The human middle ear – which houses three tiny, vibrating bones – is the key to transmitting sound vibrations to the inner ear, where nerve impulses are made that allow us to hear. Embryonic and fossil evidence show that the human middle ear evolved from the spiral of fish. However, the origin of the vertebrate coil has long been an unsolved mystery in the evolution of vertebrates. “These fossils provided the first anatomical and fossils for a vertebrate coil derived from fish gills.” – Prof. GAI Zhikun Some 20th century researchers, believing that early vertebrates should have a complete helical gill, looked for one between the mandible and the arc of the early vertebrates. However, despite extensive research that lasted more than a century, none were found in vertebrate fossils. Now, however, scientists from the Institute of Paleontology and Vertebrate Anthropology (IVPP) of the Chinese Academy of Sciences and their collaborators have found clues to this mystery from shielded shield fossils in China. Their findings were published in the journal Frontiers in Ecology and Evolution on May 19, 2022. According to Professor GAI Zhikun of IVPP, the study’s first author, researchers at the institute have found a 438-million-year-old Shuyu 3D brain fossil and the first 419-million-year-old galeaspid fossil in the last 20 years. in the first interdisciplinary chamber. The fossils were found in Changxing, Zhejiang Province and Qujing, Yunnan Province, respectively. The 3D virtual reconstruction of Shuyu. Credit: IVPP “These fossils provided the first anatomical and fossil evidence of a vertebrate coil derived from fish gills,” GAI said. A total of seven virtual intranets of Shuyu’s brain were subsequently reconstructed. Almost all the details of Shuyu’s cranial anatomy were revealed on the skull the size of his fingernails, including five parts of the brain, sensory organs and cranial nerves and blood vessels in the skull. “Many important human structures can be traced back to our fish ancestors, such as teeth, jaws, middle ears, etc. The main task of paleontologists is to find the important links that are missing in the evolutionary chain from fish to humans. “Shuyu has been considered a missing link as important as Archeopteryx, Ichthyostega and Tiktaalik,” said ZHU Min, an academician at the Chinese Academy of Sciences. The first galaxy fossil, 419 million years old, fully preserved with gill threads in the first interdisciplinary chamber. Credit: IVPP The spiral is a small hole behind each eye that opens into the mouth of some fish. In sharks and all rays, the coil is responsible for absorbing water in the mouth before being expelled from the gills. The coil is often located towards the top of the animal allowing it to breathe even when the animal is mainly buried under sediment. In Polypterus, the most primitive, living bony fish, spiracles are used to breathe air. However, spiral fish eventually replaced most non-fish species as they evolved to breathe through their noses and mouths. In the early quadrupeds, the coil appears to have developed first in the ear canal. Like the spiral, it was used to breathe and was unable to detect sound. The spiral later evolved into the ear of modern quadrupeds, eventually becoming the auditory canal used to transmit sound to the brain through tiny bones in the inner ear. This function has remained with humans throughout evolution. “Our find bridges the entire history of the spiral fissure, bringing together recent discoveries from the gill sacs of vertebrate-free vertebrate fossils, through the coils of the earliest mandibular vertebrates, to the middle ears of the first quadrupeds, which tells this remarkable story.” Said Professor Per E. Ahlberg of Uppsala University and an academic at the Royal Swedish Academy of Sciences. Reference: “The Evolution of the Spiracular Region From Jawless Fishes to Tetrapods” by Zhikun Gai, Min Zhu, Per E. Ahlberg and Philip CJ Donoghue, May 19, 2022, Frontiers in Ecology and Evolution.DOI: 10.3389 / fevo.201728.
