3D Shuyu brain box. Credit: IVPP
The human middle ear, which contains three small, vibrating bones, is key to transmitting sound vibrations to the inner ear, where they are converted into nerve impulses that allow us to hear.
Embryonic and fossil evidence proves that the human middle ear evolved from the respiratory tract of fish. However, the origin of the vertebrate respiratory system has long been an unsolved mystery in the evolution of vertebrates.
“These fossils provided the first anatomical and fossil evidence for a vertebrate animal originating from fish gills. – Prof. GAI Zhikun
Some 20th-century researchers, believing that early vertebrates must have a complete spiral gill, have sought one between the mandibular and hyoid arches of early vertebrates. However, despite extensive research spanning more than a century, none has been found in any vertebrate fossils.
However, scientists from the Institute of Paleontology and Paleoanthropology of Vertebrates (IVPP) of the Chinese Academy of Sciences and their collaborators have now found evidence of this mystery from armored fossils of a galleon in China.
Their findings were published in the journal Frontiers in Ecology and Evolution on May 19, 2022.
According to Professor GAI Zhikun of the IVPP, the first author of the study, researchers at the institute have consistently discovered 438 million years of Shuyu 3D brain corpus fossil and the first 419 million-year-old galeaspid fossil completely preserved with gill threads in the first bronchial chamber. . Fossils have been found in Changxing, Zhejiang Province and Qujing, Yunnan Province, respectively.
3D virtual reconstruction of Shuyu. Credit: IVPP
“These fossils provided the first anatomical and fossil evidence for a vertebrate animal originating from fish gills,” the GAI said.
Subsequently, a total of seven virtual endocasts of the Shuyu brain box were reconstructed. Almost all the details of Shuyu’s cranial anatomy were revealed in his claw-sized skull, including five brain departments, sensory organs and cranial nerve passages, and blood vessels in the skull.
Many important structures of human beings can be traced to our fish ancestors, such as our teeth, jaws, middle ears, etc. The main task of paleontologists is to find the important missing links in the evolutionary chain from fish to man. Shui is considered a key missing link, as important as Archeopteryx, Ichthyostega and Tiktaalik, “said Zhu Ming, an academician at the Chinese Academy of Sciences.
The first fossil of a galeaspid 419 million years old, completely preserved with gill threads in the first bronchial chamber. Credit: IVPP
The respiratory tract is a small hole behind each eye that opens to the mouth in some fish. In sharks and all rays, the respiratory system is responsible for receiving water in the buccal space before it is expelled from the gills. The respirator is often located towards the top of the animal, which allows breathing, even when the animal is mostly buried under sediment.
In Polypterus, the most primitive, living bony fish, the respiratory system is used to breathe air. However, fish respirators were eventually replaced by most non-fish species as they evolved to breathe through their noses and mouths. In early tetrapods, the respiratory tract appears to have developed first in the ear canal. Like the respiratory system, it was used for breathing and was unable to sense sound. Later, the respiratory tract evolved in the ear of modern tetrapods, eventually becoming an auditory canal used to transmit sound to the brain through small inner ear bones. This feature has remained throughout human evolution.
“Our discovery connects the whole history of the spiral slit, uniting the latest discoveries from the gill bags of fossil jawless vertebrae, through the respiration of the earliest vertebrate jaws, to the middle ears of the first tetrapods, which tells this extraordinary evolutionary story,” said Prof. Per E. Alberg of Uppsala University and an academician of the Royal Swedish Academy of Sciences.
Reference: “The Evolution of the Spiral Region from Fish without Jaws to Quadrupeds” by Jikun Guy, Min Ju, Per E. Alberg and Philip CJ Donohue, May 19, 2022, Limits in Ecology and Evolution. DOI: 10.3389 / fevo.201728 .
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