419-million-year-old Chinese fossil shows human middle ear that evolved from fish gills

The human middle ear — which houses three tiny, vibrating bones — is key to transporting sound vibrations into the inner ear, where they become nerve impulses that enable 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 vertebrate evolution.

some 20^th In the 19th century, researchers who assumed that early vertebrates would have to have a complete spiral gill searched for one between the lower jaw and the hyoid arch of early vertebrates. However, despite extensive research spanning more than a century, none have been found in vertebrate fossils.

Now scientists from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and their collaborators have found clues to this puzzle in armored galeaspid fossils in China.

Their results were published in the journal Frontiers in ecology and evolution on May 19, 2022.

According to Prof. GAI Zhikun of the IVPP, first author of the study, researchers at the institute have successively found a 438-million-year-old over the past 20 years shuyu 3D braincase fossil and the first 419 million year old galeaspid fossil fully preserved with gill filaments in the first gill chamber. The fossils were found in Changxing, Zhejiang Province and Qujing, Yunnan Province, respectively.

“These fossils provided the first anatomical and fossil evidence for a vertebrate spiracle derived from fish gills,” GAI said.

A total of seven virtual endocasts of the shuyu braincases were subsequently reconstructed. Almost all details of the skull anatomy from shuyu were revealed in his fingernail-sized skull, including five brain divisions, sensory organs, and cranial nerve and blood vessel passages in the skull.

“Many important human structures can be traced back 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 humans. shuyu was considered just as important as a key missing link Archeopteryx, Ichthyostega and tiktaliksaid ZHU Min, academician of the Chinese Academy of Sciences.

The spiracle is a small hole behind each eye that opens to the mouth in some fish. In sharks and all rays, the spiracle is responsible for taking water into the mouth before expelling it from the gills. The spiracle is often located toward the top of the animal, allowing breathing even when the animal is mostly buried under sediment.

In which Polypterus, the most primitive living bony fish, the spiracles are used to breathe air. However, fish spirals were eventually replaced in most non-fish species as they evolved to breathe through the nose and mouth. in the morning[{” attribute=””>tetrapods, the spiracle seems to have developed first into the Otic notch. Like the spiracle, it was used in respiration and was incapable of sensing sound. Later the spiracle evolved into the ear of modern tetrapods, eventually becoming the hearing canal used for transmitting sound to the brain via tiny inner ear bones. This function has remained throughout the evolution to humans.

“Our finding bridges the entire history of the spiracular slit, bringing together recent discoveries from the gill pouches of fossil jawless vertebrates, via the spiracles of the earliest jawed vertebrates, to the middle ears of the first tetrapods, which tells this extraordinary evolutionary story,” said Prof. Per E. Ahlberg from Uppsala University and academician of 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 C. J. Donoghue, 19 May 2022, Frontiers in Ecology and Evolution.
DOI: 10.3389/fevo.2022.887172