The earth was formed 4.6 billion years ago. In a dim and lightless environment, life experienced a long evolutionary history of about 4 billion years; until the Cambrian era more than 500 million years ago, this blue planet On the color planet, almost all animals, including vertebrates, quickly appeared on the scene in just a few million years. A series of creatures that were basically the same as modern animals "appeared collectively", forming a prosperous scene in which multiple categories of animals coexisted. . This rapid life evolution event is called the "Cambrian Explosion."

The Cambrian explosion posed a huge challenge to Darwin's theory of evolution and became a major mystery that has puzzled paleontologists for more than a hundred years.

Recently, another major scientific discovery was made in the "Chengjiang Fauna" in Yunnan, my country: a "unicorn shrimp" from 500 million years ago. This caused waves in the world of paleontology, and it once again confirmed the prediction of biological evolution to the world. Behind the discovery of "Kilin Shrimp" is more than ten years of painstaking research by scientific researchers.

In ancient mythology, the auspicious beast Qilin is said to be an animal that combines a lion's head, antlers, tiger eyes, an elk's body, dragon scales, and an ox's tail. From a biological perspective, animals that combine the physical characteristics of multiple animals are called "chimeric animals."

Reconstruction map of Unicorn Shrimp

On November 4, in a conference room of the Chinese Academy of Sciences on Sanlihe Road in Beijing, researchers from the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences presented to everyone It shows a "unicorn" from the Cambrian period - it looks like a shrimp, with five eyes, predatory "forelimbs", hardened skin, a multi-section trunk, and segmented legs... It combines It discovered the ancestral type of arthropods and various physical characteristics of true arthropods, and was named "Zhang's Kirin Shrimp".

Cambrian Explosion Research Group (from left: Zhao Fangchen, Zhu Maoyan, Huang Diying, Zeng Han)

The Cambrian Explosion Research Group led by Zhu Maoyan, a researcher at the Institute of Southern Ancient Sciences, Chinese Academy of Sciences, This transitional species that opened up the mystery of the origin of arthropods was found in the Chengjiang fauna, and it has therefore become a key fossil to answer the "mystery of the Cambrian explosion." At 0:00 on November 5, Beijing time, this latest research result was published in the British magazine Nature.

The Chengjiang Fossil Group Challenges "Darwin's Puzzle"

In 1859, when Darwin completed "The Origin of Species", he probably would not have imagined that a "shrimp" would solve the puzzle that he had spent his entire life solving. . The answerers were Chinese scientists. Zhao Fangchen, a researcher at the Institute of Southern Ancient Sciences, Chinese Academy of Sciences, was very proud when he mentioned this: "I started answering this question from my mentor's mentor, and I am still answering it now."

Darwin's original confusion was "540 million years" 530 million years ago, there was the Cambrian explosion of life at the beginning of the Paleozoic Era - many animals suddenly appeared in the fossil record, but had no obvious ancestors in earlier rock formations."

For more than a hundred years, this problem has troubled generations of Chinese and foreign scientists. Until 1984, near Maotian Mountain in Chengjiang, Yunnan, scientists from the Institute of Southern Ancient Sciences of the Chinese Academy of Sciences discovered the Chengjiang animal fossil group that dates back about 520 million years. Hou Xianguang, who was still an assistant researcher at the time, knocked down with his hammer and knocked out "a gorgeous world of paleontology" - these creatures are only a few millimeters in size, tens of millimeters or even larger in size, some are like sponges, and some are like Worms, jellyfish, shrimps, hats, flowers, discs... After more than 500 million years of great changes, the soft body structures of these most primitive marine animals have been preserved intact, in various poses, and lifelike. They are the first discoveries in the world. The oldest and best-preserved group of animal fossils. The discovery of animal fossils in Chengjiang has also been hailed as "one of the most amazing discoveries of the 20th century."

