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

The middle ear, which is made up of three small vibrating bones, is responsible for delivering sound vibrations into the inner ear, where they are converted into nerve impulses that allow us to hear.

The human middle ear developed from the spiracle of fishes, according to embryonic and fossil data. The origin of the vertebrate spiracle, on the other hand, has long been a conundrum in vertebrate evolution.

Some 20th-century scholars sought for a full spiracular gill between the mandibular and hyoid arches of early vertebrates, assuming that early vertebrates must have one. Despite considerable investigation spanning more than a century, no vertebrate remains have been discovered.

Scientists from the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) and its partners have now discovered hints to this riddle in armored galeaspid fossils in China.

According to the study's first author, Prof. GAI Zhikun of IVPP, the institute's researchers discovered a 438-million-year-old Shuyu 3D braincase fossil and the first 419-million-year-old galeaspid fossil totally preserved with gill filaments in the first branchial chamber during the previous 20 years. The fossils were discovered in Zhejiang Province's Changxing and Yunnan Province's Qujing, respectively.

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

The Shuyu braincase was then recreated using seven virtual endocasts. Shuyu's skull displayed almost all of its cranial anatomy, including five brain divisions, sensory organs, and cranial nerve and blood vessel pathways, in its fingernail-sized size.

“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 humans. Shuyu has been regarded as a key missing link as important as Archaeopteryx, Ichthyostega and Tiktaalik,”  remarked ZHU Min, a Chinese Academy of Sciences academician.

In certain fishes, the spiracle is a tiny opening behind each eye that leads to the mouth. The spiracle is responsible for water intake into the buccal space before being ejected from the gills in sharks and all rays. The spiracle, which is usually at the top of the animal, allows it to breathe even when it is mostly buried by dirt.

The spiracles are utilized to breathe air in the Polypterus, the most primitive extant bony fish. However, when non-fish creatures developed to breathe via their noses and mouths, fish spiracles were finally superseded. The spiracle appears to have grown first into the Otic notch in early tetrapods. It, like the spiracle, was utilized for respiration and could not detect sound.

The spiracle later developed into the ear of contemporary tetrapods, and the hearing canal, which transmits sound to the brain via small inner ear bones. This role has persisted in humans throughout their existence.

“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 of Uppsala University and academician of the Royal Swedish Academy of Sciences.