It is possible that the human outer ear evolved from the gills of ancient fish. (Photo credit: A. Martin UW Photography/Getty Images (left); photographer Basak Gurbuz Derman/Getty Images (right))
A recent study suggests that humans' outer ears may be evolutionarily related to the gills of long-extinct fish.
Gene editing experiments have revealed that the cartilage found in fish gills migrated into the ear canal over millions of years as we evolved. Going even further, it suggests that our outer ears may have evolutionary roots in ancient marine invertebrates such as horseshoe crabs, scientists say.
The study sheds new light on the mysterious origins of outer ears, which are unique to mammals. “When we began this project, the evolutionary origins of the outer ear were a complete black box,” study co-author Gage Crump, a professor of stem cell biology and regenerative medicine at the University of Southern California, said in a statement.
Researchers already knew that our middle ears — each located behind the eardrum and made up of three small bones — evolved from the jawbones of ancient fish. This example of evolution, which transforms and redefines anatomical structures, “made us wonder whether the cartilaginous outer ear might also have evolved from some ancestral fish structure,” Crump added.
Our outer ears and the ears of other mammals are made of a subtype of cartilage known as elastic cartilage. It is more flexible than the hyaline cartilage or fibrocartilage found in the human nose and the discs between our vertebrae, respectively.
The key to tracing the origins of human outer ears to fish was the discovery that elastic cartilage is also found in fish gills. “When we started the study, there was very little information about whether elastic cartilage existed outside of mammals,” Crump said.
The researchers used protein tags to find that the gills of zebrafish (Danio rerio), Atlantic salmon (Salmo salar), and three other fish species contain elastic cartilage. All of these species are modern bony fish, suggesting that elastic cartilage is a common characteristic of this group, according to a study published Jan. 9 in the journal Nature.
The scientists then looked at the evolutionary relationship between the elastic cartilage in fish gills and the outer ears of mammals. Because elastic cartilage is poorly preserved in fossils, the researchers used molecular clues. They looked for gene control elements known as “amplifiers” — short DNA sequences that can activate related genes when they interact with specific proteins.
Sourse: www.livescience.com