Ecological Restoration of Chengjiang Fauna (Drawing by Yang Dinghua)

Zhao Fangchen made the relatively boring science popularization vivid and interesting, because each number embodies the efforts of the predecessors, and each one is difficult to pronounce. Behind the scenes are the hard work of our predecessors. "Major discoveries originating from the Chengjiang fauna continue to emerge. Here, Chinese and foreign paleontologists have discovered nearly 300 species and more than 20 animal categories from ancient times, including algae in the plant kingdom, arthropods and brachiopods in invertebrates. , vermiforms, sponges, etc., and even chordates have been discovered. Among them, the most famous one is the discovery of complete Anomalocaris fossils." He subconsciously raised his voice and solemnly introduced, "In the 1990s, my mentor's mentor. , is Chen Junyuan, a researcher at the Nanguang Institute of Ancient Sciences. He and his colleagues discovered the most complete fossil of Anomalocaris so far. "

This kind of shrimp is called "the earliest sea monster". It has a pair of giant eyes with handles, a pair of giant segmented forelimbs used to quickly catch prey, a mouth with fangs shaped like a bowl and a beautiful large tail fan. Its individual size can reach up to It was more than 2 meters in size, while most other animals at that time were only a few millimeters to several centimeters in size on average. It was the top predator of the marine food chain at that time.

The discovery of this "shrimp" triggered heated discussions around the world. Why is it so popular? At that time, even Swedish provincial newspapers were reprinting and reporting on this discovery.

"However, Anomalocaris cannot be regarded as the key to solving the 'Darwin puzzle'.

Zhao Fangchen clasped his fingers and quickly recounted what happened over tens of millions of years: Fossil discoveries in the Chengjiang fauna show that almost all modern animal categories, including vertebrates, appeared 530 million years ago. And the Cambrian Since the Great Explosion, arthropods have been one of the most successful animals on earth, from trilobites and Anomalocaris, known as the "Cambrian stars", to butterflies, crabs, shrimps, spiders, etc. that live around us today. , are all arthropods, they have segmented bodies and appendages. The most numerous animals on earth today are arthropods. They account for about 80% of the species diversity in the animal kingdom and can be found in various environments such as sea, land and air. They are found everywhere.

So, how did arthropods evolve? What did their ancestors look like? "Stone". Zhao Fangchen gave an example. Crabs, shrimps, etc. are modern representatives of arthropod evolution. They all have hardened epidermis, healed head shells, multi-jointed trunks, and segmented legs. They are called "true arthropods." "Animal". If you draw an "evolutionary tree", Anomalocaris is located at the bottom of the tree. It has the characteristics of an arthropod, but its body does not have segments, nor does it have hardened epidermis and other forms; the crown is a true arthropod. “The towering tree trunk in the middle represents the unbridgeable evolutionary gap between the two. "This gap in Zhao Fangchen's mouth has also become a key missing link in exploring the "mystery of the origin of arthropods". The newly discovered Kirin shrimp is part of this link.

30 years of "baton-taking" "Exploration discovered a brand new species

How to fill the "gap" after finding it? Darwin's prediction in "The Origin of Species" points out the solution: "Between all living and extinct species, The number of species in between, in between, and in transitional links must be unimaginably large. If my theory is correct, they must have existed on earth. ”

According to Darwin’s theory of evolution, in order to solve the “mystery of origins”, scientists need to find transitional ancestral fossils in the origin of animal categories to fill in the missing gaps.

1984 So far, generations of scientists have never given up on the study of Chengjiang fauna. More than 30 years of "continuous" exploration have promoted the study of Chengjiang fossils to the world.

In early 2004, Zhao Fangchen was still studying for a master's degree. , followed his mentor Zhu Maoyan to Chengjiang, Yunnan, which was his first scientific expedition.

“Field work is the top priority of geological research. "When Zhu Maoyan was still a student, his mentor Chen Junyuan always kept this sentence on his lips. After he became a mentor, Zhu Maoyan told Zhao Fangchen.

Field fossil collection site

“Listening is one thing, but truly understanding is in the wilderness. "Zhao Fangchen recalled that no matter how harsh the field environment was, Teacher Zhu would always be full of enthusiasm and devote himself to field work. He would often need to walk for more than ten kilometers, and he would still personally carry the samples out on his hands and shoulders, just like the hands of others. An example is worth a thousand words. This is how generations of scientific researchers have developed excellent field capabilities.

