30 years of research has proved that Yunnan insects are the most primitive vertebrates

Ecological restoration map of insects in Yunnan (zhao fangchen guidance, Yang Dinghua map)

The ancestor of mankind turned out to be a fish!

While a bit incredible, previous research has confirmed that life evolved from aquatic animals, to amphibians, to terrestrial animals, and finally to the highest animal on Earth, human beings.

It is not difficult to find that in the evolution of life, the evolution of the human lineage- vertebrates is one of the most magnificent chapters, and the question of the origin of vertebrates has always been the most attractive scientific hotspot.

In the 1990s, Hou Xianguang, then a researcher at the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (hereinafter referred to as the Nanjing Institute of Paleontology), first discovered the yunnan insect fossil in the Tianshan Mountains of Chengjiang, Yunnan Province, and speculated that it may be the earliest vertebrate, which also set off a cross-century question and debate in the academic community.

Now, this cross-century debate can finally be conclusive. China’s scientific research team found that the Yunnan insect produced by the Cambrian Chengjiang fauna 518 million years ago has a unique cellular cartilage structure of vertebrates, which confirms that yunnan insect is the most primitive group of vertebrates. The results of the study were published in Science on July 8.

The results were completed by zhu Maoyan’s team, a researcher at the Nanjing Institute of Paleontology, and a research group of Professor Jiang Baoyu of Nanjing University. Zhao Fangchen and Jiang Baoyu, researchers of the Nanjing Institute of Paleontology, are the corresponding authors of the paper, and Tian Qingyi, a doctoral student co-supervised by the two, is the first author, which is also Tian Qingyi’s first Science.

Controversial question of origin

Evolutionary biologists usually speculate that the ancestors of vertebrates are a class of hindquarters with a chordal cord, dorsal neural tube, and gill fissures, based on the characteristics of the living cephalozoa Wenchang fish. However, this hypothesis has never been supported by fossil evidence.

In 1909, foreign researchers discovered the Burgess shale fauna in Canada dating back about 505 million years, and then produced the cephalic pika fish and the basal vertebrate post-Splicer fish. Since 1991, in the Chengjiang fauna in Yunnan, China, about 518 million years old, researchers at the Nanjing Institute of Paleontology have discovered a variety of chordates, including the basal vertebrate Kunming fish and the Yunnan insect with a 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.

“Chordates include tail cords, cephalic cords, and vertebrates.” Zhao Fangchen told China Science Daily, “Unlike the Kunming fish with typical vertebrate characteristics, the Yunnan insect morphology is closer to that of the head animal Wenchang fish, and its location on the vertebrate origin and the tree of the posterior mouth animal system has been controversial.” ”

In 1995, based on newly collected Yunnan insect fossils, Chen Junyuan, a researcher at the Nanjing Institute of Paleontology, and others published a paper in Nature, interpreting Yunnan insects as the earliest ancestors of chordates for the first time.

Yunnan insect specimen, fossil body length 3.9 cm (Zhao Fangchen courtesy photo)

After the paper was published, the New York Times and Science Weekly made special comments on the topics of “From Yunnan Worms to You” and “The Road to the Backbone” respectively.

Because the fossil site of the Maotianshan Mountains at that time had been protected and difficult to excavate, Chen Junyuan set his sights on the town of Haikou in Kunming, dozens of kilometers away. There are sedimentary rocks of the same era as the Hat Tian Shan, and it is very likely that there are a large number of similar fossils.

Sure enough, Chen Junyuan and others found a large number of new specimens of Yunnan insects in the Haikou area of Kunming. They developed a more elaborate anatomy on these specimens, and based on this study, classified yunnan insects as primitive cephalic species, believing that their evolutionary location was between the cephalic Wenchang fish and the vertebrate lamprey eel.

After the above results were published in Nature in 1999, a great discussion was set off in the academic community about the taxonomic location of Yunnan insects.

Different scholars have different interpretations of the details of the different soft tissues preserved by yunnan insects. Since its first report, Yunnan insects have been classified as vertebrates, cephalics, hemizoans, hindquarter stems, and even primitive symmetrical animals on both sides.

Zhao Fangchen pointed out that due to the controversy over the taxonomic location of Yunnan insects, it has seriously affected the research on the origin of vertebrates based on such key fossils.

127 specimens are revealed from the microscopic level

In 2018, Tian Qingyi began to engage in research related to Yunnan insects, and he found that there were many research papers on Yunnan insects, at least 20 to 30, and each paper reflected a high level of research. He told China Science Daily: “Almost most of the macroscopic structure has been repeatedly described and studied by predecessors. So I was very confused at the beginning of the study, and I stagnated for a long time and did not find a breakthrough point. ”

In view of the mystery of the classification location of Yunnan insects, since conventional morphological research cannot reach a consensus, through repeated communication and consultation with his mentors Zhao Fangchen and Jiang Baoyu, Tian Qingyi decided to start with the microstructure. “We use a variety of modern experimental techniques such as three-dimensional X-ray tomography microscopy, Fourier infrared spectroscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy, hoping to solve this puzzle from the microscopic anatomy.”

