New research has unveiled a significant connection between human outer ears and the gills of ancient fish, dating back 400 million years. Gene-editing experiments indicate that cartilage in fish gills may have evolved to form the outer ear structures present in mammals today.
According to a study published in Nature, researchers led by Gage Crump, Professor of Stem Cell Biology and Regenerative Medicine at the University of Southern California, aimed to uncover the origins of mammalian outer ears. Elastic cartilage, the primary component of human outer ears, is unique to mammals and more flexible than other types of cartilage found in the human body. The study revealed that this cartilage type is also present in the gills of modern bony fish, including zebrafish and Atlantic salmon.
In experiments, human genetic enhancers related to outer ear development were inserted into zebrafish genomes, triggering activity in the fish gills and suggesting a genetic link. Conversely, introducing zebrafish enhancers into mouse genomes resulted in activity in the mice's outer ears, further reinforcing the connection.
The research also indicated that reptiles and amphibians inherited gill-related structures from fish. Evidence from green anole lizards showed that elastic cartilage began migrating from gills to ear canals approximately 315 million years ago. Additionally, a gene control element in horseshoe crabs, which emerged 400 million years ago, activated activity in fish gills, suggesting deeper evolutionary ties.
Researchers concluded that these findings illustrate the adaptive reuse of ancestral gill structures in the development of mammalian ears throughout evolutionary history.