“Every day, they walk several kilometers or even more than ten kilometers under the scorching sun. , you have to climb rocks on the cliffs to take samples, and hammer out the fossils hidden in the rocks. "Zhao Fangchen said with a smile, "Actually, we repeat an action every day - 'knocking'. Every time the hammer hits, we don't know what will happen. Every time we knock, there is a little more expectation in my heart, until I see the exquisite fossils. Revealed in the rock. "

Zhao Fangchen (first from right) at the field scientific research site

After a hard day, carrying 20 kilograms of fossils down the mountain, Zhao Fangchen's legs began to tremble, "I was really exhausted after walking the last few kilometers. , but my heart felt like honey. "Zhao Fangchen was smiling all over his face.

Hard work will eventually pay off. In 2005, during the quantitative collection of fossils, Zhao Fangchen and others excavated a pit of more than 50 square meters and a depth of more than 2 meters. "Order What surprises people is that there are tens of thousands of fossil individuals buried in the pit. ”

In this “fossil pit” formed by yellow-green mud shale, he observed the microfacies of sedimentary rocks and found that this set of rock layers that preserved exquisite fossils was caused by the rapid formation of flocculent mud caused by storms. Sedimentation was formed. He speculated that during the Cambrian period, the creatures here most likely experienced a heavy rainstorm or tsunami. "The sediment brought from the land was sorted by gravity to form fine mud, and a group of small animals quickly Buried... After tens of millions of years of diagenesis and geological changes, fossils were formed, and they were not seen again until 520 million years later. ”

Kirin shrimp fossils

A large number of fossil specimens were brought back to the laboratory for repair and arrangement. A fossil with a corner cut off caught Zhao Fangchen’s attention: the exposed fossil From one corner, a small, helmet-like head is vaguely exposed, and a pair of curved "front claws" covered with small spines. "These are a pair of very special front appendages that immediately remind us of the strange. shrimp. "Recalling the scene of discovering the fossil, Zhao Fangchen's tone was full of excitement, "Similar to Anomalocaris, this pair of 'claws' are also segmented, and each segment has small spines. However, from the head that appears, Judging from its internal characteristics, it is different from the Anomalocaris that had been reported at that time.

"

Unicorn Shrimp Fossil

Zhao Fangchen immediately began to repair the fossil. As the repair progressed, the figure of a strange "little shrimp" gradually appeared on the yellow-green rock. Presentation - In addition to having a pair of spiny "front claws" like Anomalocaris, it also has a multi-segmented body, segmented legs, and hardened skin... "It looks like everything, but looks like nothing at all." "It not only has the characteristics of ancient animals, but also has similarities with the shrimps we eat today..." Zhao Fangchen said half-jokingly, "We may have found the 'four different' of the Cambrian." "However, since the body of the shrimp finally presented was incomplete, he still needed to find more fossils of the same animal.

This is very likely to be a brand new species! In order to confirm his conjecture, Zhao Fangchen started In the following ten years, he visited almost all Cambrian stratigraphic fossil producing areas around Kunming and collected four other fossil specimens with the same shape. They are still in a natural history museum. A well-preserved fossil was found. "This kind of 'four-like' animal fossil is really rare in the Chengjiang fauna, and it is very rare to find 6 of them. The smallest among them is only about 1 centimeter, and the fossil collected by the museum is the largest, measuring 7 centimeters. But to say that the most complete and exquisite one is the one that was sorted out in the "Fossil Pit". " Zhao Fangchen gently held up the fossil. On the palm-sized yellow-green rock, the purple-red body of the "little shrimp" was clearly visible.

Using a microscope to examine this 5-centimeter "little shrimp" "While conducting structural research, he and his colleagues made a new discovery - there are five eyes on the top of the helmet-like head of "Shrimp"! Zhao Fangchen explained that these five eyes are in two rows, with a pair of larger ones in front. Eyes, with the other three eyes arranged behind. "This feature can be found in the arthropod ancestor types of the Cambrian period. The most typical one is the 'Cambrian monster'-the Opabin sea scorpion." "

However, it is not enough to just see these morphological structures from fossils. The research team also needs to confirm that these structures of the "shrimp" really exist. With the help of a variety of technical means, the research team The team conducted detailed anatomical research on each fossil.

