The outer ear is unique to mammals, but its evolutionary origin has remained a mystery. According to a new study published in Nature from the USC Stem

An earful of gill: USC Stem Cell study points to the evolutionary origin of the mammalian outer ear

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2025-01-12 00:30:05

The outer ear is unique to mammals, but its evolutionary origin has remained a mystery. According to a new study published in Nature from the USC Stem Cell lab of Gage Crump, this intricate coil of cartilage has a surprisingly ancient origin in the gills of fishes and marine invertebrates.

“When we started the project, the evolutionary origin of the outer ear was a complete black box,” said corresponding author Crump, professor of stem cell biology and regenerative medicine at the Keck School of Medicine of USC. “We had been studying the development and regeneration of the jawbones of fishes, and an inspiration for us was Stephen Jay Gould’s famous essay ‘An earful of jaw,’ which laid out how fish jawbones transformed into the middle ear bones of mammals. This made us wonder whether the cartilaginous outer ear may also have arisen from some ancestral fish structure.”

The first clue toward cracking this mystery was the team’s discovery that gills and outer ears are both composed of a relatively rare tissue type: elastic cartilage. “When we started the study, there was very little out there about whether elastic cartilage existed outside of mammals,” said Crump. “So it wasn’t really known if fish had elastic cartilage or not. It turns out that they do.”