himself believes in this regard, this means that the time of sharp depletion of barites in oxygen-17 may be a marker of the duration of the period when the forming barites were deprived of access to such oxygen. Although the scientist believes it is premature to name the exact reasons for the isotopic depletion, he says that it can already be used to date the period of “global thawing.” According to calculations, such anomalously depleted oxygen-17 barites are typical for a period of no more than 0.00–0.99 million years.
The authors of the study link the restoration of normal levels of the oxygen isotope-17 with the restoration of a normal atmosphere. In their opinion, in order to emerge from the climate knockout and melt, the Earth needed 350 times more carbon dioxide than it does today. They believe that this concentration was accompanied by a small amount of oxygen in the atmosphere or its almost complete absence. After volcanoes delivered excess amounts of carbon dioxide into the atmosphere, which there was no one on the frozen Earth to consume, super-intense global warming began with a positive feedback loop. During the period of warming and restoration of the normal state of the planet, the authors of the work theorize, the content of oxygen-17 should have been minimal.
In other words, the normal state of affairs was restored by geological standards quickly - in less than a million years. Extremely quickly, given the catastrophic nature of glaciation and the associated process of mass extinction of organisms. Strictly speaking, the nuclear winter scenario promoted in the 1980s is not as severe as what happened during the Snowball Earth era.
“No matter what happens on Earth, it will recover, and very quickly,” emphasizes Hui Ming Bao. - The planet survived, and life continued even after this murderous event. The only thing that has changed is the composition of life. In other words, no matter what people do to the Earth, life will endure. But will people remain a part of it..."
The research report was published in the journal Red dots indicate locations of formations indicating glaciation, whose age corresponds to the supposed period of "Snowball Earth". As you can see, they are found all over the world (illustration by New Scientist) A new experiment by geologists was supposed to answer the main question that arises among its opponents: how did the planet then thaw, because the snow and ice cover reflects the rays well, further increasing the cooling?
Diagram showing the movement of sea glaciers during the "Snowball Earth" era, which eventually led to the accumulation of dust over a large part of the planet's surface. Previously, the thaw was explained by the appearance of large amounts of carbon dioxide in the atmosphere from volcanoes. However, recent research shows that CO 2 levels at that time were only a tenth of the amount required to melt ice (illustration by Goodman, Pierrehumbert/University of Chicago) Dorian Abbot and Raymond Pierrehumbert used climate modeling to study the effects of dust released into the atmosphere by volcanic eruptions and rock weathering.
They found that the Earth's surface was becoming polluted quite quickly at that time, especially in those regions where snow rarely fell. At the same time, its reflective properties changed so much that huge areas of the planet could absorb sunlight and gradually melt the ice.
Thus, scientists say, the mystery of the thaw can be easily solved if we recognize that our planet was more of a “mudball” than a “snowball.” Geologists intend to test this hypothesis by searching for fossil dust in sediments from that period. The article by Chicago specialists was published in the Journal of Geophysical Research – Atmospheres, and you can read it (PDF document). About 700 million years ago, when global glaciation was so powerful that ice reached the equator, small areas of the ocean remained free. The key to the survival of the biosphere at one of its most critical moments was discovered by scientists from Britain and Australia.
7.10.11 Some researchers believe that two or three times in the history of our planet there came a period, conventionally designated “Snowball Earth,” when ice almost completely covered the surface of the Earth. The last time this happened was about 635 million years ago. Then, for a number of reasons, the greenhouse effect occurred and the planet thawed.
However, an international group of scientists has questioned the spike in atmospheric carbon dioxide concentrations during those times. According to new data, the greenhouse effect was not powerful enough to melt the thick ice. Therefore, the Earth did not turn into a big snowball.
The main evidence in favor of the hypothesis is glacial deposits that were located in the equator region 635 million years ago. Above them is a layer of "cap carbonates", which are believed to have formed when the glaciers melted or shortly thereafter, that is, when there was an excess of carbon dioxide in the atmosphere.
The Snowball Earth period is thought to have ended when carbon dioxide levels in the atmosphere rose. The cause could be volcanic activity. The factors that normally remove carbon dioxide from the atmosphere were blocked by the ice. In addition, the cold did not allow weathered rocks to absorb carbon dioxide to form bicarbonates. All this led to the accumulation of greenhouse gases in the atmosphere.
The researchers decided to find out how much carbon dioxide was in the atmosphere at that time. To do this, they analyzed the chemical composition of Brazilian rocks of that time and the fossilized organic substances inside them. Specialists were interested in the ratio of isotopes.
Both rocks and organic matter (mostly algae) extract carbon from carbon dioxide dissolved in the ocean. A decrease in gas concentration leads to the fact that algae begin to adhere to the heavier isotope. On the other hand, the ratio of carbon isotopes in carbonate rocks does not change regardless of the concentration of carbon dioxide.
Comparisons of rock and organic matter showed that the concentration of carbon dioxide in the atmosphere was much lower than previous estimates. They said 90 thousand parts per million, but the new analysis says it was less than 3,200 parts per million. It is possible that the concentration was close to today's (about 400 ppm).
Red-brown, iron-rich glacial deposits in the Ogilvie Mountains (Yukon Territory, Canada). They formed 716.2 million years ago, when the planet may have been almost completely covered in ice. (Photo by Francis Macdonald.)
“And since there was no high concentration of carbon dioxide in the atmosphere, it means there could not have been a Snowball Earth, otherwise the Earth would have been frozen to this day,” sums up study author Magali Ader from the Geophysical Institute in Paris (France).
She cautions, however, that many uncertainties remain. It is possible, for example, that the rocks were dated incorrectly. There is also a possibility that the greenhouse effect was caused not by carbon dioxide, but by methane...