From Gills to Ears: Unraveling the Evolutionary Journey of Mammals

The outer ear is distinct to mammals, yet its evolutionary beginnings have remained unclear. A study disseminated in Nature from the USC Stem Cell laboratory of Gage Crump reveals that this intricate structure of cartilage has surprisingly ancient roots in the gills of fish and aquatic invertebrates.

“At the start of the project, the evolutionary source of the outer ear was wholly unknown,” stated corresponding author Crump, a professor of stem cell biology and regenerative medicine at the Keck School of Medicine of USC.

“We were examining the development and regeneration of jawbones in fish, and we drew inspiration from Stephen Jay Gould’s renowned essay ‘An earful of jaw,’ which described how fish jawbones evolved into the middle ear bones of mammals. This led us to ponder whether the cartilaginous outer ear might also derive from some ancestral fish structure.”

The initial hint toward solving this enigma was the team’s finding that both gills and outer ears consist of a relatively uncommon tissue type: elastic cartilage. “When we initiated the study, there was scant information regarding the existence of elastic cartilage outside mammals,” Crump noted. “So, it wasn’t clearly known if fish possessed elastic cartilage or not. As it turns out, they do.”

Gills and outer ears present different appearances and functions. They also do not undergo mineralization, which means they are seldom found in the fossil record. Consequently, a novel approach was essential to determine if they had an evolutionary link.

The study’s lead author, Mathi Thiruppathy, a Ph.D. candidate in the Crump lab, concentrated on gene regulatory elements known as enhancers. While the genes influenced by these enhancers often play roles in the development of many unrelated tissues and organs, enhancers usually demonstrate a high degree of tissue specificity.

The researchers were capable of integrating enhancers responsible for forming the elastic cartilage of the human outer ear into the genomes of zebrafish. Intriguingly, human outer ear enhancers exhibited activity exclusively in the gills of these transgenic zebrafish.

The scientists also achieved the reverse experiment, producing transgenic mice with genomes that included zebrafish enhancers typically associated with gill formation, and observed their activity in the mice’s outer ears. These enhancers were crucial in linking structures that initially seem quite dissimilar.

In collaboration with others, the researchers examined whether the human outer ear and fish gill enhancers could be utilized to trace the evolution of gills into outer ears through intermediary species, such as amphibians and reptiles.

The results showed that when either human ear or fish gill enhancers were integrated into the genomes of tadpoles, the enhancers displayed activity in their gills. However, upon the emergence of reptiles, the elastic cartilage of gills transitioned to the ear canal, a finding demonstrated through various experiments with green anole lizards.

This cartilage eventually underwent further modifications to create the prominent outer ears of early mammals.

Another unexpected finding was that the elastic cartilage of gills may have originated much earlier than previously believed. Earlier studies had identified cartilage-like tissues in the gills and tentacles of various marine invertebrates, such as horseshoe crabs, which have remained relatively unchanged since they first appeared close to 400 million years ago.

The researchers conducted DNA sequencing on individual cells of the horseshoe crab gills and uncovered a crab enhancer that, when introduced into the zebrafish genome, exhibited gill activity.

This indicates that the very first elastic cartilage, akin to what is found in our outer ears, may have first appeared in ancient marine invertebrates.

“This research adds a new chapter to the evolution of the mammalian ear,” stated Crump.

“While the middle ear developed from fish jawbones, the outer ear originated from cartilaginous gills. By analyzing how the same gene regulatory elements can facilitate the development of both gills and outer ears, the researchers provide a novel approach to revealing how structures can dramatically shift over evolution to adopt new and unforeseen functions.”