A fascinating new study reveals that the mammalian outer ear has its evolutionary roots in the gills of ancient fish.
This surprising discovery sheds light on how structures can adapt over millions of years to serve entirely new functions.
Published in Nature by researchers at the Keck School of Medicine of USC, the study connects the elastic cartilage in mammalian ears to the same rare tissue found in fish gills.
To explore this link, scientists used gene enhancers — DNA sequences that control tissue development.
Remarkably, when human ear enhancers were introduced into zebrafish, they activated specifically in the gills.
Conversely, zebrafish gill enhancers activated in the outer ears of transgenic mice, further solidifying the evolutionary connection.
The study also traced the journey of elastic cartilage across species. In reptiles, this cartilage began transitioning from gills to the ear canal, ultimately forming the prominent outer ears of mammals.
Evidence suggests this tissue first emerged over 400 million years ago in marine invertebrates like horseshoe crabs.
"When we started the project, the evolutionary origin of the outer ear was a complete black box," said the author of the study, Gage Crump, professor of stem cell biology and regenerative medicine at the Keck School.
"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."
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