◎Our reporter Jin Feng
The research team analyzed the branchial arch structures of 127 specimens of Yunnan worms produced in the Haikou area. For the first time, micro-nano-scale three-dimensionally preserved stacked disc-like cell structures and protein microfibril structures were found on the pharyngeal arch of Yunnan worms.
This proves that Yunnan worm has a pharyngeal arch that is unique to vertebrates and is composed of cellular cartilage, and belongs to primitive vertebrates.
Where did modern vertebrates come from?
What did the oldest vertebrates look like?
These major scientific questions have always been fascinated by paleontologists.
The Chengjiang Fauna, located in Yunnan, my country, preserves a large number of ancestral groups of vertebrates.
Previously, it was generally believed that the oldest vertebrate was the Kunming fish from the Chengjiang fauna, and it was speculated that the ancestor of the vertebrates was a class of post-stomach animals with notochord, dorsal neural tube and gill slits.
However, this hypothesis has never been supported by fossil evidence.
Now, the mystery has finally been solved.
On July 8, the international authoritative academic journal "Science" published the research results of the "Earth-Life System Early Evolution" team led by Professor Jiang Baoyu's research group from Nanjing University and Zhu Maoyan, a researcher at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, confirming that my country's 5.18 The yunnanensis in the Chengjiang fauna was the most primitive group of vertebrates 100 million years ago.
The identity of the Yunnan bug was once controversial
The Burgess Shale fauna in Canada about 505 million years ago and the Chengjiang fauna in Yunnan, China about 518 million years ago have abundant and beautifully preserved animal fossils characterized by soft body structures.
Among them, the Burgess shale fauna, which had been discovered in 1909, produced fossils of the cephalopod Pika and the backbone vertebrate post-sprigger. Scientists found in the Chengjiang fauna, including the backbone vertebrate Kunming Fossils of various chordates, including fish and Yunnan worms of questionable taxonomic location.
These fossils are by far the oldest chordates in the world and provide valuable material for unraveling the mystery of the origin and early evolution of vertebrates.
From the morphological point of view, Yunnan worms are somewhat similar to the current worms, their bodies are flat on the side, generally only 3 cm to 4 cm long.
"Different from Kunming fish, which has typical vertebrate features, the morphotype of the yunnanensis is closer to the cephalopod amphioxus, but its position in the vertebrate origin and deuterostomia phylogeny has been controversial. Co-corresponding author of the paper, Nanjing University Professor Jiang Baoyu introduced.
In 1995, the journal "Nature" published a study that identified Yunnan worm as the earliest chordate.
"The New York Times" and "Science Weekly" also made a special review.
After that, Chinese scholar Chen Junyuan and others discovered a large number of new specimens of Yunnan worm in the Haikou area of Kunming.
Based on the finer anatomical structure of these specimens, they classified the yunnanensis as a primitive cephalopod, and believed that its evolutionary position was between the cephalopod amphioxus and the vertebrate lamprey.
After this result was published in the journal "Nature" in 1999, a great discussion about the taxonomic position of the yunnanensis was set off in the academic circles.
"Because different scholars have different interpretations of the details of the preserved soft tissues of P. yunnanensis, since it was first reported in 1991, P. yunnanensis has been classified into vertebrates, cephalopods, hemipora, and deuterostomes. Gan group, or even primitive bilaterally symmetrical animals." Jiang Baoyu introduced that the controversy over the taxonomic location of Yunnan worms has seriously affected the research on the origin of vertebrates based on these key fossils.
The special features of the pharyngeal arch help it to identify the body
In response to the mystery of the taxonomic location of the worms in Yunnan, when conventional morphological studies could not reach a consensus, the research team used various modern The experimental techniques are used for research, hoping to solve this puzzle from the microscopic anatomical structure.
Based on the important clue that the branchial arch of Yunnan worm may have cytocartilage proposed by Chen Junyuan and other scholars in 2003, the research team analyzed the branchial arch structure of 127 samples of Yunnan worm produced in the Haikou area. Three-dimensionally preserved stacked disk-like cellular structures and protein microfibril structures were discovered at the micro- and nano-scale.
