The North Atlantic Ocean current stopped flowing. Within a few days, the icebergs in the North and South poles melted and a large amount of fresh water was injected into the ocean. Rare snow falls in India, hailstones hit Tokyo, tornadoes swept Los Angeles, and floods flooded into new york ... This is the doomsday scene outlined by the American film The Day After Tomorrow with the theme of global climate upheaval and serious natural disasters caused by greenhouse effect. It should be pointed out that although it is a science fiction film, its content is by no means a fabrication. According to the research of paleontologists, such a scene did happen in the history of the earth, and it was called extinction at least five times. What is extinction? Here, let's look back at those bleak years in the history of the earth and see how the pictures in the movies appeared one by one in history. The English word "Extinction" means extinction and disappearance, but in the Chinese biological dictionary, it corresponds to a somewhat odd noun-"extinction", which means the same as extinction, but is literally adjusted one by one. In the long process of evolution, long-term stability period and short-term drastic change period appear alternately. Extinction occurs on different scales. In the stable period, the average newborn rate is much higher than the average extinction rate, and the overall average extinction rate remains at a low level. This low-level extinction is called background extinction. Accordingly, in the period of drastic changes, many biological species became extinct in a short time, and the process of biological evolution was suddenly interrupted, so that the extinction rate suddenly increased, while the newborn rate decreased very low. This massextinction is called mass extinction or mass extinction. At present, there are about 654.38+75 million species recorded by human beings on the earth, and in fact there may be 5 million to 654.38+0 million species. But according to paleontologists, there are already 4 billion species of plants and animals on the earth, which shows that most species are out in the process of evolution. The origin of life on earth can be traced back to at least 3.5 billion years ago, but for a long time, the life forms on earth were only microorganisms and bacteria. This phenomenon lasted until the early CAMBRIAN about 530 million years ago. In CAMBRIAN, a great event happened in the history of biological evolution. Multicellular animals suddenly appeared in the ocean and quickly developed into various types with complex structures, which paleontologists called the "Cambrian explosion". It's like an evolutionary experiment. Life has made various attempts, and many animal classes have emerged, including trilobites, brachiopods, mollusks, chordates and so on. Of special significance, scientists also found the most primitive vertebrates-"Haikou fish" and "Kunming fish" in the fossils of this period. However, with the end of the "experiment", the failed products are naturally eliminated, and the survivors still have to save energy for the next "experiment". According to statistics, there have been 15 obvious extinction events since CAMBRIAN. Five of them were extinct at the turn of Ordovician-Silurian, Late Devonian-Famen, Permian-Triassic, Triassic-Jurassic and Cretaceous-Tertiary. There have been five mass extinctions in the history of the earth: the biological extinction at the turn of Ordovician to Silurian (439 million years ago). During this period, most creatures have soft tissues, and the chances of forming fossils are very small. Only those animals with shells or hard tissues left more clues, so we can't figure out what happened at that time and which species were affected. It is estimated that about 100 biological families are extinct, and the extinction rate is higher at the level of genus and species. For example, the extinction rate of brachiopods is 60%, and the extinction rate of species can reach 85%. Trilobites were hit hard in this extinction, and they could never regain their previous prosperity. This extinction event has a great impact on the organisms in the tropical areas at low latitudes, but relatively little impact on the organisms in the deep waters at high latitudes, making it the third largest species extinction event in the history of the earth. Paleontologists believe that this extinction was caused by global climate cooling. Large glaciers make ocean currents and atmospheric circulation cool, the temperature of the whole earth drops, glaciers lock up water, sea level drops, and the original vibrant coastal biosphere is destroyed, leading to a large number of species extinction. Biological extinction at the turn of the late Devonian Frazier-Famen period (367 million years ago) Although scientists can confirm that the extinction occurred in the late Devonian, we don't know how long it lasted. It may last for 500,000 years or 6.5438+0.5 million years. Moreover, we don't know whether there was a mass extinction or several successive smaller-scale extinctions, and experts haven't reached a consensus on several important facts at that time. After this extinction, 70% species disappeared. Species in the ocean are more affected than those in fresh water, and many invertebrates such as corals, brachiopods, ammonites and sea lilies have suffered great losses. However, on land, the plants of new species and emerging amphibians are much less affected. We know little about the cause of this extinction. Judging from the disproportionate disappearance of species in the warm water ocean, climate change should have occurred at that time, and global cooling may be an important factor. At the same time, there are signs that the oxygen content in shallow water also decreased at that time. At the turn of Permian-Triassic, the Great Extinction (250 million years ago). In Permian, there was a thriving scene on the earth. Sea lilies, ammonites, corals and fish flourish in the ocean, and amphibians and reptiles migrate inland. This relatively stable period lasted about 1 billion years. By the end of Permian, about 245 million years ago, the largest cluster extinction event occurred in the history of the earth. According to statistics, this extinction event reduced the number of biological families by 52% and the number of species by more than 90%, and marine organisms, especially benthic organisms and salinity organisms, were most affected. 