The CAMBRIAN explosion of life is called a big unsolved problem in paleontology and geology, which has been puzzling the academic circles such as evolution since Darwin. "Cambrian" is an era in geological history, named after a hill in England, which lasted about 540 million years to 5. 1 100 million years ago. Its name does not contain special meaning, but it is a special era.
The earth was born about 4.6 billion years ago. In the four billion years after the birth of the earth, life on the earth, including plants and animals, left almost no substantial traces. But in the short period from 5 million to 6.5438+million years, that is, about 600 million years ago, it was called the beginning of Cambrian in geology, and most invertebrates appeared in this short period of several million years. This kind of invertebrate fossils (arthropods, mollusks, brachiopods, annelids, etc. ) appeared almost simultaneously and suddenly in the CAMBRIAN strata, but no animal fossils were found in the older strata before CAMBRIAN for a long time, which was called "CAMBRIAN life explosion" by paleontologists. Darwin mentioned this fact in his book The Origin of Species and was puzzled by it. He thinks this fact will be used as strong evidence against his theory of evolution. But at the same time, he explained that CAMBRIAN animals must have come from the ancestors of Precambrian animals and were produced through a long evolutionary process. The "sudden" of CAMBRIAN animal fossils and the lack of Precambrian animal fossils are due to incomplete geological records or because the old strata are submerged in the ocean.
Ancient trilobites
1882, Darwin talked about ancient trilobites in the last letter of his life. In his view, it is inconceivable that complex animals like trilobites suddenly appeared in the Cambrian strata.
Trilobite is an extinct primitive arthropod that lived 500 million to 200 million years ago. Its body is flat and wide, with a protrusion in the middle of the back and two longitudinal grooves in the back, which divides the body into trilobites, hence its name.
In the past, CAMBRIAN was usually called "trilobite age" because there were a lot of trilobite fossils in CAMBRIAN strata, but no mineralized mollusk fossils were found in CAMBRIAN strata.
Scientists found some fossil remains of single-celled organisms in ancient rock formations before CAMBRIAN, but they all disappeared soon. It's like drawing a dividing line in the history book of life. It was just a warm-up exercise before, and then the Cambrian began to really evolve.
Everything started suddenly, and the form of life suddenly became colorful and full of vitality. This sudden life innovation event is often called "Cambrian life explosion". It was during this period that life chose its own basic form, and there was no big change in the future. When scientists search the history of CAMBRIAN fossil records, there are mysteries and blank stages in the evolution process that are difficult to solve everywhere. When new life forms appear in the fossil record, people can't find the corresponding primitive species, and when they suddenly become extinct, there is no obvious evolutionary body left.
"Why haven't we found thousands of transitional fossils buried in the earth's crust?" Darwin painfully realized that such fossils did not exist in stratigraphic faults. This fact has been bothering Darwin, so much so that he devoted a whole chapter in his book to discussing "the incompleteness of geological records".
Discovery of Burgess Fauna
1In August, 909, Wilcarter, director of the US Geological Survey, led his family to Burgui Mountain in the Canadian Rockies for a field geological trip. On the way back, a stone tripped over his wife's mount. Will Carter picked up this troublesome stone and a miracle appeared. A fossil with a mollusk appeared in front of Wilcarter.
Later, after large-scale excavation and collection, Burgess fauna caused great shock in the scientific community at that time. It makes scientists clearly realize for the first time that there are only a few skeletal animals in the Cambrian ocean, and most of them are mollusks that are not easy to preserve, which corrects people's misunderstanding that there are only a few hard animals such as trilobites in the Cambrian.
Fossils record the amazing richness of invertebrates in the middle CAMBRIAN period 50./kloc-0. 5 billion years ago, which is regarded by the scientific community as an important basis for the origin and early evolution of life and provides important clues for solving the mystery of the CAMBRIAN life explosion.
Due to the defects of geological preservation, Burgess fauna fossils have no three-dimensional hierarchy, and many animal forms can only be guessed.
198 1 year, the Burgess fauna in Canada was approved by UNESCO as a "world cultural heritage" and became a sacred place for paleontologists all over the world.
The representative of CAMBRIAN life explosion in Yunnan-Chengjiang fauna
There is a small town more than 50 kilometers southeast of Kunming. There is a famous Fuxian Lake in the territory, the water quality is clear and pure, and it can be directly drunk, so it is named Chengjiang. However, no one expected that under the humble Maotianshan Mountain in Chengjiang, there was a sealed book that cracked the life of the earth. For the paleontologist Professor Hou Xianguang, his great discovery in 1984 Chengjiang Maotianshan Mountain still makes him fidgety.
