In addition to the two most famous extinctions - the one that deprived the world of dinosaurs, and the largest, Permian - there were at least three large-scale extinctions that destroyed a huge number of species. Today, a number of scientists believe that we are living in the period of the sixth extinction.
This extinction is considered the oldest - it happened 440 million years ago. Life on the planet in the Ordovician period became more difficult, the seas were filled with primitive octopuses, trilobites, corals, starfish, eels and jaw-fishes. Plants on land also joined the struggle for existence.
In the percentage ratio of all surviving and dead species, this extinction takes an "honorable" third place. There were at least two waves of extinction, with an interval of about a million years. Their sad result was the death of 60% of marine invertebrates: bivalve molluscs, brachiopods, bryozoans, echinoderms - almost all of them have sunk into oblivion. Only the great Permian extinction killed more marine organisms.
The Ordovician-Silurian extinction is often associated with the southern movement of the ancient supercontinent of Gondwana. A drop in ocean levels, coupled with cooling, has destroyed habitual biological niches and led to a decrease in biodiversity. Among the main versions, scientists also consider the fall of an asteroid and large-scale volcanic eruptions.
According to one of the hypotheses, the Ordovician-Silurian extinction occurred due to the outbreak of gamma radiation from a supernova located 6 thousand light years from us. It has reduced the ozone layer of the atmosphere and powerful ultraviolet radiation has destroyed millions of terrestrial organisms. The supernova could be in the nearest arm of the Milky Way.
The catastrophe that happened about 360 million years ago is named after the Devonian period, which became the time of the evolution of some fish, whose strong fins allowed them to move on land. Trilobites at this time lose their dominance in the sea, and on land the plants become more complex.
Extinction could have had two stages, which took place 374 and 359 million years ago. According to other versions, there were not two stages, but much more. Be that as it may, the number of marine species decreased by 50%, and specifically in the first period, almost all jawless were destroyed. Terrestrial and freshwater organisms were practically not affected, but the reef system was badly shabby.
Scientists find it difficult to name the main reason for the Devonian extinction. Someone again associates it with the fall of an asteroid, someone with an increase in temperature and evaporation of water, others point to the evolution of plants. There is also a theory according to which there was no mass extinction at the end of the Devonian, but the formation of new species greatly slowed down.
Sedimentary analyzes have shown that the environment has changed dramatically in the Late Devonian. A sharp decrease in the oxygen content in the oceans (anoxia) was observed, while the rate of carbon deposition, on the contrary, increased. Anoxia prevented the decay of organisms, and there was more and more organic matter.
Great Permian extinction
The largest known extinction occurred long before the death of the dinosaurs - 252 million years ago. It became the point of demarcation of the Permian and Triassic periods. The planet was then dominated by species that lived and laid eggs on land. However, such a competitive advantage did not save them. 70% of terrestrial vertebrate species and 96% of all marine species became victims of extinction.
The catastrophe happened in just 60 thousand years.Many parareptiles (primitive tetrapods), arthropods and fish, 83% of all insect species, have gone into oblivion. But thanks to this event, the ancestors of the dinosaurs, which for a long time remained in the shadow of evolution, were able to develop.
The reasons for the Permian extinction are also far from unambiguous and are hotly discussed in the scientific community. All of them are similar to the previous ones: an asteroid fall, volcanic eruptions, and a large-scale drought. Recently, another confirmation was found by the most popular of the theories - volcanic. Stony deposits (at the end of the Permian period, they were the seabed) of the United Arab Emirates, which have hardly changed over millions of years, helped to answer some questions. The reason for the death of species could be the saturation of the atmosphere with carbon dioxide after the eruption of Siberian volcanoes.
The extinction that occurred 199 million years ago is used as the boundary between the Triassic and Jurassic periods. At that time, relatively large dinosaurs were already walking on the earth, which, nevertheless, experienced competition with other reptiles.
As a result of the catastrophe, conodonts, which constituted 20% of all marine families, died out, archosaurs, therapsids and amphibians were badly damaged. The extinction took place in 10 thousand years, providing an opportunity for dinosaurs to dominate the Earth in the subsequent Jurassic period.
