Discussion on the causes of CAMBRIAN life explosion
The CAMBRIAN explosion has attracted countless paleontologists and evolutionists to look for evidence to explore its causes. The evidence since 100 has produced two basic viewpoints to explain the CAMBRIAN explosion. One view is that the CAMBRIAN explosion is an illusion, which is held by some Darwinists or neo-Darwinists. Because evolution is gradual, the so-called "eruption" only means the first discovery of organisms that have existed and developed widely as early as Precambrian in the fossil record, while other biochemical groups may be "missing files" because of incomplete geological records. The reason for this "missing file" is that the Precambrian strata experienced high temperature and high pressure, and the fossils in them were destroyed. This explanation is no longer convincing because a large number of simple prokaryotes such as bacteria and cyanobacteria have been found in Precambrian fossil sediments. Another view is that the Cambrian explosion represents a real event in the process of biological evolution, and scientists explain this phenomenon from the changes of physical environment and ecological environment.
1965, two American physicists proposed that the CAMBRIAN explosion was caused by the physical factor of oxygen level in the earth's atmosphere. They believe that there was little or no free oxygen in the atmosphere of the early earth, and oxygen was the product of photosynthesis of Precambrian algae and gradually accumulated. Metazoa need a lot of oxygen, on the one hand, for breathing, on the other hand, oxygen also absorbs a lot of harmful ultraviolet rays in the atmosphere in the form of ozone to protect metazoa from harmful radiation.
Biologists discuss this problem from the ecological relationship of organisms themselves, because geological evidence denies this view of oxygen theory. /kloc-There were a large number of severely oxidized rocks in a wide range of sediments from 0/100 million to 2 billion years ago, indicating that there were enough oxygen conditions for life explosion during this period. Therefore, biologists explore the causes of the CAMBRIAN explosion from the emergence of sexual reproduction and the emergence of biological harvesters.
According to fossil data, eukaryotic algae had sexual reproduction about 900 million years ago. In fact, sexual reproduction appeared earlier. The occurrence of sexual reproduction plays an extremely important role in the evolution of the whole biological world. Because sexual reproduction provides genetic variation, it may further increase the biodiversity of organisms, which is one of the reasons for the Cambrian explosion.
The biological reaper hypothesis was put forward by American ecologist Stanley, which is an ecological theory to explain the Cambrian explosion, that is, the harvesting principle. Stanley believes that for most of the Precambrian 2.5 billion years, the ocean was an ecosystem composed of simple primary producers such as prokaryotic cyanobacteria. The community in this system belongs to a single and unchangeable community in ecology, and the trophic level is also simple and unique. Because the physical space is tenaciously occupied by this small but huge biological community, the evolution of this community is very slow and has never been rich in diversity. The key to the CAMBRIAN explosion is the emergence and evolution of herbivorous harvesters, that is, protozoa (cyanobacteria) that eat prokaryotic cells. Harvesters create more diversity space for producers, and the increase of producer diversity leads to the evolution of more specific harvesters. The pyramid of trophic level develops rapidly in two directions: producers at the lower level have added many new species, enriching species diversity, while at the top, new "reapers" have been added, enriching the diversity of trophic level. Thus, the biodiversity of the whole ecosystem is continuously enriched, which eventually leads to the explosion of life in CAMBRIAN.
At present, scientists have not found direct evidence to prove the correctness of the "harvest theory", however, some indirect evidence supports this theory. One of the indirect evidences comes from Precambrian stromatolites, which are composed of algae and preserve the richest producer communities in Precambrian. Today, stromatolites only exist in a large number in harsh environments that lack metazoan harvesters, such as salt lakes with excessive salt content. The abundance of algae in Precambrian strata may reflect the poverty of harvesters at that time. In addition, studies in the ecological field also provide some indirect evidence. Studies have shown that putting carnivorous fish into artificial ponds will increase the diversity of plankton. Removing sea urchins from a diverse algae community will make an algae dominant in the community and reduce its diversity.
As the second unsolved case in geological history, the Cambrian explosion has always been concerned by people. With the continuous discovery of fossils and the establishment of new theories, the mystery of this local history will eventually be revealed to the world.
Since the 7th issue of Biology Bulletin in 2000
American scientists have recently found evidence that calcium, which appears in large quantities in the ocean, may have played a great role in the CAMBRIAN explosion. About 530 million years ago, the animals on the earth experienced a rebirth, and the ancestors of all living animals evolved bones and shells at this time. For a long time, scientists have been puzzled about what prompted life on earth to suddenly present colorful diversity in CAMBRIAN.
Why did animals evolve shells? The most obvious reason is to avoid being eaten. Another view may help explain why this difficult part of life first appeared in the early CAMBRIAN: the increase of oxygen in the atmosphere allowed animals to build more complex body structures. In 1970s, scientists put forward a new hypothesis: marine organisms began to evolve shells in the early CAMBRIAN environment in order to digest calcium that suddenly appeared in the body. However, there is a lack of direct evidence of marine chemical changes.
Small pieces of early CAMBRIAN seawater are hidden in the rocks. Now, three geochemists have measured the calcium content in these packets. Sean Brennan of the US Geological Survey, Tim Lowenstein of new york State University and Juske Horita of Oak Ridge National Laboratory have tested 24 kinds of salt crystals that may have formed on the seabed during the Precambrian period 544 million years ago, and also tested 14 Precambrian samples formed about 565.438 million years ago. They broke the crystal and analyzed the composition of tiny water droplets in it. In the June issue of Geology, they reported that the calcium content in the early Cambrian was at least three times higher than that in the original generation.
The researchers pointed out that this kind of calcium should come from the calcium sprayed into hot seawater during the eruption of the Mediterranean ridge volcano. They stressed that the high level of crustal activity, calcium injection and the "Cambrian explosion" seemed to support the view that early life evolved shells and bones to solve the potential toxicity caused by the rising calcium content in seawater.
Robert Goldstein, a geologist at the University of Kansas in the United States, said that the idea that the chemical properties of seawater affect the evolution of life is exciting. He said that although more work is needed to understand whether calcium in the ocean is the cause or result of life evolution, calcium should be an important factor. Other environmental factors are also important.
(Source: Science Times, June 4, 2004)