Shuyu’s 3D brain. Credit: IVPP
The human middle ear, which houses three small, vibrating bones, is key to transporting sound vibrations to the inner ear, where they become nerve impulses that allow us to hear.
Embryonic and fossil evidence shows that the human middle ear evolved from the spiracle of fish. However, the origin of the vertebrate spiracle has long been an unsolved mystery in the evolution of vertebrates.
“These fossils provided the first anatomical and fossil evidence of a vertebrate spiracle originating from fish gills.” – Prof. GAI Zhikun
Twentieth-century researchers, believing that the first vertebrates must have a complete spiral gill, looked for one among the mandibular and hyoid arches of the first vertebrates. Although extensive research spans more than a century, none have been found in vertebrate fossils.
Now, however, scientists at the Institute of Paleontology and Paleoanthropology of Vertebrates (IVPP) of the Chinese Academy of Sciences and their collaborators have found clues to this mystery from armored galaspid fossils in China.
His findings were published in the journal Frontiers in Ecology and Evolution on May 19, 2022.
According to GAI Professor Zhikun of IVPP, the first author of the study, the institute’s researchers have successively found a 438-million-year-old Shuyu 3D brain fossil and the world’s first galleospid fossil. 419 million years. completely preserved with gill filaments in the first gill chamber. The fossils were found in Changxing, Zhejiang Province and Qujing, Yunnan Province, respectively.
Shuyu’s 3D virtual reconstruction. Credit: IVPP
“These fossils provided the first anatomical and fossil evidence of a spiracle of vertebrates originating from fish gills,” GAI said.
A total of seven virtual endocasts of Shuyu’s brain box were subsequently reconstructed. Almost every detail of Shuyu’s cranial anatomy was revealed in his nail-sized skull, including five brain divisions, sensory organs and cranial nerves and the passages of the skull’s blood vessels.
“Many important human structures date back to our ancestral fish, such as our teeth, jaws, middle ears, and so on. The main task of paleontologists is to find the important trunks that are missing in the evolutionary chain from fish to humans. Shuyu has been regarded as a lost link as important as Archeopteryx, Ichthyostega and Tiktaalik, “said ZHU Min, a scholar of the Chinese Academy of Sciences.
The first 419 million-year-old galaspid fossil is completely preserved with gill filaments in the first gill chamber. Credit: IVPP
The spiracle is a small hole behind each eye that opens into the mouth in some fish. In sharks and all rays, the spiracle is responsible for ingesting water into the oral space before being expelled from the gills. The spiracle is often at the top of the animal, allowing it to breathe even while the animal is mostly buried under sediment.
In Polypterus, the most primitive living bony fish, spiracles are used to breathe air. However, fish spiracles were eventually replaced in most non-fish species as they evolved to breathe through their nose and mouth. In early tetrapods, the spiracle appears to have first developed in the optic notch. Like the spiracle, it was used in breathing and was unable to detect sound. Later, the spiracle evolved into the ear of modern tetrapods, eventually becoming the auditory canal used to transmit sound to the brain through the small bones of the inner ear. This function has been maintained throughout evolution to humans.
“Our finding unites the entire history of the spiracle cleft, gathering recent discoveries from the jaw bags of jawless fossil vertebrates, through the spiracles of the first jaw vertebrates, to the middle ears of the first tetrapods, which explains this extraordinary evolutionary history. “said Professor Per E. Ahlberg of Uppsala University and a member of the Royal Swedish Academy of Sciences.
Reference: “The Evolution of the Spiracular Region From Jawless Fishes to Tetrapods” by Zhikun Gai, Min Zhu, By E. Ahlberg and Philip CJ Donoghue, May 19, 2022, Frontiers in Ecology and Evolution.DOI: 10.3389 / fevo.2022.887172