419 Million Year Old Chinese Fossil Shows Human Middle Ear Evolved From Fish Gills

Shuyu’s 3D puzzle. Credit: IVPP

The human middle ear, home to three tiny vibrating bones, is essential for carrying sound vibrations into the inner ear, where they become nerve impulses that allow us to hear.

Embryonic and fossil evidence proves that the human middle ear evolved from the spiracle of fish. However, the origin of the vertebrate stigma has long been an unsolved mystery in vertebrate evolution.

“These fossils provided the first anatomical and fossil evidence of a vertebrate spiracle from fish gills.” — Professor GAI Zhikun

Around twentye century researchers, believing that early vertebrates must possess a complete spiracular gill, looked for one between the mandibular and hyoid arches of early vertebrates. Despite extensive research spanning over a century, none have been found in vertebrate fossils.

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

Their findings were published in the journal Frontiers in Ecology and Evolution May 19, 2022.

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

Shuyu 3D Virtual Reconstruction

Shuyu’s 3D virtual reconstruction. Credit: IVPP

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

A total of seven virtual endocasts from Shuyu puzzles were then reconstructed. Almost every detail of the cranial anatomy of Shuyu were revealed in his fingernail-sized skull, including five brain divisions, sensory organs, and passages of cranial nerves and blood vessels in the skull.

“Many important structures of human beings 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 man. Shuyu was considered a missing link as important as Archeopteryx, Ichthyostega and Realsaid ZHU Min, Academician of the Chinese Academy of Sciences.

Fully preserved 419 million year old Galeaspid fossil with gill filaments

The first fully preserved 419-million-year-old galeaspid fossil with gill filaments in the first gill chamber. Credit: IVPP

The spiracle is a small hole behind each eye that opens into the mouth of some fish. In sharks and all rays, the spiracle is responsible for bringing water into the mouth space before being expelled from the gills. The spiracle is often located towards the top of the animal allowing for breathing even when the animal is mostly buried under sediment.

In the polyptera, the most primitive living bony fish, the spiracles are used to breathe air. However, fish stigmas were eventually replaced in most non-fish species as they evolved to breathe through their noses and mouths. At first[{” 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

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