Chinese fossils show human middle ear evolved from fish gills

The human middle ear—which houses three tiny, vibrating bones—is key to transporting sound vibrations into the inner ear, where they become nerve impulses that allow us to hear.

Chinese scientists have eventually found clues to the mystery in fossils unearthed in the provinces of Zhejiang and Yunnan, which provided anatomical and fossil evidence for the origin of vertebrate spiracles from gills.

According to Gai Zhikun, a Chinese Academy of Sciences researcher and the first author of the article "The Evolution of the Spiracular Region From Jawless Fish to Tetrapods," there is ample embryonic and fossil evidence that the human middle ear evolved from a fish's spiracle.

"The discovery of these ancient fish fossils is proof that our middle ears originated from fish gills. It explains why human ears don't breathe today, but they're still connected to the mouth, because they used to be the organ of fish's respiratory system," said Gai.

The "ancient fish kingdom" of Qujing in southwest China's Yunnan Province was ocean from Silurian to Devonian. During excavations of fossils in the early Devonian strata, the research team collected the first fossil material of broad-shelled turtle with intact gill-filaments marks. It provides the most accurate anatomical and fossil evidence for the theory that the spiracle of vertebrates such as fish originates from degenerated gills.

"Many human body structures can be traced back to the ancestors of fish, such as our teeth, jaws and middle ears. Paleontologists are filling in important processes in the chain of evolution from fish to humans. The fossil of Qujing ancient fish kingdom provides important evidence for the improvement of this chain," said Zhu Min, researcher of Institute of Vertebrate Paleontology and Paleoanthropology under CAS.

The research team proposed the evolution process of the spiracle from jawless fish to tetrapod on the basis of the fossils, thus establishing the evolutionary sequence of the spiracle from the gills of jawless fish to the middle ear of human.

Experts believe that as fish evolved and landed on ground, they had to develop new senses to survive better in the air. The spiracle, which had lost its ability to breathe, gradually evolved into the middle ear, becoming the hearing canal used for transmitting sound to the brain via tiny inner ear bones. Thus, humans evolved acute hearing.