Scientists recently announced that a new mass extinction has begun on Earth: due to human activities, the rate of extinction of species of vertebrates has approached the level that was 66 million years ago, when the dinosaurs became extinct. Did the flora and fauna of the Earth really change as a result of disasters? The impact of a meteorite, global warming, incinerating fires - contrary to the usual beliefs, the laws of evolution are much stronger than universal cataclysms.
The history of our planet keeps traces of several major disasters that literally reformatted the ecological systems of those times. After each global extinction, new classes of living beings appeared on the evolutionary scene, and only petrified remains of the previous ones remained. Scientists of the A. A. Borisyak of the Russian Academy of Sciences seems to have groped the mechanism of these changes. And insects helped them in this - one of the most successful categories of animals, some of which are able to survive even near the epicenter of a nuclear explosion.
Apocalypse Alpha
The words "global extinction" usually bring to mind the extinction of dinosaurs. But there was in the history of the Earth a catastrophe of a much larger scale, known to specialists as the "Great Extinction". At the border of the Permian and Triassic periods, 252 million years ago, 96 percent of marine and more than 70 percent of terrestrial animal species disappeared as if by magic. Neither before nor after on our planet has there been such a large-scale devastation of the biosphere.
The circumstances of that tragedy are well understood and could easily form the basis of a Hollywood blockbuster. Why is there a movie - it seems that it was the Great Extinction that inspired the author of The Revelation of John the Theologian, who described in detail the death of our own civilization. Here are just a few of the factors whose combination seems to have brought about this epic catastrophe.
At the end of the Permian period, over the vast expanses of Siberia, magma outbursts of fantastic volumes to the Earth's surface occurred. The frozen lava fields, known as Siberian traps, cover a total of two million square kilometers - the area of Greenland or India. The volume of the erupted rocks is estimated today at about five million cubic kilometers. Of course, such a mass of lava noticeably warmed up the earth's atmosphere, adding a fair amount of greenhouse gases to it, which led to even greater - global - warming.
Basalt rocks of the Putorana plateau formed by traps
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Image Credit Flickr xandreani
Around the same time, a large asteroid fell on the territory of modern Antarctica, on Wilkes Land, the crater from which (500 kilometers in diameter) is still visible. Note that the diameter of the Mexican crater Chicxulub from the meteorite, which is considered to be a dinosaur killer, is only 175 kilometers.
Finally, a mysterious event took place in the ocean, releasing huge masses of methane - the most important component of greenhouse gases. Either this methane accumulated for millions of years in the bottom layers of water, or archaea of the genus Methanosarcina began to release it en masse, but the composition of the earth's atmosphere has changed a lot.
The result of all these troubles is a test version of the Apocalypse: the sky was covered by a veil of volcanic dust, with which the dust from the asteroid impact was mixed. The air was hot and dry, and the ocean turned into a solution of acid, completely unsuitable for the life of most of its former inhabitants.
Diversity crisis
Few living creatures are able to survive in such terrifying conditions, many modern paleontologists believe. Therefore, the fauna was massively dying out, and only when the atmosphere cleared, and the ocean water turned from acidic to just salty, the rare surviving creatures began a new stage of evolution. But, as it turned out, this logical and understandable scheme, unfortunately, has nothing to do with reality!
Russian paleontologist, doctor of biological sciences, head of the arthropod laboratory of the Paleontological Institute, Alexander Pavlovich Rasnitsyn, has been studying insects of the Permian and Triassic periods for many years. And according to his data, it turns out that there was actually no catastrophic Great Extinction.
The crisis at the border of the Permian and Triassic was considered so deep that the famous British paleontologist Michael Benton even published a book called When Life Almost Died. Another scientist, Jack Sepkoski from the University of Chicago, presented a graph in one of his works, which shows how the rises in animal diversity are gradually replaced by a relatively stable number of species and deep, sharp dips in extinctions. And Russian scientists found out that there is some decline on the border of the Permian and Triassic, but long and not as deep as depicted by Sepkowski. This dip in the Permian and Triassic, very long, lasted for more than half of the Permian period - tens of millions of years.
Species diversity of the three main types of marine fauna of the Phanerozoic: Cambrian, Paleozoic, modern
Chart: J. John Sepkoski, Jr. / Contributions to Zoology
To understand in more detail the events of that distant time, Rasnitsyn and his colleagues applied their own methodology. They work at the family level, because insects have not enough orders, but there are too many genera, and most importantly, they are very short-lived. As a result, a curve was constructed based on several indicators, the most important of which are the first and last finds of representatives of specific families. Appearance in the geological record and disappearance from it has been interpreted as occurrence and extinction. It turned out that the total number of families (that is, the actual diversity of the fauna) decreases very slowly throughout the entire Middle and Upper Permian, not reflecting any sharp extinction. In the Lower Triassic, there are practically no insects, but the trouble isthat in the Middle and Upper Triassic a very significant part of the Paleozoic families reappears. “That is, about extinction - this is not the story at all,” notes Rasnitsyn.
