The earth is a special planet in the solar system, and it is also the only planet with life in the vast known universe. Although human beings have never given up their efforts to find life on other planets, so far no signs of life have been found. At present, the earliest known life form on earth appeared 3.8 billion years ago, and it is the simplest bacteria or organic matter. The turning point of the major development of the biosphere was the emergence of higher organisms with tissue and organ differentiation 500 million to 600 million years ago. Of course, from single cell to multi-cell, from simple to complex, from boneless to boneless itself is also a huge change.
Mass extinction refers to the sudden extinction of many kinds of organisms in geological history, and its extinction amount is much higher than that of normal biological background, which is obviously catastrophic and occurs almost simultaneously. In recent years, it is considered that cluster extinction is a mutation form of biosphere evolution and development, which may be mainly caused by the comprehensive attack of catastrophic events in a specific geological period and the drastic environmental changes caused by it.
The research on the extinction of biological clusters and its causes has attracted great attention of scientists all over the world, and is considered as the second hotspot of geoscience research after plate tectonics. With the development of modern science and social civilization, human beings have shown unprecedented great concern for their own living environment, destiny and the future of the earth. Studying the evolution and extinction of organisms in geological history, especially the cluster extinction and its catastrophic impact, can not only make us better understand the evolution of the planet where human beings live, the interaction of various circles and the deep geodynamic process reflected, but also better guide the exploration of mineral resources; Moreover, we can get an important enlightenment from the interaction between biology and environment-protecting the environment is to protect human beings themselves.
Since the emergence of life on the earth, the biological world has undergone a long evolution and many major changes, forming the colorful biosphere we see today. In the process of biological evolution and development, environmental change always plays an important role: on the one hand, it promotes the development and evolution of organisms, on the other hand, it also leads to the decline and extinction of organisms. Since Neoproterozoic, the biological world has experienced many extinction events. Among them, there were 7 recognized extinctions of major biological groups, which occurred at the end of Neoproterozoic (600Ma? ), Late Cambrian (5 10Ma), Ordovician (439Ma), Late Devonian (F/F, 367Ma), Permian (250Ma), Triassic (2 10Ma) and Cretaceous (66ma); In addition, there are other important extinction events, such as late Early Jurassic, late Middle Jurassic, late Late Jurassic, late Early Cretaceous, middle Late Cretaceous, late Eocene and late Pliocene. Among them, the Neoproterozoic/Cambrian, Permian/Triassic and Cretaceous/Tertiary mass extinctions, which had the greatest impact on biosphere changes, marked the end of Proterozoic, Paleozoic and Mesozoic respectively, and heralded the arrival of a new stage of earth evolution.
The extinction of mollusks represented by Ediacara fauna at the end of Proterozoic is considered to represent an unsuccessful attempt in the history of biological evolution. These extinct creatures are fundamentally different from those after Phanerozoic. At present, the study of thousands of specimens found all over the world shows that at the level of genus and species, no biota survived to Phanerozoic and all of them became extinct. At the phylum level, according to the gradual change theory, they all belong to the known biological categories (coelenterates, worms, arthropods, etc.). However, it is now considered that these creatures are completely different from those known in modern times. Then came the rapid biological radiation in the early CAMBRIAN-the door-building era.
At the turn of P/T, the worst cluster extinction occurred in the history of biological evolution. More than 54% of families, 75% ~ 82% of genera and about 94% of species were extinct, and the biological world was devastated. Some important Paleozoic organisms, such as trilobites, corals, plate corals, ancient cups, reptiles, soft-tongued snails and jellyfish, were completely extinct; Other categories have been hit hard.
At the turn of K/Ter, about 52% ~ 70% of genera and more than 80% of species became extinct in a short time. The ammonites, dinosaurs and sessile clams are all extinct; Arrow stone and ancient abdomen foot declined rapidly; Foraminifera, coral, ultramicro and sea urchin were completely replaced.
Accompanied by the three major disasters in the biosphere, great changes have taken place in the lithosphere, hydrosphere and atmosphere of the earth almost simultaneously.
In the past, there were two completely different views on the reasons for the extinction of biological clusters: internal cause theory and external cause theory. In recent years, it generally tends to be caused by external factors-the catastrophe of external environment; The focus of debate is whether this environmental catastrophe is caused by external factors (star collision, supernova explosion, solar flare, annual cycle of the Milky Way and solar companion) or internal factors (volcanic eruption, plate activity, mantle plume, sea level change, magnetic pole reversal, anoxic events, salinity change, temperature change and food chain interruption). Among them, the most influential hypotheses are "star collision" and "volcanic eruption", and both sides have sufficient evidence. But one thing is clear, that is, the deterioration of the environment is the direct cause of the extinction of biological clusters. Judging from the research results in recent years, the theory of the cause of the impact of extraterrestrial stars is more advantageous. P/T event (Siberian overflow basalt activity) and T/J event (CAMP large igneous area), which used to be considered as the typical volcanic genesis of the earth, also provided important new evidence for the genesis of extraterrestrial meteorite impact, which challenged the volcanic genesis theory of these two events (Beck et al., 2001; Kaiho et al, 2001; Olson et al., 2002).
The periodicity of the extinction of biological clusters is also an important scientific issue. Whether there is a cycle is an important controversial issue; In addition, there are different views on the understanding of periodicity. Strict periodicity is difficult, which is also related to geological dating. Geologically speaking, the period cannot be as strict as that in mathematics. Phenomenologically, there seems to be a certain periodicity. There are also objections and doubts. At present, there are two opinions on periodicity: ① Raup & Sepkoski (1984 ~1993): 26 ~ 27ma; ② Rampino & Stosur (1984 ~1994): 30 ~ 33ma.
At present, the main point of view about the extinction of periodic biological clusters is that it may be related to the periodic passage of the solar system through the galactic plane, which coincides with the time, about 33.5Ma. Another way of saying this is that the solar system may be a binary star system. In addition to the sun, there is another star (Nemesis), which is at perihelion every 26Ma, thus disrupting the balance of Oort stars in the past and making some comets enter the solar system. However, the existence of companion stars has not been observed. 1998 science and technology newspaper reported an amazing news that a companion star of the sun was discovered. But other properties have not been clarified.
Recent studies have also suggested that there is a certain periodicity at the lower level of biological groups. 1996 "Science" published a research paper on ammonites, mainly on the study of late Paleozoic ammonites and also found periodicity. But not very violent. It shows the occurrence and substitution of some small groups. From June, 5438 to October, 2000, Science published a new scientific research achievement. According to the evolution of planktonic foraminifera since Mesozoic, it also shows a period of 30Ma.
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