From the first fish on the earth to the most prosperous group of vertebrates at present, compared with the ups and downs of the development history of dinosaurs and other reptiles, the evolution of fish seems to be so long and uneventful. shock. In fact, this process implies two major revolutions in the evolution of vertebrates, the emergence of the jaw and the occurrence of landing. Two large branches developed among the bulky swimmers in the Paleozoic oceans. One branch further adapted to life in the water, and finally evolved into today's various fish, becoming complete conquerors of the earth's waters; the other branch left Waters, developing towards land with more diverse and challenging living conditions, becoming today's four-legged animals.
There is a rule in the evolution of living things, that is, the higher the morphological level, the faster the evolution. With the advent of the Cambrian period, the fire of life gradually formed a prairie fire. In the vast ocean, not only bacteria, cyanobacteria and single-cell, colony-cell or multi-cell plants live, single-cell animals and multi-cell invertebrates such as sponges, coelenterates and worms live, but also the body wall Animals composed of three germ layers, they are increasingly higher invertebrates, such as bryozoans, brachiopods, molluscs, arthropods, and echinoderms. Some of them were floating in the water, and some were fixed on the seabed or crawling on the seafloor. The situation of explosive prosperity of organisms appeared. It was the beginning of the Paleozoic Era.
On November 4, 1999, the British "Nature" magazine, one of the internationally renowned scientific magazines, published a research paper written by a Chinese scholar that caused a strong sensation in the academic community. The article reports on the earliest known vertebrates discovered in the Early Cambrian Chengjiang biota in my country - Kunming fish and sea fish. The same magazine published a comment by a French scholar titled "Catching the First Fish": This major discovery from China shows that "vertebrates have already begun to differentiate in the Early Cambrian." The first fish under the earth has been found!
Chengjiang County is 63 kilometers away from Kunming City. In June 1984, Hou Xianguang from the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences went alone to Maotian Mountain in Chengjiang for a paleontological investigation and discovered some well-preserved invertebrate fossils with strange shapes. Subsequently, more than 10 scientists, including Hou Xianguang and Chen Junyuan, came here to conduct systematic excavation and research on various fossil sites. More than 50,000 paleontological fossils were collected, including more than 80 species and belonging to more than 40 phyla. This discovery shocked the world and was called one of the most amazing discoveries of the 20th century.
The main reason why the Chengjiang fauna has attracted great attention is that the Chengjiang fauna is not only diverse and well-preserved, but also of great scientific significance. In 1946, the world-famous Ediacaran biota fossils were discovered in Australia in the late Precambrian strata about 600 million years ago, which mainly include jellyfish, sea gills and worms. In 1909, the fossils of the Burgess fauna discovered in the mid-Cambrian strata of Canada about 510 million years ago are some higher metazoans, such as arthropods, micronematodes, trachobranchs and coelenterates. categories and other new categories. The discovery of the Chengjiang fauna suddenly increased the number of fossils known to people from more than 20 categories to more than 40 categories, causing a sensation around the world. Chengjiang Maotian Mountain was listed as a scientific site by the United Nations. The interval between the Ediacaran biota and the Burgess fauna is 85 million years, and there has been no fossil evidence of transitional types between the two. The Chengjiang fauna is right in the middle of these two fossil groups, connecting the past and the future. In addition, although many species in the Chengjiang fauna have long been extinct, a large part have continued to evolve to this day and constitute the diversity of living organisms. In other words, the ancestors of important categories of modern animals can be found in the Chengjiang fauna.
There are currently divergent opinions on whether Darwin's theory of evolution is still applicable to the "Big Bang" issue represented by the Chengjiang fauna. One opinion is that millions of years are short compared to the 4.6 billion-year history of the earth. Therefore, the fossils of Chengjiang fauna with many categories in the strata are the result of a catastrophe, and thus question Darwin's theory. . Another opinion is that the Cambrian explosion of life is a natural phenomenon, which is in line with Darwin's natural law of natural selection through mutation and inheritance, promoting the evolution of life from low-level to high-level, from simple to complex. It's just that due to the incompleteness of fossil records in the strata, people still don't know the truth about "pregnancy".
Kunming fish and Haikou fish were discovered by Chinese scholars in 1998 in the Early Cold Period strata in Haikou near Dianchi Lake in Kunming. The two jawless vertebrates are similar in shape, both fish-shaped and about 3 centimeters long. They have already differentiated their heads and preserved the zigzag-shaped myotomes that structure the gill sacs of the jaws. Like hagfish and chess eels, they are completely naked and have not yet put on an exoskeleton. This fact suggests that the absence of exoskeletons in extant jawless animals is not due to secondary degeneration, but a primitive feature. There are only two types of extant jawless eels, hagfishes and lampreys, with about 50 species. However, in the oceans of the early Paleozoic era, they were numerous in number and species, and they were the true overlords of the oceans.