In addition to two of the most famous extinctions – that have deprived the world of dinosaurs, and the largest Perm was still at least three large-scale extinction, which destroyed a large number of species. Today, a number of scientists believe that we live in the age of the sixth extinction.
This extinction is considered the most ancient – it happened 440 million years ago. Life on earth in the Ordovician period became harder and the seas were filled with primitive octopus, trilobites, corals, starfish, eels, and jawed fish. In the struggle for existence involved and plants on land.
The percentage of all survivors and victims of this species extinction is “honorable” third place. There were at least two waves of extinction with a gap of about a million years. Sad the outcome was the death of 60% of marine invertebrates: bivalves, brachiopods, bryozoans, echinoderms, almost all of them have sunk into Oblivion. Only the great Permian extinction killed more marine organisms.
Ordovician-Silurian extinction is often associated with the southern movement of the ancient supercontinent Gondwana. The decrease in the level of the ocean combined with cold destroyed the usual biological niches and led to the reduction of biodiversity. Among the major versions scientists also consider the asteroid impact and massive volcanic eruptions.
According to one hypothesis, the Ordovician-Silurian extinction occurred due to the flash of gamma radiation of a supernova, located off of us 6 thousand light years. It has reduced the atmospheric ozone layer and intense UV radiation have destroyed millions of earth organisms. A supernova could be the nearest arm of the milky Way.
The Devonian extinction
What happened about 360 million years ago, a catastrophe is named after the Devonian period, which became the time evolution of some fish, strong fins which allowed them to move on the land. The trilobites at this time lose its dominance in the sea and on the land the plants become more difficult.
Extinction could have two phases, which took place 374 and 359 million years ago. In other versions, the steps were not two, and much more. Anyway, the number of marine species decreased by 50%, specifically in the first period were destroyed, nearly all jawless. Terrestrial and freshwater organisms is practically not affected, but the reef system was badly battered.
Scientists find it difficult to name the main cause of the Devonian extinction. Someone again associates it with the fall of an asteroid, someone with increasing temperature and evaporation of water, others point to the evolution of plants. There is a theory that in the late Devonian mass extinction was not, but greatly slowed the formation of new species.
Analyses of sediments sedimentary rocks has shown that in the late Devonian environment has changed dramatically. There was a sharp decrease in the content of oxygen in the oceans (anoxia), and the rate of deposition of carbon, on the contrary, increased. Anoxia prevented decay organisms, and organic matter became more and more.
The great Permian extinction
The most massive of the known extinctions occurred long before the death of the dinosaurs – 252 million years ago. It has become a point of differentiation of the Permian and Triassic periods. On the planet then dominated the types who lived and lay their eggs on land. However, this competitive advantage did not save them. Victims of extinction became 70% of terrestrial vertebrate species and 96% of all marine species.
The accident occurred just 60 thousand years. Gone a lot of parareptilia (primitive tetrapods), arthropods and fish, 83% of all insect species. But through this event was able to develop the ancestors of the dinosaurs, has long remained in the shadow of evolution.
The causes of the Permian extinction is also not clear and hotly debated in the scientific community. They are all similar to the previous one: the fall of an asteroid, volcanoes, and large-scale drought. Recently one more confirmation found the most popular of theories is volcanic. To answer some of the questions helped stony deposits (at the end of the Permian they were the bottom of the sea) desert of the United Arab Emirates, which is almost unchanged for millions of years. Cause of death species could be the saturation of the atmosphere with carbon dioxide after the eruption of the Siberian volcanoes.
Triassic-Jurassic extinction event
Occurred 199 million years ago extinction is used as the boundary of the Triassic and Jurassic periods. On the ground then walked a relatively large dinosaurs, which, however, had experienced competition with other reptiles.
The disaster became extinct conodonts, which constituted 20% of all marine families, suffered the archosaurs, therapsida and amphibians. The extinction happened over 10 thousand years, allowing the dinosaurs take over the Earth in the subsequent Jurassic period.
