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

3D Shuu brain enclosures. Credit: IVPP

The human middle ear – which contains three tiny, vibrating bones – is key to transmitting sound vibrations to 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 a fish ear. However, the origin of vertebrates has long been an unsolved mystery in vertebrate evolution.

“These fossils provided the first anatomical and fossil evidence of the vertebrate soul originating from fish gills. – prof. GAI Zhikun

Some 20th century researchers, believing that early vertebrates must possess a complete spiral gill, sought one between the mandibular and hyoid arches of early vertebrates. Despite extensive research that has lasted more than a century, none has been found in a single vertebrate fossil.

Now, however, scientists from the Institute of Paleontology and Paleoanthropology of Vertebrates (IVPP) of the Chinese Academy of Sciences and their associates have found traces of this mystery from armored fossils of galeaspids in China.

Their findings were published in the journal Boundaries in ecology and evolution May 19, 2022

According to GAI professor Zikun from IVPP, the first author of the study, researchers from the institute have found 438 million years old in the last 20 years. Shuiu The 3D fossil of the meninges and the first 419 million-year-old galeaspid fossil completely preserved with gill filaments in the first granial chamber. Fossils were found in Changxing in Zhejiang Province and Zhujing in Yunnan Province.

Shuiu 3D virtual reconstruction

3D virtual reconstruction of Shuu. Credit: IVPP

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

A total of seven virtual endocasts Shuiu the meninges were subsequently reconstructed. Almost all the details of the anatomy of the skull Shuiu they were found in the nail-sized skull, including five sections of the brain, sensory organs, and cranial nerve and blood vessel passages in the skull.

“Many important structures of human beings can be traced back to our fish ancestors, such as our teeth, jaws, middle ear, etc. The main task of paleontologists is to find important links that are missing in the evolutionary chain from fish to humans. Shuiu is considered a key link that is missing just as important as Archaeopterik, Ichthiostega i Tiktaalik“, Said ZHU Min, an academician of the Chinese Academy of Sciences.

A 419 million year old Galeaspid fossil completely preserved with gill filaments

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

The choke is a small hole behind each eye that opens in some fish’s mouth. In sharks and all rays, the suffocation is responsible for bringing water into the buccal space before it is expelled from the gills. The trachea is often located towards the top of the animal allowing it to breathe even while the animal is mostly buried under sediment.

U Polypterus, the most primitive, living bony fish, spiracles are used to breathe air. However, fish nozzles were eventually replaced in most non-fish species because they evolved to breathe through the nose and mouth. In the early[{” 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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