Unicorn shrimp adds strong evidence of biological evolution.

As the research progresses, the restoration of the "little shrimp" is also revealed. Continuously improving, one image after another was brought home by Zhao Fangchen, which aroused his daughter's curiosity, "Dad, what is this?" "The little girl asked.

Sketch of Kirin Shrimp

"This is the 'Kilin' from the Cambrian period. You see, it has five eyes, and these are its claws. Connected to the nervous tissue..." Zhao Fangchen took out the computer. On the screen, there was a red energy spectrum analysis image, and dots of bright light intersected to form a curved "pipe." "This long one is its intestines. "

In fossils, soft body structures such as eyes, intestines, digestive glands, and nervous tissue of animals are usually the most difficult to preserve. "To prove the existence of these soft body structures, you need to scan Through electron microscopy and energy spectrum analysis, we can see the elemental differences in the fossils. " Zhao Fangchen said that the exquisite preservation state of the fossil provided them with a wealth of information. Through energy spectrum analysis, the element distribution of the "shrimp" body was presented on the computer. The digestive system is mainly preserved with carbon elements, and during degradation or mineralization During the process, due to factors such as different degradation rates, the carbon element remains "in situ", retaining the original shape of the digestive system.

A new model of the origin and early evolution of arthropods revealed by Kirin shrimp (Zeng). Han Cartography).

After obtaining the element distribution image, we need to compare it with modern biological anatomy and combine it with the known morphological characteristics of arthropods to determine the distribution position of the digestive system and nervous system of the "small shrimp" "In contrast to vertebrates, arthropods have their digestive system on their back and their nervous system on their abdomen. "Zhao Fangchen pointed to an element distribution map. On the image, the elements are distributed in strips, extending into different branches, connecting eyes, legs, appendages, etc. Comparative anatomy research shows that the body of "little shrimp" is divided into Twenty-five segments, each segment has a pair of double-limbed legs, including paddle-shaped outer limbs for swimming and inner limbs for walking; the head looks like a helmet, with five eyes looking straight ahead. , a pair of claw-like appendages covered with small spines protrude from the bottom of the head, indicating its identity as a "predator"

The body has obvious characteristics of a true arthropod, but the head is different. Combining the morphological structure of the ancestors of arthropods, this strange "little shrimp" is like the "four different" auspicious beast "Qilin" in traditional Chinese mythology, with its body incorporating the morphological characteristics of multiple animals. Therefore, Zhao Fangchen. and others named it "Zhang's Unicorn Shrimp", "Zhang's" is a tribute to the donor of the fossil from the museum.

Where is the evolutionary position of the Unicorn Shrimp? Is it the "transitional" fossil predicted by Darwin?

In order to reveal the significance of Kirin shrimp in the long history of evolution, Dr. Zeng Han, a member of the research team, conducted many years of observation and research on tens of thousands of arthropod fossils from the Chengjiang fauna in China and the Burgess Shale in Canada. Basically, it integrates more than twenty morphological data sets in the field of "Arthropod Origin and Early Evolution" and encodes the latest large-scale data matrix in this field - including 81 taxa and 283 characters, using a variety of models and parameters An evolutionary tree reconstruction was performed. "The results of phylogenetic analysis consistently show that the evolutionary position of the Kirin shrimp is at the very root of the euarthropods, between Anomalocaris and other euarthropods."

"The Kirin shrimp represents the evolution predicted by Darwin's theory of evolution An important transitional fossil adds strong evidence to the theory of biological evolution. "On November 5, the research results that condensed the efforts of several generations of scientists were published in a well-known international journal, attracting global attention. However, Zhao Fangchen did not stop - he returned to the field scientific research site in Chengjiang, trying to find more answers to the "Darwin puzzle".

In order to celebrate the team's latest discovery, he changed his WeChat avatar to a "Qilin picture" drawn by his daughter. He was very happy that his daughter, who is in elementary school, was very interested in his research. "In the future, I hope these 'little Darwins' , can unlock more secrets of this huge evolutionary tree of life..."