(ORDO NEWS) — An international team of scientists has found evidence that our middle ear came from the gills of a fish.
The human middle ear is part of the mammalian auditory system, which at one time developed from the bones of the lower jaw and provides the transformation of air vibrations into vibrations of the fluid that fills the inner ear. In the human middle ear there are three tiny vibrating bones that help transmit these vibrations.
Embryonic and fossil evidence shows that our middle ear evolved from the spiracle, a vestigial gill opening preserved in some modern fish.
Believing that early vertebrates must have had complete respiratory gills (i.e., spiracles), they looked for them between the mandible and the hyoid arch, but did not find them.
Scientists from the universities of the Chinese Academy of Sciences (China), Uppsala (Sweden) and Bristol (UK) have found the key to solving this mystery in fossil armored galeaspids (an extinct taxon of jawless marine and freshwater fish) found in China. They presented their findings in the journal Frontiers in Ecology and Evolution.
The study was conducted on two fossils aged 438 and 419 million years from the provinces of Zhejiang and Yunnan. The remains belong to an extinct genus of jawless vertebrates, Shuyu, which is considered a key missing link as important as Archeopteryx, Ichthyostega and Tiktaalik.
Although Shuyu’s skull is the size of a fingernail, thanks to its good preservation, scientists have been able to see minute details in it, including five brain regions, gill filaments, passages of cranial nerves and blood vessels. A spiracle was also found behind each eye of the galeaspid, allowing it to breathe.
As evolution has progressed, many land-dwelling species have shed their spiracles in order to breathe through their mouths and noses. In early tetrapods, the spiracles probably first developed into an auditory notch and later evolved into the ear, becoming an auditory canal that helps transmit sounds to the brain.
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