According to the research of scientists for the entire existence of life on Earth, several periods are distinguished during which there were mass extinctions of living organisms.
Extinction is a phenomenon in biology and ecology, which consists in the disappearance (death) of all representatives of a certain biological species or taxon. Extinction can be natural or anthropogenic. With especially frequent cases of extinction of biological species in a short period of time, one usually speaks of mass extinction. During mass extinctions, the rate of extinction of species was much faster than usual.
The duration of the extinction is usually estimated to within 1 million years. The causes of mass extinctions have not been precisely established, but there are many different theories.
The main causes of extinctions:
- changes in the environment;
- disasters;
- the size of the population (in animals that breed only in groups, for example, a wandering pigeon);
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- genetic diversity (can threaten cheetahs);
- life expectancy;
- predators;
- competition with other species of animals in the same living space;
- attacks of parasites (any disease);
- human factor.
Some scientists are of the opinion that we are living during one of the mass extinctions. It was named Holocene.
The age of the Earth, according to scientists, is 4.54 ± 0.05 billion years. The earliest indisputable evidence of life on Earth is estimated to be at least 3.5 billion years old.
Scientists have identified six of the largest extinctions in Earth's history:
1. Ordovician-Silurian - 440 million years ago, more than 60% of marine invertebrate species disappeared. The third in the percentage of extinct genera out of the five strongest extinctions in the history of the Earth and the second in terms of losses in the number of living organisms.
The main hypotheses of the causes: prolonged cooling, fluctuations in the level of the world ocean, an outbreak of gamma radiation, volcanism and erosion.
2. Devonian - 364 million years ago, the number of species of marine organisms decreased by 50%. The first (and strongest) peak of extinction occurred at the beginning of the Famennian age - the last century of the Devonian period, about 374 million years ago, when almost all jawless suddenly disappeared. The second impulse ended the Devonian period (about 359 million years ago). In total, 19% of families and 50% of genera died out.
The main hypotheses of the causes: the extinction took place over a long period, so it is very difficult to isolate a single cause. Among the hypotheses - environmental change, meteorite fall, plant evolution, erosion effect.
3. Great Permian - 251.4 million years ago there was the most massive extinction of all, which led to the extinction of more than 95% of the species of all living beings. During this period, 96% of all marine species and 70% of terrestrial vertebrate species became extinct. The disaster was the only known mass extinction of insects, as a result of which about 57% of genera and 83% of species of the entire class of insects became extinct. Due to the loss of such a number and diversity of biological species, the restoration of the biosphere took a much longer period of time compared to other disasters. According to researchers from the Massachusetts Institute of Technology, 96% of aquatic species and 70% of terrestrial species became extinct in just 60 thousand years.
The main hypotheses of the causes: changes in the environment, increased volcanic activity, the fall of meteorites, the release of methane from the seabed.
4. Triassic - 199.6 million years ago, at least half of the currently known species that lived on Earth at that time became extinct. This event freed up ecological niches, allowing dinosaurs to dominate since the Jurassic. The Triassic extinction occurred in less than 10,000 years and occurred just before Pangea began to fall apart. Statistical analysis of losses to marine life at this time suggests that the decrease in diversity was associated with a decline in the rate of speciation rather than an increase in extinction.
The main hypotheses of the causes: a gradual change in climate, the fall of an asteroid, massive volcanic eruptions, the release of methane.
5. Cretaceous-Paleogene - 65.5 million years ago, one sixth of all species, including dinosaurs, became extinct. Along with the dinosaurs, marine reptiles became extinct, including mosasaurs and plesiosaurs, flying dinosaurs, many mollusks, including ammonites and belemnites, and many small algae. In total, 16% of families of marine animals (47% of genera of marine animals) and 18% of families of land vertebrates died. Presumably, some dinosaurs (Triceratops, theropods, etc.) existed in western North America and India for several million years at the beginning of the Paleogene after their extinction in other places.
The main hypotheses of the reasons: an asteroid fall, a supernova explosion or a close gamma-ray burst, a collision of the Earth with a comet, an increase in volcanic activity, a sharp drop in sea level, a change in average annual and seasonal temperatures, a sharp jump in the Earth's magnetic field, an excess of oxygen in the Earth's atmosphere, a sharp cooling of the ocean, a change in the composition of sea water, a massive epidemic, a change in the type of vegetation, the appearance of the first predatory mammals.
6. Eocene-Oligocene - 33.9 million years ago there were significant changes in the composition of marine and terrestrial flora and fauna. It was inferior in scale to the first five mass extinctions.
The main hypotheses of the causes: collisions with asteroids, eruptions of supervolcanoes, climate change and partial shading of the Earth by hypothetical rings of the Earth.
Hypothetically, we are living in the next largest extinction period, called the Holocene, which began about 13 thousand years ago with the disappearance of large mammals, the so-called megafauna. Extinction is thought to be mainly due to human activity.
This extinction encompasses numerous families of plants and animals, including mammals, birds, amphibians, reptiles, and arthropods. 875 extinctions occurring between 1500 and 2009 were documented by the International Union for Conservation of Nature and Natural Resources. The vast majority of cases are not documented. In theory, the current rate of extinction could be up to 140,000 species per year.