Chinese fossils show human middle ear evolved from fish gills

Fig. 1 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.

Some 20th-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 Frontiers in Ecology and Evolution May 19.

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

Chinese fossils show human middle ear evolved from fish gills

Fig. 2 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 of Shuyu’s puzzle were then reconstructed. Almost every detail of Shuyu’s cranial anatomy has been 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 humans. Shuyu said considered a key missing link as important as Archeopteryx, Ichthyostega and Tiktaalik,” said Zhu Min, Academician of the Chinese Academy of Sciences.

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.

Chinese fossils show human middle ear evolved from fish gills

Fig. 3 The first fully preserved 419-million-year-old galeaspid fossil with gill filaments in the first gill chamber. Credit: IVPP

In Polypterus, the most primitive living bony fish, the spiracles are used to breathe air. However, the spiracles of fish were eventually replaced in most non-fish species as they evolved to breathe through their noses and mouths. In early tetrapods, the spiracle seems to have first developed in the otic notch. Like the spiracle, it was used in respiration 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 via tiny bones in the inner ear. This function remained throughout evolution until man.

“Our discovery connects the entire history of the spiracular cleft, bringing together recent discoveries from the gill pouches of fossil jawless vertebrates, through the spiracles of early jawed vertebrates, to the middle ears of early tetrapods, which tells this story. extraordinary scalability,” said Professor Per E. Ahlberg of Uppsala University and Academician of the Royal Swedish Academy of Sciences.

Palaeospondylus: longstanding vertebrate evolution mystery solved using powerful X-rays

More information:
Zhikun Gai et al, The evolution of the spiracular region from jawless fishes to tetrapods, Frontiers in Ecology and Evolution (2022). DOI: 10.3389/fevo.2022.887172

Provided by Chinese Academy of Sciences

Quote: Chinese fossils show human middle ear evolved from fish gills (Jun 17, 2022) Retrieved June 18, 2022 from middle-ear.html

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