In total, there are five stages of mass extinctions of animals on Earth. The most famous completely destroyed the dinosaurs, but it was not the biggest disaster. The largest extinction deprived the Earth of 95% of all animals, but made it possible for the emergence of new species that have formed the modern zoosphere.
Let's find out more about them …
1. Ordovician-Silurian extinction
The very first mass extinction of animals occurred about 450-440 million years ago. It is impossible to name the exact cause of the extinction, but most scientists are inclined to believe that the movement of Gondwana, a huge supercontinent, which included almost all of the Earth's land, was to blame.
And all because Gondwana - a giant continent from which Africa, South America, Australia, and Antarctica later "hatched" - went into a drift and headed exactly to the South Pole. Water boundaries have changed, and with them the habitual habitats of all kinds of brachiopods and molluscs. It all ended with a global cooling - water and land. What is today the Sahara Desert was then a solid glacier. The ice significantly changed the terrain: the water level in the ocean dropped sharply. In short, 60% of marine invertebrates have not been able to pass on their genes.
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2. Devonian extinction
It happened 374 and 359 million years ago. The Devonian extinction consisted of two peaks, during which the Earth lost 50% of all existing genera and almost 20% of all families. During the Devonian extinction, almost all jawless animals disappeared (only lampreys and mixins survived to this day).
The extinctions were accompanied by widespread oceanic anoxia, that is, a lack of oxygen, which prevented the decay of organisms and predisposed to the preservation and accumulation of organic matter. This effect, combined with the ability of spongy reef stones to hold oil, has made the Devonian rocks an important source of oil, especially in the United States.
3. Great Permian extinction
The same mass extinction of animals that has ever happened on our planet. Some scientists call the Permian extinction the greatest mass extinction of all time. About 250 million years ago, 70% of all land animals disappeared. In the ocean, things were even worse - 96% of marine species died. During the Great Permian Extinction, more than 57% of genera and 85% of insect species died. This is the only known extinction that has affected insects.
Because of the loss of this abundance and diversity of species, the recovery of the biosphere took a much longer period of time than other extinction disasters.
After the Permian extinction, the fauna was restored for 30 million years (some scientists believe that the restoration of the biosphere lasted 5 million years). Animals that were previously overshadowed by stronger species became widespread. So, this time is considered the period of formation of archosaurs (ancestors of modern crocodiles and extinct dinosaurs). From them came the birds, which could not have existed, if not for the Great Perm Extinction.
4. Triassic extinction
The Triassic extinction happened 200 million years ago. About 20% of all marine animals died, many archosaurs (which became widespread after the Permian extinction), and most amphibian species. Scientists estimate that half of all known animals that lived at that time died during the Triassic extinction.
A feature of the Triassic extinction is the transience. It happened within 10 thousand years, which is very fast on a planetary scale. At this time, the disintegration of the Pangea supercontinent into separate continents began. It is possible that the cause of the decay was a large asteroid that changed the weather on the planet, causing an extinction. But there is no evidence of this theory, so far not a single large crater of the Triarian period has been found.
Today in science there are several versions of the extinction that happened. The most common hypothesis about the so-called. "Methane hydrate gun", which is the most plausible. Due to volcanism and accumulation of carbon dioxide in the atmosphere, methane began to be released from bottom clathrates in huge quantities. The toxic emissions of this unpleasant greenhouse gas played the role of a trigger for a sharp global warming, which destabilized the climate on the planet and caused a total Achtung.
5. Cretaceous-Paleogene extinction
The most famous extinction occurred about 65 million years ago. It is famous for the fact that dinosaurs died out on Earth at that time. More than 15% of families of marine animals and 18% of families of land animals also died.
Many explanations have been proposed - from fantastic (dinosaurs were exterminated by little green men hunting them on flying saucers) to very plausible (climate change destroyed their ecological niche). The most famous theories say that the Earth collided with a large asteroid or got into the radiation zone from a supernova explosion.
The most interesting explanation links the extinction of the dinosaurs with the appearance of flowering plants, which is believed to have occurred 65 million years ago - just when the dinosaurs disappeared. The implication is that before that, dinosaurs ate mainly pine needles and similar foods rich in natural oils, and when they had to switch to grass, they all died of constipation!
Another very interesting theory is that they were exterminated by the first mammals, which destroyed the clutches of dinosaurs, preventing them from reproducing. This is supported by the fact that some dinosaurs lived for quite a long time in the territory of modern North America and India, where, possibly, "dangerous" mammals appeared later.
Researchers from Stanford University assure that the Earth is currently experiencing one of the early stages of another mass extinction. They shared their observations in a review that was published in the latest issue of the scientific journal Science. Their publication caused a wide response and attracted the attention of many popular science portals, such as LiveScience.
According to scientists, the planet is now experiencing a peak in biodiversity for 3.5 billion years of life on it.
However, alarming signals began to arrive long before the start of the industrial revolution: since 1500, about 320 species of terrestrial vertebrates have become extinct. And the populations of the remaining species have decreased by a quarter. Moreover, of the living vertebrates, about 33% of the species are endangered. The situation is especially dangerous with representatives of the megafauna - elephants, giraffes, hippos and other large animals.
Despite the fact that there are not so many large animals on the planet, their disappearance is accompanied by catastrophic consequences. So, in Kenya, in areas where elephants, giraffes and zebras used to live, rodents found shelter. And this, in turn, contributes to the uncontrolled eating of grass and bushes.
The result of this is soil compaction, and as a result, a decrease in yield, and subsequently desertification of vast territories. And the huge population of rodents carries many diseases. Since there are no predators, there is no one to control the number of rodents and to destroy sick individuals.
Scientists have not forgotten about invertebrates: over the past 35 years, their number has decreased by 45%. This trend is especially dangerous in light of the fact that insects pollinate about 75% of the world's food crops. In addition, insects play an important role in nutrient cycling and organic degradation. The United States alone spends $ 4.5 billion annually on the fight against predators that destroy such insects, which helps to understand the scale of the problem.
“In a global sense, extinction is the loss of a particular ecosystem. However, now the extinction of certain species is directly threatening people. While there is time to try to change something,”says Professor Rodolfo Dirzo, who is one of the authors of the review.
The previous study on the sixth mass extinction was published in the journal Nature. Then scientists were sure that the mass extinction had not yet begun, but would certainly happen in the next 2 thousand years. The researchers named 300 years as the earliest date. Be that as it may, people need to try to change the current situation now.