In 2003, Chen Junyuan and his collaborators proposed that the gill arch of Yunnan insects may have cellular cartilage. “Based on this important clue, we analyzed the gill arch structure of 127 Yunnan insect specimens produced in Haikou area, and for the first time found a three-dimensional preservation of stacked disc-shaped cell structure and protein microfibrils on the yunnan insect pharyngeal arch.” Tian Qingyi introduced.

It is these two characteristics that prove that Yunnan insects have a pharyngeal arch composed of cellular cartilage that is unique to vertebrates, and thus indicate that Yunnan insects belong to primitive vertebrates.

Further findings confirm that Yunnan worm is in the most basic position of the vertebrate lineage, between tail cords and other vertebrates (including living and fossil species).

The evolutionary position of Yunnan insect and the pharyngeal morphology of various chordates (from top to bottom, respectively, Wenchang fish, Yunnan insect, post-Spliger fish, lamprey eel, shark) (A) Evolution tree and pharyngeal morphological trait distribution. (B) Side view (C) ventral view. (Tian Qingyi drawing)

For the previously controversial first pair of pharyngeal arches of Yunnan insects, in the position where the new specimen corresponds to the first pair of pharyngeal arches, the research team found the same stacked disc cell structure, gill filaments and protein microfibrils as other gill arches in the posterior part. The research team determined that Yunnan worms have 7 pairs of pharyngeal arches that are similar to each other from front to back and have cellular cartilage.

Similar pharyngeal arches also appear in another Cambrian basal vertebrate, the post-Splician fish.

As early as the 19th century, anatomists had already proposed the theory of homology of pharyngeal arch in vertebrates, and the discovery of 7 pairs of pharyngeal arches similar to Yunnan insects supported this hypothesis. That is, the gill arch of fish is the prototype of the jaw arch and the tongue arch, and the jaw arch and tongue arch of the vertebrate are homologous to the gill arch behind the series.

Tian Qingyi explained that although the pharyngeal arches of living vertebrates at different locations will develop into different forms of bones such as jaw arch, tongue arch and gill arch, in the early stage of vertebrate evolution, the pharyngeal arches at different locations are similar to each other.

Yunnan insect fossil pharyngeal structure (A) overall photo of pharynx; (B–E) Microstructure of the Pharyngeal Arch: Optical Photograph of the Pharyngeal Arch: (B) Schematic Diagram of the Structure (C) Three-dimensional X-ray CT rendering of a single Pharyngeal Arch (D) Scanning electron microscopic photograph of carbonaceous residues in the Pharyngeal Arch showing the morphological structure of protein microfibrils preserved in fossils (E) (Tian Qingyi)

“It’s a milestone”

After the Spree fish research paper was published in 1993, researchers finally discovered the vertebrate-like head through the study of about 100 new specimens, which was identified as a vertebrate in 2014; A research paper on the Yunnan insect was published in 1991, and researchers conducted microscopic-scale studies of 127 specimens, which were identified as primitive vertebrates in 2022.

Why does it take so long from discovery to identification as a vertebrate? Tian Qingyi explains: “Because Yunnan insects have always lacked the macroscopic characteristics of universally recognized vertebrates, decades later, with cutting-edge analysis technology, we have finally found key microscopic features. ”

“A lot of life on Earth hasn’t been preserved, and we’re lucky because we found evidence of life — fossils.” Zhao Fangchen hopes that Tian Qingyi, who is studying in the UK, can use China’s natural fossil treasure house to do more cutting-edge research while broadening his horizons.

“This Science paper not only identifies the oldest close relative of living vertebrates, yunnan worm, but also provides key evidence for revealing the origin and early evolution of vertebrates, which will have a profound impact on the exploration of the evolution of vertebrate jaws and other key features.” Zhu Maoyan pointed out that from another perspective, the study once again shows that the fossils of the Chengjiang fauna have the potential to preserve the fine biological structure of the micro-nano scale.

The reviewers spoke highly of the study: “This is a significant contribution to science and will be a key paper in paleovertebrate zoology, presenting the long-awaited and most credible evidence of anatomical structured stones and resolving the Cambrian stem group vertebrate controversy.” For many years to come, even higher-resolution research will be difficult to surpass. This is a milestone work. (Source: China Science Daily Shen Chunlei)

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