Among the possible causes of extinction, the so-called "methane hydrate gun" hypothesis is often mentioned, according to which rising ocean temperatures liberate methane from sediments located under the seabed. Methane is a greenhouse gas, so temperatures start to rise in leaps and bounds, leading to more methane release. It looks like a vicious circle, and it is impossible to stop the process, just as it is impossible to stop the shot if the trigger is already pressed. Other versions are also actively discussed.
It was this cataclysm, which occurred 65 million years ago, that killed dinosaurs, marine reptiles and flying dinosaurs. But there are other hypotheses, which, as a rule, complement the main one - the asteroid one. In addition to the well-known tyrannosaurs, triceratops, ankylosaurs and other lizards, small mammals actively spread during the Cretaceous. They were destined to inherit the world.
In total, 16% of families of aquatic animals and 18% of families of land vertebrates became victims of the disaster. Experts find it difficult to say whether the extinction went in stages or happened in a short period of time. It is believed, for example, that the herbivorous Triceratops could have existed for several million more years.
A recent study by experts from Princeton University, Massachusetts Institute of Technology, the University of Lausanne and Amravati University leans towards the version of volcanic eruptions. Analysis of the geological formations in the Deccan Plateau traps helped to find out when they began and how long they continued. It turned out that large-scale eruptions began to occur 250 thousand years before the fall of the alleged asteroid and lasted for 500 thousand years. During this time, the released carbon dioxide acidified the World Ocean, which led to the death of many species and disruption of food chains.
None of the mass extinctions has generated as many hypotheses as the Cretaceous-Paleogene. In addition to popular scientific versions (volcanoes, asteroids, predatory mammals, etc.), semi-fantastic ones also appeared. Some seriously argue that the discovered "graveyards" of dinosaurs, including the bones of many individuals, testify to the man-made destruction of lizards.
Chronicle of the Earth
As you can see, the greatest mass extinctions occurred at different periods and at different time intervals. So, between the Ordovician-Silurian and Devonian extinctions lies 76 million years, and the Triassic and Cretaceous-Paleogene are separated by 134 million years.
However, this is if we take it on faith that such extinctions really took place.Perhaps new species emerged more slowly, and the extinctions themselves were not pronounced. And each of the mass extinctions could represent a series of smaller disasters, or the number of large-scale cataclysms themselves was higher.
Where did this uncertainty come from? We still know very little about the history of the Earth. Charles Darwin developed the concept of incompleteness of the fossil record. In the work "The Evolution of Taxonomic Diversity", authored by A. Alekseev, V. Yu. Dmitriev and A. G. Ponomarenko, it is indicated that modern science knows only 1-2% of the species that existed on Earth. Simply put, we judge mass extinctions by looking at the few organisms found in fossils. This is how scientists determine what percentage of species and genera did not survive until the next period.
Science doesn't know enough to answer all of these questions. We can not only confidently name the causes of the disasters, but also understand whether they were in reality. In any case, in the form in which people present them.
Commonality and differences
But let's try to isolate similarities and differences. Before us are five mass extinctions (six if we take into account the Eocene-Oligocene). It is logical to assume that many of them had similar reasons. At the same time, the two most popular versions - volcanoes and falling celestial bodies - are most criticized. It is known that the most powerful volcanic activity took place during the Cretaceous-Paleogene and Permian extinctions. However, if we consider all known cases of extinctions (and there are at least eleven of them), it turns out that large-scale geological processes can be correlated with only six.
A similar situation is with asteroid falls. The death of the dinosaurs coincides in time with the fall of a giant asteroid near the island of Yucatan. From it was the Chicxulub crater with a diameter of 180 km and an initial depth of up to 20 km. The energy generated from the fall was 2 million times higher than the explosion energy of the thermonuclear Tsar Bomb, and this could be enough to change life on Earth. But with the Triassic-Jurassic extinction, it is more difficult: scientists have not yet found craters that could explain it.
But perhaps the reasons for the extinctions should be sought elsewhere? Recently, this possibility was announced by scientists from the University of Western Sydney, working under the direction of Professor Miroslav Filipović (Miroslav Filipović). They drew attention to the timeline of the movement of the solar system. Our Sun orbits the center of the Milky Way galaxy in 200 million years. On its way, the system passes through galactic spiral arms, where the density of stars and interstellar gas is higher. The constructed model helped to find out that mass extinctions coincide with the passage through these arms. This applies to the Cretaceous-Paleogene, Triassic, Permian, Late Devonian and Late Ordovician extinctions.
