Chinese fossils show human middle ear evolved from fish gills

Fig. 1 Shuyu’s 3D braincase. Credit: IVPP

The human middle ear – which houses three tiny vibrating bones – is key to carrying 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 the spiracle of fish. However, the origin of the vertebrate blowhole has 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, however, none have been found in any vertebrate fossils.

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

Their findings were published in Frontiers in Ecology and Evolution on the 19th of May.

According to Professor Gai Zhikun of IVPP, the study’s first author, researchers at the institute have found successively in the last 20 years a 438-million-year-old 3D Shuyu fossil and the first 419-million-year-old Galeaspid fossil. completely preserved with gill filaments in the 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 blowhole originating from fish gills,” said Gai.

A total of seven virtual endocasts of Shuyu’s braincase were later reconstructed. Nearly every detail of Shuyu’s cranial anatomy has been revealed in his fingernail-sized skull, including five brain divisions, sensory organs, and cranial nerve and blood vessel passages in the skull.

“Many important structures of humans can be traced back to our fish ancestors, such as our teeth, jaws, middle ears, etc. was considered a missing link as important as Archeopteryx, Ichthyostega and Tiktaalik,” said Zhu Min, an academician at the Chinese Academy of Sciences.

The blowhole is a small hole behind each eye that opens into the mouth in some fish. In sharks and in all rays, the blowhole is responsible for the entry of water into the mouth space before being expelled by the gills. The blowhole is usually located at the top of the animal, allowing it to breathe even when the animal is 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, spiracles are used to breathe air. However, fish spiracles were eventually replaced in most non-fish species as they evolved to breathe through the nose and mouth. In early tetrapods, the blowhole appears to have developed first in the optic notch. Like the blowhole, it was used in breathing and was unable to feel sound. Later, the blowhole evolved into the ear of modern tetrapods, becoming the ear canal used to transmit sound to the brain via tiny bones in the inner ear. This function remained throughout evolution until humans.

“Our discovery links the entire history of the spiracular cleft, bringing together recent findings from the gill pouches of jawless fossil vertebrates, through the spiracles of early jawed vertebrates, to the middle ears of early tetrapods, which tell this extraordinary evolutionary story.” Prof. Per E. Ahlberg of Uppsala University and scholar of the Royal Swedish Academy of Sciences.

Palaeospondylus: Long-standing mystery of vertebrate evolution 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 / Feb.2022.887172

Provided by the Chinese Academy of Sciences

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

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