"The stacked disc-like cell structure is a unique arrangement of chondrocytes, and the protein microfibrils are the fibrous structure of the intercellular matrix common to vertebrate cartilage. These two features prove that Yunnan worm has a unique vertebrate and is composed of cellular cartilage. The pharyngeal arch, indicating that the Yunnan worm belongs to the primitive vertebrates." Jiang Baoyu said.
In order to further determine the specific evolutionary position of yunnanensis, the research team integrated the latest trait matrix including deuterostomes and early vertebrate fossils, added newly observed yunnanensis traits, and used Bayesian algorithm for reconstruction calculation. .
The results of the analysis support that the yunnanensis is in the most basic position of the vertebrate lineage, between the caudate and other vertebrates.
"The origin of vertebrate jaws has also puzzled the academic community for a long time. In the early 19th century, some scientists proposed that the gill arch evolved into the jaw arch and the lingual arch during the long evolution, but no fossil record evidence has been found before." Jiang Baoyu introduced that subsequent scientific discoveries have called into question this hypothesis.
"The genes that control the first two branchial arches of the jawless vertebrates hagfish and lamprey did not develop into jaw and lingual arches, but developed into upper and lower lips, and no traces of branchial arches can be seen during development. , which is different from the gill arches of jawed vertebrates." Jiang Baoyu said that in this study, after confirming the "identity" of the Yunnan worm, the researchers found a pair of pharyngeal arches in its position equivalent to the first pair of pharyngeal arches In the rear, there are other structures such as stacked disc-like cells, gill filaments and protein microfibrils like other branchial arches.
Therefore, Yunnan worm possesses 7 pairs of pharyngeal arches that are similar to each other from anterior to posterior and also have cellular cartilage.
Pharyngeal arches that are similar to each other also appear in another Cambrian backbone vertebrate, the post-sprigger fish.
"The discovery of 7 pairs of similar pharyngeal arches in Yunnan worm supports the hypothesis of the homology of vertebrate pharyngeal arches proposed by anatomists as early as the 19th century, that is, the branchial arches of fish are the prototype of the jaw and lingual arches. The jaw and lingual arches of vertebrates are a series of homologous structures with the branchial arches behind them." Jiang Baoyu said, this means that although the pharyngeal arches in different positions of living vertebrates will develop into jaw arches, lingual arches and branchial arches, etc. Bones with different shapes, but the pharyngeal arches at different positions in the early stages of vertebrate evolution are similar to each other.
Basket-shaped pharynx confirms pharyngeal morphology in vertebrate ancestor
Jiang Baoyu believes that another important new discovery of this study is that the seven pairs of pharyngeal arches of Yunnan worm are connected by pairs of horizontal cartilages at the ventral and dorsal ends, forming a basket-like pharynx.
Pharyngeal skull refers to a group of bone fragments around the digestive tract of vertebrate heads, which play a supporting and protective role. The pharyngeal skull of living vertebrates is composed of bones of pharyngeal arches such as jaw arch, lingual arch and branchial arch.
"The basket-like pharynx is typical of extant jawless vertebrates and fossil jawless euphorbias, whereas post-Cambrian sprigfish and jawed vertebrates have pharyngeal craniums separated from each other by a series of Cartilage rods." Jiang Baoyu said that this pharyngeal feature of the post-sprigger fish has caused controversy about the pharyngeal morphology of the ancestors of vertebrates.
The pharyngeal cranial features of A. yunnanensis, which appeared earlier and located at the most basal position of vertebrates, support the idea that the basket-shaped pharyngeal cranial is an older ancestral feature.
It can be seen that, as the oldest close relative of living vertebrates, yunnanensis provides key evidence for revealing the origin and early evolution of vertebrates, and will have a profound impact on the exploration of the evolution of vertebrate jaws and other key features.
Jiang Baoyu believes, "From another perspective, this study once again demonstrates the potential of Chengjiang fauna fossils to preserve fine biological structures at the micro and nano scales."
The results were also highly praised by the reviewers of the journal Science: "This is a major contribution to science, and the paper will become a key paper in vertebrate paleontology, demonstrating the long-awaited and most promising results. The fossil evidence of the anatomical structure of the letter resolves the Cambrian stem group vertebrate controversy. The information they present is a milestone that will be difficult to surpass even by higher resolution studies for many years to come."