There is a strange-looking building halfway up Maotian Mountain in Chengjiang, which is the field workstation of Nanjing Institute of Paleontology, Chinese Academy of Sciences. It was designed by Qi Kang, a famous architect in China, and its design was inspired by the CAMBRIAN creature Cinderella worm discovered in Maotian Mountain.
1In July, 984, researcher Hou Xianguang of Nanjing Institute of Geology and Palaeontology discovered the first early Cambrian animal fossil-Naluokelongwei in Maotianshan Mountain, Chengjiang County, Yunnan Province. This discovery can be said to be a very accidental event. Hou Xianguang's original intention was to go to Chengjiang to look for the fossils of golden bugs. Unexpectedly, he accidentally got the fossil of this long-tailed worm.
In June of that year, under the guidance of his mentor Zhang Wentang, Hou Xianguang went to Yunnan to collect paleoostracods and write a master's thesis.
1, 1, in July, 0984, around 3 pm, Hou Xianguang picked up a piece of stone and hammered it down-a lifelike bug appeared. This is an early Cambrian invertebrate, belonging to an important member of the Burgess fauna discovered by the famous Canadian paleontologist Wilcarter in 1909. Foreign scientists believe that worms are one of the earliest hard life, first discovered in the Asian continent, and still have appendages. This discovery means that the evidence of the Cambrian explosion of life is at your feet. Later, this day became the anniversary of Chengjiang animal fossils. Professor Serach, a world-renowned paleontologist and German, said: "The discovery of Chengjiang fauna is as shocking as the information from outer space." The New York Times said: "The discovery of Chengjiang fauna in China is one of the most amazing discoveries in this century".
Hou Xianguang returned to Nanjing with this great achievement, wrote an excavation report, and co-authored a research paper on animal fossils in Chengjiang, Discovery of Nero Insects in Asian Continent, with his tutor Professor Zhang Wentang. In the article, teachers and students formally named the paleontology discovered in Chengjiang as "Chengjiang Animal Fossil Group". Since then, the prelude to the world's continuous surprises in the next 20 years has been opened.
A series of field excavation and research work began. Constant discovery and exploration have revealed the life world 530 million years ago.
After the news came out, groups of geologists and paleontologists flocked at home and abroad, and groups of fossils with different shapes were excavated. People have never seen such a well-preserved biological fossil: not only the skeleton of the organism is preserved, but also various soft tissues such as epidermis, cilia, eyes, stomach, digestive tract, mouth and nerves. Among them, the tentacle, radial tube, annular muscle, central cavity and mouth structure of jellyfish fossils are clearly visible, the body ring, snout structure, digestive tract and tail thorn of worm fossils are intact, and brachiopod fossils present thick fleshy stems protruding from the shell. The Nalox fossils buried in a curled, reclining or lying posture have well preserved their soft body structure, and even the food in their intestines can be clearly seen, indicating that they had a full meal before they died. Micronets with reticular bone fragments are not easy to stand even if they are alive, and some can even stand after death. These fossils are rare in other parts of the world where the body structure is soft. 500 million years ago, the soft structure of living things could actually become fossils, which existed in rock layers. This is the most unique feature of Chengjiang fossils.
Experts found from Chengjiang paleontological fossils that all the animal phyla in the world appeared at the same time during this period, and no new phyla was produced after that. The life forms of this period are very similar to those of today's creatures, from starfish with star symmetry to crustaceans with left and right symmetry, and animals with embryonic spines. Among the animal fossils found in Chengjiang County, Yunnan Province, even if the extinct phylum is not included, there are still as many as 20 phyla, but now there are only 35 phyla of tens of millions of living creatures on the earth. This shows that the basic system of animal diversity was established in the Early Cambrian.
1989, a truly complete microfauna fossil appeared in Maotianshan Mountain, which surprised scientists. It turns out that these bone fragments, which have troubled paleontology for decades, grow in a worm-like creature and grow on both sides of the body.
Micronetworms are extinct multi-legged and slow-moving animals. It is 4 to 6 cm long, with a short and small head and a long tail. It has a pair of legs on both sides of its abdomen. It is not good at walking and clings to other creatures with its claws. It is a rare fossil of Chengjiang fauna.
The discovery of micro-network fossils by Professor Chen solved the problem of classification and location of fossils containing reticular bone fragments. Professor Gould of Harvard University in the United States wrote: The discovery and research of complete fossils of micro-networms have made the history of human science a step forward.
Another great contribution of Chengjiang Fauna Fossil Group is the discovery of complete strange shrimp fossils, which confirmed the existence of carnivores in the Early Cambrian. "Strange shrimp" is not a real shrimp, it is the largest and fiercest carnivore in the Cambrian ocean. 1992, when the expressway was built on the northern slope of Maotian Mountain, a large section of the road was pushed out by bulldozers, and the most complete shrimp fossils finally appeared.