And an absolutely remarkable thing - the Late Permian extinction is traditionally associated with Siberian traps, with the outpouring of a colossal amount of magmatic matter in Siberia. It is argued that the release of huge amounts of gases, the burning of accumulated coal and peat, and other dramatic events turned the planet into a perfect gas chamber. But the fact is that the Tunguska lava outpourings were not simultaneous, and from the intertrappean sedimentary layers (for example, from the Tunguska and Babi Kamen in Kuzbass), a variety of insects and plants are known. That is, these lava fields did not destroy the biosphere - at least the flora and entomofauna.
So what happened in this case with the variety of insects while the rest of the fauna was globally extinct?
“The intensity of extinction of insects in the Middle-Upper Permian, at the height of the Great Extinction, freezes at the same level. But at the same time, the appearance of new families is sharply reduced. It turns out that the decrease in diversity is not due to an increase in extinction, but due to a decrease in the rate of emergence of new families, and it is this parameter that determines the entire dynamics of diversity in insects. That is, extinction, which depends mainly on external factors - volcanism, traps, asteroids - at large and most interesting time intervals turns out to be a constant value, and the dynamics of appearance varies, which is largely determined by the internal properties of organisms and the processes taking place in the biosphere. This is a completely different ideology! - exclaims Dr. Rasnitsyn.
Alexander Rasnitsyn
Photo: lenta.ru
So, studies of Permian and Triassic insects show that there is no need to talk about any sharp extinction provoked by a global catastrophe. The long-term and gradual decline in the diversity of Permian insects is more reminiscent of the last millions of years of the Cretaceous period, when the number of known genera of dinosaurs gradually and inexorably decreased from hundreds at the beginning of the Cretaceous to 10-30 at the end. The reason for this impoverishment of diversity, at least in insects, is evolutionary stasis, when new families simply stop appearing, and the old ones continue to live as before and die out at the same rate.
The tragedy of perfection
But how can evolution stop? This question will inevitably arise for any reader who has mastered the school course in biology. It is not known what Charles Darwin thought about this, but modern scientists believe that not only can, but in certain cases simply must do so.
“If we restrict ourselves exclusively to the genetic approach to evolution, within the framework of the very synthetic theory that is passed in school, then creatures in which genetics change more intensively should evolve faster than others: those who reproduce faster, have a larger population, and higher fertility. In other words, bacteria. And the largest, slowest breeding forms will evolve the slowest. So, in fact, elephants and whales evolve the fastest, and unicellular ones evolve most slowly. Everything is exactly the opposite,”says Rasnitsyn.
Why it happens? Scientists do not yet have an exact answer, but if there is no doubt about the correctness of genetics, then there is only one thing left to assume: the reserves of changes in elephants and whales provided by genetics are quite sufficient to ensure the fastest evolution. But in others, evolution is somehow inhibited and does not go, or does not go at full speed. The question is what is holding back evolution. Rasnitsyn proposed the concept of an adaptive compromise based on the fact that if everything in the body is coordinated and harmonious, then it is very difficult to change one thing in it without affecting other parameters according to the correlative rules. And correlative changes will almost always be non-adaptive and therefore harmful.
Permian insects
In order for evolution to proceed in such a situation, a finely balanced organization of a living being is required. A simple toughening of conditions will lead to its extinction, and a softening will only cause increased reproduction, an increase in mortality dependent on density and the restoration of the entire system at the same level. Apparently, overcoming the adaptive compromise is possible under some unilateral change in conditions, when in fact only some of the functions are subjected to adaptive control. For example, an organism finds itself on an island where there are no predators or dangerous parasites for it, and the whole problem comes down to learning how to eat what is there. By the way, it is well known that it is on the islands that rapid evolution often occurs and that animals take grotesque forms. So evolution proceeds when such unbalanced conditions arise,when entering a new ecological niche. Over time, she will again create a completely balanced creature, after which evolutionary stasis will again come. What will happen if in the fauna all its components reach a very high perfection, a powerful compromise, so to speak, become stagnant in their perfection? Then, when conditions change, they will simply die out - until the habitat becomes free, until conditions soften, adaptive control decreases and evolution becomes easier. We see all this on the example of Paleozoic insects.ossify in their perfection? Then, when conditions change, they will simply die out - until the habitat becomes free, until conditions soften, adaptive control decreases and evolution becomes easier. We see all this on the example of Paleozoic insects.ossify in their perfection? Then, when conditions change, they will simply die out - until the habitat becomes free, until conditions soften, adaptive control decreases and evolution becomes easier. We see all this on the example of Paleozoic insects.
“The extract from our research is that insects did not have any mass extinctions and the dynamics of their diversity is determined by the dynamics of the emergence of new groups, and not the dynamics of extinction. On land, the impact and scale of the Permian-Triassic catastrophe was much less than in the sea, which is completely amazed if we consider that the causes of the catastrophe are in atmospheric changes, asteroid impact and volcanism. And since our approach has not been applied to other groups of animals, I cannot say that the situation with insects is specific. The development of insect diversity from the Carboniferous to the present day is very similar to how the total diversity of other animals changed during this period. Therefore, it is not at all excluded that these trends may turn out to be universal, and the Permian-Triassic extinction was actually caused by biological,and not by geological factors,”Rasnitsyn sums up.
Dmitry Samarin