New research indicates that the elastic cartilage that makes up our ears shares lineage with the gills of ancient fish.
A breakthrough carried out by USC Stem Cell researchers and published in Nature revealed a fascinating connection between the ears mammalian exteriors and ancient gills. Research shows how the elastic cartilage in our outer ears shares a lineage with the gill cartilage of ancient sea creatures.
The research was led by Professor Gage Crump of the Keck School of Medicine of USC. “When we started the project, the evolutionary origin of the external ear was a complete black box,” Crump explained.
Inspired by Stephen Jay Gould’s essay “An Earful of Jaw,” which traced the bones of the middle ear to the jaws of fish, Crump’s team explored the possibility that mammalian ears also evolved from a ancestral structure of fish, refers to .
The study focused on the unique tissue that makes up mammalian ears: the elastic cartilage. Found in both ears and gills, elastic cartilage does not fossilize, making direct evidence scarce. To investigate further, the researchers turned to gene enhancers – DNA sequences that regulate gene activity and are highly tissue-specific.
Using a new approach, author Mathi Thiruppathy introduced human external ear enhancers into zebrafish genomes. Remarkably, enhancers have become active in zebrafish gills.
The team then reversed the process, incorporating the zebrafish gill enhancers into mice, where they found activity in the external ears. Similar experiments on tadpoles and green anoles have shown consistent resultslinking ancient gills to the evolution of mammalian ears.
Further exploration revealed that the origins of elastic cartilage date back to ca. 450 million years. The team identified an enhancer in horseshoe crabs – among the oldest species on Earth – and tested its activity in zebrafish, finding that it was also linked to the gills. This discovery makes the evolutionary timeline of elastic cartilage much deeper than previously thought.
“This work provides a new chapter for evolution of the mammalian ear,” Crump noted. While the middle ear originated from the jaw bones of fish, the outer ear arose from the cartilaginous gills. The study not only reveals a remarkable evolutionary journey, but also introduces a new method for studying structural adaptations between species.