96% of marine species are extinct, including trilobites. On land, more than three-quarters of vertebrates have disappeared, and lizards, amphibians and theropods have also decreased dramatically. Scientists have put forward many explanations: sea level fluctuation, ocean salinity change and volcanic activity. The most important factor seems to be climate change. The extinction at the turn of Triassic-Jurassic (208 million years ago) was considered relatively small by many scientists. Studies have shown that this disaster caused the extinction of marine life of 60 families, and the extinction rate of families was about 1/4. Some studies also believe that there were at least two extinction periods at the end of Triassic, with an interval of1200-17 million years. However, whether it is a single extinction or several small-scale extinctions in succession, during this period, conodonts all became extinct, and many categories such as ammonites, sponges, cephalopods, brachiopods, insects and terrestrial vertebrates all came to the end of evolution. Although the loss of this extinction is relatively small, it has freed up many "niches" and provided favorable conditions for the emergence of many new species. It was at this time that dinosaurs began their journey to rule the earth. Perhaps it is because this extinction has attracted less attention, and we don't know why, but climate change still seems to be an important factor, especially the increase of rainfall. The extinction at the turn of Cretaceous-Tertiary (65 million years ago) was the second largest extinction event in the history of the earth, and the end of the dinosaur era here made it the most widely known extinction event. According to statistics, there were 2868 genera in the biosphere at the end of Cretaceous, and only 1502 genera remained at the beginning of Tertiary, with an extinction rate of 52% and a species extinction rate of 85%. Dinosaurs on land and plankton in marine life are most affected, including some marine benthos. Its extinction rate is 97% for freshwater organisms, 58% for marine plankton, 565,438+0% for marine benthos and 30% for marine swimming organisms. In addition to the extinction of dinosaurs, the ammonite that survived the first four extinctions was finally extinct. For some reason, some species are basically unaffected, and crocodiles, turtles, lizards, mammals and birds have all successfully survived this crisis. The disappearance of dinosaurs and their relatives provided opportunities for the emergence of mammals and humans. Perhaps it is precisely because this extinction is so attractive that scientists have conducted the most thorough research on it. Experts analyzed it from the perspectives of volcanic eruption, climate change, environmental pollution and cosmic rays. At present, an explanation generally accepted by foreign scientific circles is that this extinction was the direct result of a huge asteroid or comet falling to the earth, causing drastic changes in the earth's ecosystem. Extinction is the result of various disasters. Scientists have been studying the causes and laws of extinction and put forward many explanations, such as meteorite impact, cosmic ray enhancement, volcanic eruption, climate change, atmospheric composition change, ocean salinity change, geomagnetic change and so on. Although these views sound irrelevant, they can be roughly divided into two parts, namely, the influence from inside and outside the earth. We already know that environmental changes such as volcanic eruption, marine pollution and greenhouse benefits will lead to extinction. If one or several factors are accumulated in large quantities, it may cause serious ecological disasters. From the above analysis of the causes of extinction, I am afraid that the drastic changes in climate and environment are the most direct reasons, and the harsh environment makes a large number of creatures unable to adapt. Some factors that are considered very important in the process of normal evolution (including normal extinction), such as survival adaptation and survival of the fittest, may not play a role at all in extinction events. Whether it can survive extinction depends largely on its ability to protect and shield things around it, its reproductive ability, its genetic mode and some physiological characteristics. This also explains why some creatures disappear from the whole category, while others are basically unaffected. Here, Darwin's theory of continuous and gradual natural selection is no longer applicable, and it has been replaced by leap-forward big mutation evolution. At present, many scientists believe that the extinction of dinosaurs is related to a meteorite that landed on Mexico's Yucatan Peninsula. Whispers' visit led to volcanic eruption, the earth was covered with a lot of volcanic ash, the climatic conditions deteriorated, the ecosystem was destroyed, and a large number of animals and plants died. In May this year, scientists also discovered that a crater site with a diameter of about 200 kilometers may be hidden underwater near the northwest coast of Australia. They speculate that the biggest extinction of species 250 million years ago may also be related to the impact of meteorites on the earth.

Please accept it, thank you!