The oldest vertebrate on Earth has been identified! It also has a "photo" →

The evolution of vertebrates is one of the most magnificent chapters in the history of life evolution. The origin of vertebrates has always been one of the most attractive scientific hotspots.

The research team of Professor Jiang Baoyu from the School of Earth Sciences and Engineering of Nanjing University and the "Early Evolution of the Earth-Life System" research team led by Researcher Zhu Maoyan from the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences worked closely together to confirm that Yunnanzoa in the Chengjiang fauna of my country 518 million years ago was the most primitive group of vertebrates. The results were published in the international authoritative academic journal Science on July 8.

Yunnan insect ecological restoration map. The paper's co-corresponding author, researcher Zhao Fangchen from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, directed the map, and Yang Dinghua drew the map

Yunnan worm, which is more similar to lancelet in appearance, was once controversial in its identity

"Evolutionary biologists usually infer that the ancestor of vertebrates was a type of deuterostome with a notochord, dorsal neural tube and gill slits based on the characteristics of the living cephalochordate amphioxus. However, this hypothetical ancestor of vertebrates has never been supported by fossil evidence," said Jiang Baoyu, co-corresponding author of the paper and professor at Nanjing University.

Jiang Baoyu said that unlike the Kunming fish which has typical vertebrate characteristics, the Yunnan worm is morphologically closer to the cephalochordate lancelet, but its position in the vertebrate origin and deuterostome phylogenetic tree has always been controversial.

Previously, Chinese scholar Chen Junyuan and others discovered a large number of new specimens of Yunnanworm in the Kunming Haikou area. Based on the more detailed anatomical structure of these specimens, they classified Yunnanworm as a primitive head class, and believed that its evolutionary position was between the cephalochordate lancelet and the vertebrate lamprey. After the publication of this result, a great discussion on the classification position of Yunnanworm was set off in the academic community.

"Due to the great differences in the interpretations of different scholars on the different details of the soft tissues preserved in Yunnanzoon, Yunnanzoon has been classified as a vertebrate, cephalochordate, hemichordate, deuterostome stem group, and even a primitive bilaterian since it was first reported in 1991." Jiang Baoyu introduced that the controversy over the taxonomic position of Yunnanzoon has seriously affected the definition of the origin of vertebrates by this type of key fossils.

Yunnan worm specimen, the fossil body is 3.9 cm long. Photo provided by Zhao Fangchen

Special features on the pharyngeal arch of Yunnan insect help to identify its true identity

In order to solve the mystery of the classification of Yunnanworm, the research team used a variety of modern experimental equipment such as 3D X-ray tomography microscopes and scanning electron microscopes to re-study a class of animals whose classification was previously questionable from a microscopic perspective: Yunnanworm. In terms of morphology, Yunnanworm is somewhat similar to modern worms, with a flat body and generally only 3 to 4 centimeters long.

By reanalyzing 127 specimens of Yunnan worm, researchers discovered for the first time the three-dimensionally preserved discoid cell structures and protein microfibrils on the pharyngeal arches of Yunnan worm. These two fine microscopic structures are unique to vertebrate cell cartilage, proving that Yunnan worm is a primitive vertebrate.

The research team also used evolutionary analysis methods to further pinpoint the precise evolutionary position of Yunnanworm. The results of the analysis confirmed that Yunnanworm is at the most basal position in the vertebrate lineage, between urochordates and other vertebrates.

The structure of the pharynx of the Yunnan worm fossil. (A) Overall photo of the pharynx. (B–E) Microstructure of the pharyngeal arches: optical photo of the pharyngeal arches (B), structural schematic (C), 3D X-ray CT rendering of a single pharyngeal arch (D), scanning electron microscope photo of the carbonaceous residues in the pharyngeal arches, showing the morphological structure of the protein microfibrils preserved in the fossil (E). The first author of the paper, Tian Qingyi, made the map.

As for the controversial first pair of pharyngeal arches of Yunnanworm, the research team found the same stacked cell structure, gill filaments and protein microfibrils as the other gill arches in the back in the new specimen. Therefore, Yunnanworm has 7 pairs of pharyngeal arches that are similar to each other from front to back and have cellular cartilage.

"The discovery of seven pairs of pharyngeal arches similar to Yunnanworm supports the hypothesis of the homology of pharyngeal arches in vertebrates proposed by anatomists as early as the 19th century, that is, the gill arches of fish are the prototypes of the maxillary arches and lingual arches, and the maxillary arches and lingual arches of vertebrates are serial homologous structures with the posterior gill arches," said Jiang Baoyu.

Comparison of the evolutionary position of Yunnanzoon and the morphology of the pharyngeal skulls of various chordates (from top to bottom, they are lancelet, Yunnanzoon, metasprigger, lamprey, and shark). The first author of the paper, Tian Qingyi, made the map

The basket-shaped pharyngeal skull of Yunnanworm confirms the morphology of the ancestral pharyngeal skull of vertebrates

Jiang Baoyu believes that another important new discovery of this study is that the seven pairs of pharyngeal arches of Yunnanworm are connected by pairs of horizontal cartilages at the ventral and dorsal ends, forming a basket-like pharyngeal skull.

"The basket-shaped pharyngeal skull is a typical feature of living jawless vertebrates and fossil jawless euphaloid fishes, but the pharyngeal skulls of Cambrian post-Spriggish fishes and jawed vertebrates are composed of a series of separate cartilaginous rods." Jiang Baoyu said that this pharyngeal skull feature of post-Spriggish fishes has caused controversy about the morphology of the pharyngeal skulls of vertebrate ancestors. The pharyngeal skull features of Yunnan worms, which appeared earlier and were located at the most basal position of vertebrates, support that the basket-shaped pharyngeal skull is a more ancient ancestral feature.

It can be seen that as the oldest relative of living vertebrates, Yunnanzoon 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 characteristics.

This achievement was also highly praised by the reviewers of Science magazine: "This is a major contribution to science and will become a key paper in paleontology. It presents the long-awaited and most credible fossil evidence of anatomical structure and resolves the controversial issue of Cambrian stem vertebrates. The information they presented is a milestone that will be difficult to surpass even with higher-resolution research for many years to come."

Source: Science and Technology Daily

Author: Jinfeng Zhang Ye

Editor: Wu Qiong

Review: Wang Fei

Final judge: Chen Lei