Life on Earth arose almost immediately after its formation. Since water is the basis of life, it can be assumed that after the planet cooled down to acceptable temperatures, there were no obstacles to the emergence of the first forms of life.
Science estimates the age of the Earth at 4 billion years, of which about 3.6 billion years of life has existed on it. However, you need to understand that evolution is a very long and complex process, and most of the time that life has existed, it had a completely different view from which we are used to.
Scientists use the term "implicit life"; it means that traces of organisms of that era could not leave (as a rule, due to their small size) any noticeable traces in the fossils. The implicit period lasted about three billion years, and only in the last 500-600 million years we can observe the manifestations of life on our planet in an "explicit" form - in the form of fossils or waste products of organisms.
Approximately 540 million years ago, an event occurred that radically changed everything that happens on our planet - the so-called "Cambrian explosion". Suddenly (in a very short period, only a few million years), a huge number of different types of living organisms appear on the earth. The reasons for this phenomenon can be very diverse - from improving environmental conditions to the emergence of a stable mechanism of inheritance of beneficial mutations in animals. One thing is certain - if it were not for this phenomenon, the Earth would still be inhabited by unicellular bacteria, and there could be no question of any intelligent life.
Neither before nor after this event has our planet experienced anything like this. A sharp increase in biomass, not only in quantitative but also in qualitative terms, happened to life on Earth only once. On the other hand, global events that occurred at different times after it led to the opposite results - mass extinctions of not just individual species, but also entire kingdoms of flora and fauna.
In the 500 million years that followed the Cambrian explosion, the Earth experienced five major extinctions alone and about fifteen small ones. It should be noted here that for quite a long time, scientists did not even assume that there were extinctions. That is, fossils of extinct animals were found constantly, but this was attributed to the peculiarities of natural selection. However, the fact that the reason for the departure from the scene of some species and the arrival of others may be due not only to evolution or adaptability has created a real sensation in the scientific world.
No less surprising was the fact that the time of the extinctions practically coincided with the geological annals of our planet. That is, all the extinctions fit into the already existing picture of the geological division of time into eras or eras (Paleozoic era, Mesozoic, and so on); and did not just fit in, but occurred just at the junctions of these time intervals. This pattern was discovered only 40 years ago by the University of Chicago scientists D. Sepkoski and D. Raup; they were also the discoverers of all major extinctions of animals and plants on our planet.
The first mass extinction occurred about 440, the last about 65 million years ago. At first glance, there is no dependence of the extinction time on any factors, that is, it was not possible to predict the time of this or that event of this kind - everything happened quite spontaneously. However, as scientists got new information, some patterns were still obtained.
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Three extinctions have a periodicity of about 150 million years - these are Devonian, Permian and Cretaceous-Paleogene. Each of them is quite remarkable, not like the others. For example, the Devonian extinction affected mainly the inhabitants of the seas. With him, the most primitive vertebrates died out. The Permian extinction is one of the most dangerous for life on Earth. Under him, about 95% of aquatic and 75% of terrestrial species died. As for the extinction at the junction of the Cretaceous and Paleogene eras, thanks to it, large reptiles (dinosaurs) disappeared, allowing mammals to become the dominant class on our planet. Roughly speaking, if it weren't for this extinction, mankind would not exist either.
What could be the cause of such cataclysms? The scale and transience of the extinctions indicated that there was some factor of external irresistible force that the then life on Earth could not cope with. There are opinions that similar events were caused by phenomena of a purely terrestrial origin: for example, eruptions of basalt from the depths of the Earth or a decrease in the level of the world ocean due to the release of huge amounts of methane and hydrogen sulfide from its depths. However, the scale of such events does not allow us to speak about their global significance for the biosphere of the planet. A separate group of versions consists of those that consider the activity of life itself as the cause of cataclysms. For example, the emergence of bacteria that emit large amounts of carbon dioxide, which led to the global greenhouse effect.
On the other hand, there were about 15 basalt eruptions, and all of them led to local extinctions, and gas emissions or changes in the composition of the atmosphere could not significantly change the ecosystems of the entire planet. Hence, the reason must be sought outside the Earth; the threat could come from outer space and have a very diverse nature: from the fall of giant meteorites to a significant increase in cosmic radiation reaching the earth's surface.
Some scientists believe that the period in which we live now is also a period of mass extinction (and even the term was invented for it - Holocene). And the main reason is not cosmic or geological reasons at all. The main reason is the anthropogenic factor, that is, human activity. Will we be able to survive as a result of this "extinction", will we not kill ourselves? This option is quite possible, as well as the fact that in a few million years other intelligent creatures will analyze the cause of another extinction and not find it …