The first kiwi shrimp fossil was discovered in Canada as early as 1892. In fact, this is just a forelimb of the strange shrimp, but it was mistakenly thought to be the body of some kind of animal similar to shrimp at that time, but it is puzzling that the shrimp head has never been found. It was not until three of the most complete strange shrimp fossils were unearthed in Maotian Mountain of Chengjiang that the misunderstanding of paleontology for a century was finally clarified. It turned out that the "strange shrimp" discovered by Burgess was just the "hand" of a large carnivore. /kloc-for 0/00 years, scientists have always believed that strange shrimp is a big monster without legs, and the strange shrimp fossils unearthed in Chengjiang are paired thick legs.
Kiki shrimp is a giant carnivore with a pair of stem-shaped eyes at the back of the head and a pair of strong giant forelimbs at the front of the mouth, which are used to catch prey. According to its fossil fragments, the animal's body length can reach more than 2 meters, and its mouth diameter can reach 25 centimeters. At that time, the average size of animals was only a few millimeters to a few centimeters. In contrast, kiwi shrimp is an invincible giant. Later, Japanese scientists made a model of this strange shrimp based on the fossil restoration model and research results. We see that the animal simulated by scientists has long tentacles and whiskers, a shrimp-like shell and tail, and a square mouth under its soft body, which is used to filter seawater and get food.
The appearance of kiwi shrimp shows that in the early CAMBRIAN, the marine ecosystem had a fairly complete pyramid-shaped food chain system, and kiwi shrimp was the creature at the top of this pyramid.
199 1 year, Professor Hou Xianguang discovered a worm-like animal fossil named "Yunnan worm" in Maotian Mountain. At first, because some characteristics were difficult to explain, it was placed in the classification of worms with uncertain position. Four years later, Professor Chen from Nanjing Institute of Geology and Paleontology first proposed the oldest chordates of Yunnan Insecta in the British magazine Nature, and unveiled their mystery. However, Professor Shu Degan of Northwest University drew different conclusions from the same clue. "This is beyond the scope of chordates. However, I think because it has no sarcomere, there is no real chordate, that is, it does not have the two basic characteristics of a real chordate. " Chen refuted the argument of "semi-chordates" and firmly supported that Yunnan worms are chordates. Argument is often an opportunity for discovery. From April 65438 to/kloc-0 to April 999, Professor Chen and others discovered the close relative of Yunnan worm "Haikou worm" on the hillside of Ercai village in Haikou, Kunming, which pushed the study of Chengjiang fauna to a climax. Haikou worm provides the world with a clear and indisputable snapshot of chordates in the early stage of the outbreak. In June 5438+065438+ 10 of the same year, Professor Shu Degan of Northwest University and others discovered the well-preserved "Kunming Fish" fossil in Haikou, Kunming, which once again triggered a huge shock in the scientific community. The discovery of rare fossils such as "Haikou worm" and "Kunming fish" in Chengjiang fauna has formed the knowledge of paleontology. From the end of Precambrian to the beginning of CAMBRIAN, a great innovation event of biological category did happen on the earth, and invertebrates and vertebrates in the animal kingdom participated in this explosion.
In the study of these animal fossils, the most exciting thing is an animal named "Yunnan worm", which is the oldest chordate. Its discovery and research have pushed forward the history of vertebrates, including humans, for more than 654.38+0 billion years. Chordata dinosaurs of Mesozoic in geological age, Chordata mammals of Cenozoic in geological age and later humans, their ancestors were "Yunnan worms".
According to scientists' inference, in order to gain the ability to swim in the sea, Yunnan insects develop muscles at every node, and use muscle contraction to make their bodies show wave motion. Its body has a hose running through the front and back, as a spine, and a pair of small side folds, which later evolved into mammalian limbs and our human limbs.
On June 5438+1October 65438+May, 2002, Yunnan paleontologists Luo Huilin, Liang Zhongchang and Hu discovered two new species of fossils, Cathaysian fish and Sino-Singapore fish, in the early Cambrian Chengjiang fauna of Kunming. This discovery fills the gap in the evolution from cephalopods to vertebrates, and the spectacular scene of the Cambrian life explosion and the complete life evolution chain have been clearly presented to people. The picture shows the newly discovered Sino-Singapore fish fossils. From this ancient fossil, the head, dorsal fin, mouth and eyes of the fish can be clearly distinguished. In fact, this fossil "fish" belongs to protozoa and is the most primitive mollusk vertebrate.