Among the possible causes of extinction often referred to the so-called hypothesis of “metamoderation gun”, according to which the increase in ocean temperature releases methane from deposits located beneath the seabed. Methane is a greenhouse gas, so the temperature begins to increase abruptly, leading to further release of methane. It’s like a vicious circle, and stop the process impossible, as it is impossible to stop the shot if the trigger is pushed. Actively discuss other versions.
The Cretaceous-Paleogene extinction
It is this cataclysm that occurred 65 million years ago killed the dinosaurs, marine reptiles and flying lizards. But there are other hypotheses that are generally complementary to the primary asteroid. Besides the well-known tyrannosaurs, Triceratops, ankylosaurs and other dinosaurs in the Cretaceous period, actively spread small mammals. They were destined to inherit the world.
All the victims were 16% of the families of aquatic animals and 18% of families of terrestrial vertebrates. Experts find it difficult to say there was extinction gradually or happened in a short period of time. It is believed, for example, that herbivores Triceratops could exist for several million years.
A recent study by experts at Princeton University, mit, University of Lausanne and University Amravati declines to a version of the eruptions. Analysis of geological formations in traps of the Deccan plateau helped to figure out when they started and how many continued. It turned out that large-scale eruptions began to occur for 250 thousand years before the time of the alleged fall of the asteroid and continued for over 500 thousand years. During this time, the released carbon dioxide sakizli the World’s oceans, which led to the death of many species and disruption of food chains.
None of the mass extinctions have not generated as many hypotheses as Cretaceous-Paleogene. In addition to popular scientific editions (volcanoes, asteroid, predatory mammals, etc.) appeared and semi. Some seriously argue that about man-made destruction of dinosaurs were discovered in “graveyards” of dinosaurs, including the bones of many individuals.
The Chronicle Of The Earth
As you can see, the greatest mass extinction occurred at different periods and in different time intervals. So, between the Ordovician-Silurian and Devonian extinctions lies 76 million years, and Triassic and Cretaceous-Paleogene share 134 million
However, if it is to take it on faith that such extinction really was. Perhaps new species have formed more slowly, but by themselves extinctions were not of a pronounced character. And each of the mass extinctions could be a series of smaller catastrophes, or the number of large-scale disasters themselves was higher.
Where did this uncertainty? We still know very little about the history of the Earth. The concept of the incompleteness of the fossil record developed even Charles Darwin. In the work “Evolution of taxonomic diversity”, which became A. S. Alekseev, V. Yu Dmitriev and A. G. Ponomarenko, States that modern science knows only 1-2% of the species that existed on Earth. Simply put, we judge the mass extinctions based on the analysis of the few found in the fossil remains of organisms. So, scientists can determine what percentage of species and genera did not survive to the next period.
Science knows enough to answer all these questions. We can not only confidently name the reasons for the disasters, but to understand whether they have been in reality. At least in the form in which people represent them.
The similarities and differences
But we will try to identify similarities and differences. Before us five mass extinctions (six if you consider the Eocene-Oligocene). It is logical to assume that many of them had similar reasons. At the same time, the two most popular versions – the volcanoes and the fall of the heavenly bodies subjected to the most criticism. It is known that the most powerful volcanic activity took place in the period of the Cretaceous and Permian extinctions. However, if we consider all the known cases of extinctions (and there are at least eleven), it turns out that large-scale geological processes can be correlated only with six.
The situation is similar with falling asteroids. The death of the dinosaurs coincides with the fall of a giant asteroid, near the island of Yucatan. He left the Chicxulub crater with a diameter of 180 km and an initial depth of 20 km, Resulting from falling energy was 2 million times the energy of explosion of a thermonuclear “Tsar Bomba”, and that could be enough to change life on Earth. But with the Triassic-Jurassic extinction is more complicated: scientists have not yet discovered craters that could explain it.
But, perhaps, the causes of the extinctions must be sought elsewhere? Recently about this possibility, said researchers from the University of Western Sydney, working under the direction of Professor Miroslav Filipovic (Miroslav Filipović). They drew attention to the timetable of the Solar system. Our Sun makes a full rotation around the center of the milky Way galaxy over 200 million years. On the way the system passes through the galactic spiral arms, where the higher density of stars and interstellar gas. The model helped to find out that mass extinctions coincide with passage through these sleeves. This applies to the Cretaceous-Paleogene, Triassic, Permian, and late Devonian extinctions postneonatal.