According to the authors, a coincidence is possible, but its probability is very small. Scientists do not undertake to say what exactly ruined terrestrial organisms. In theory, the passage through the spiral arms of the Galaxy increases the chances of a nearby supernova explosion with all the ensuing consequences. But the researchers themselves prefer the version of the gravitational influence associated with the passage through a dense star cluster. In this case, the cometary cloud located on the periphery of the system may lose stability, thereby increasing the risk of collision of the planet with celestial bodies.
All of the above are just hypotheses. However, they should not be neglected, because now the Sun is in one of these spiral arms. There are many other concerns as well.
The Eocene-Oligocene extinction is called the sixth in the list of mass catastrophes. It happened later than others - 33, 9 million years ago, and was not so destructive. For 4 million years, about 3.2% of marine animals died out. Half of the extinct families were foraminifera and sea urchins. Terrestrial organisms also suffered.Among the possible reasons, as in other cases, indicate the possibility of a collision with a celestial body, volcanic activity or climatic changes.
Will humanity die out?
Undoubtedly. The only question is when it will happen. People can die along with the planet, the Sun, the Galaxy or the Universe. In a million or, for example, ten billion years. But there is also a much more pessimistic scenario.
The hypothesis that the sixth mass extinction has already begun on Earth has existed for several years. Now a group of scientists led by the famous ecologist Paul Ehrlich of Stanford University has found new evidence of the validity of this assumption. The frequency of extinction of animals and plants during the periods of past extinctions was analyzed in detail, as well as the dynamics that was observed in the intervals between them. Before humans began to play an important role in the terrestrial ecosystem, two species of mammals died out on our planet every ten thousand species that existed at that time. But already in the XX century, this figure increased 114 times. In just a hundred years, as many species have become extinct as usually perish in ten thousand. If we talk about vertebrates, then there is an analogy with the Cretaceous-Paleogene extinction, when the dinosaurs disappeared.
It is interesting that the authors themselves call such a forecast “optimistic”, since they proceeded from conservative forecasts. Now, according to Paul Ehrlich, 40% of amphibian species are on the verge of destruction, and besides that, a quarter of mammals may sink into oblivion. Extinction will also affect humans, because they are entirely dependent on their native Earth.
By the way, it is the anthropogenic factor that scientists call the main reason for a possible new extinction. Ehrlich believes that humans cannot sit back and must fight for endangered species. He recommends preserving the natural habitat of species and preventing global climate change.
The thesis of a new extinction is indirectly confirmed by European scientists. Claire Régnier of the National Museum of Natural History (France) believes that extinction may be an indicator of extinction. According to new data, in the Anthropocene epoch, 10% of snail species have already died, and several more main species of these creatures are on the verge of death. The survival threshold of snails is very high, and their disappearance is a bad sign. The situation is aggravated by the fact that humanity is not even aware of many species. Therefore, they are not covered by statistical accounting.
Studies have shown that over 200 thousand years of its existence, man has destroyed about a thousand species. If we take a shorter period, then since 1500, people have killed about 320 species of animals. This list includes the wandering pigeon, the Tasmanian tiger and the freshwater baiji dolphin that lived in China. The restoration of the terrestrial ecosystem, according to experts, will take millions of years.
Populations of other animals are also declining. The mass death of bees was recently confirmed in the United States. From April 2014 to May 2015, 40% of all bee colonies were destroyed. Both in America and on the territory of the EU countries, bee "families" have been massively reduced since 2006. If the dynamics do not change, then these insects could completely disappear by 2035. The reasons for the phenomenon are not known for certain. They can be both natural (mites of the genus Varroa parasitizing on bees) and anthropogenic (chemical preparations for the destruction of pests). It is also unknown how the decline in bees will turn out for other animals and plants. For example, populations of pollinated plants can be severely affected.
So what are the causes of mass extinctions? Is it possible to identify common signs of these disasters? There is still no unequivocal answer to these questions. If we talk about the sixth mass extinction (taking into account the Eocene-Oligocene - the seventh), then its causes will differ from those responsible for the previous five: apparently, it is caused by human activity.And only people themselves can prevent this.