New research has highlighted a fascinating link between human outer ears and the gills of ancient fish. Gene-editing experiments have revealed that the cartilage in fish gills may have migrated to form the outer ear structures seen in mammals today. Scientists believe this evolutionary transformation took place millions of years ago, suggesting that the origins of elastic cartilage in human ears may date back to early marine invertebrates such as horseshoe crabs.
Study Highlights Evolutionary Link
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, sought to uncover the mysterious origins of mammalian outer ears. Elastic cartilage, the primary component of human outer ears, is unique to mammals and is more flexible than other types of cartilage found in the human body. The researchers found that this type of cartilage is also present in the gills of modern bony fish, such as zebrafish and Atlantic salmon.
Gene-Editing Experiments Provide Insight
As reported in Live Science, in experiments, human genetic enhancers associated with outer ear development were inserted into zebrafish genomes. The enhancers triggered activity in the fish gills, suggesting a genetic link between the structures. The reverse experiment, involving the introduction of zebrafish enhancers into mouse genomes, showed activity in the mice’s outer ears, reinforcing the connection between fish gills and mammalian ears.
Ancient Marine Connections
The team further demonstrated that reptiles and amphibians also inherited gill-related structures from fish. Evidence from green anole lizards indicated that elastic cartilage had begun migrating from gills to ear canals by the time reptiles appeared approximately 315 million years ago. Additionally, a gene control element in horseshoe crabs, organisms that emerged 400 million years ago, activated activity in fish gills, pointing to even deeper evolutionary ties.
As per the researchers, these findings highlight the adaptive reuse of ancestral gill structures in the development of mammalian ears over evolutionary history.