According to the authors, a coincidence is possible, but its probability is very small. Scientists do not undertake to say what killed terrestrial organisms. Theoretically, the passage through spiral arms of the Galaxy increases the chances of a close supernova explosion with all the consequences. But the researchers prefer the version about the associated with passing through a dense star cluster gravitational influence. In this case, located on the periphery of the cometary cloud system may lose stability, thereby increasing the risk of collision of the planet with the heavenly bodies.
All of the above is just hypothesis. However, they should not be neglected, because now the Sun is in one of these spiral arms. There are many other things to worry about.
The Eocene-Oligocene extinction called the sixth mass disasters. It happened later than the other – 33.9 million years ago, and it was not as devastating. For 4 million years extinct order of 3.2% of marine animals. Half of extinct families made up of foraminifera and sea urchins. Suffered and terrestrial organisms. Among the possible reasons, as in other cases, indicate the possibility of collision with a celestial body, volcanic activity or climate change.
Humanity will die out?
Of course. The only question is when it will happen. People may die along with the planet, Sun, Galaxy or Universe. In a million or perhaps ten billion years. But there is much more pessimistic scenario.
The hypothesis that the Earth has already begun the sixth mass extinction, there the first year. Now a group of scientists under the guidance of renowned ecologist Paul Ehrlich (Paul Ehrlich) from Stanford University found new evidence of the validity of this assumption. Was analysed the frequency of extinction of animals and plants in the periods of past extinctions, as well as dynamics, which was observed in the intervals between them. Before man began to play an important role in terrestrial ecosystem on our planet every hundred years, died out two mammal species, for every ten thousand of the then-existing species. But in the XX century this figure had increased to 114 times. For some hundred years extinct as many views as are usually killed ten thousand. If we talk about vertebrates, there is an analogy with the Cretaceous-Paleogene extinction, when the dinosaurs disappeared.
Interestingly, the authors themselves call this forecast “optimistic”, since they came from conservative projections. Now, according to Paul Ehrlich, on the brink of destruction are 40% of amphibian species, and in addition may sink into Oblivion a quarter of mammals. The extinction will affect the person because he depends entirely on his native Land.
By the way, human factor scientists call the main cause of a new extinction. Ehrlich believes that man can not sit idly by and have to fight for endangered species. He recommends to preserve the natural habitat of the species and prevent global climate change.
The thesis of the new extinction indirectly confirmed by European scientists. Claire Regnier (Régnier Claire) from the National Museum of natural history (France) said that the rate of extinction may be the extinction of invertebrates. According to new data, the anthropocene has already killed 10% of the species of snails, and a few basic types of these creatures are on the verge of death. The threshold of survival of the snails is very high, and their disappearance is a bad sign. The situation is aggravated by the fact that many of the types of humanity does not even know. Therefore, they are not subject to statistical accounting.
Studies have shown that over 200 thousand years of its existence, people have destroyed thousands of species. If you take shorter period, with 1500 people killed around 320 species of animals. This list includes the passenger pigeon, Tasmanian tiger and lived in China a freshwater Dolphin bicsi. For the restoration of terrestrial ecosystems, according to experts, it will take millions of years.
Reduced population and other animals. Recently, the United States reaffirmed a mass death of bees. From April 2014 to may 2015, was destroyed 40% of all bee colonies. In America and in the countries of the European Union bee “family” massively reduced since 2006. If the dynamics does not change, these insects can completely disappear by 2035. The cause is not known. They can be worn as a natural (parasitic bee mites of the genus Varroa) and anthropogenic (chemicals to kill pests). It is unknown what the reduction in bees will turn to other animals and plants. So, for example, may suffer population pollinated plants.
So what causes mass extinctions? Can you identify the common signs of these disasters? The unequivocal answer to these questions is not so far. If we talk about the sixth mass extinction (including the Eocene-Oligocene – seventh), it reasons will differ from the perpetrators of the previous five: apparently, it is caused by human activity. And only the people themselves can stop this.