A new study has revealed surprising information about the evolutionary background of the mammalian outer ear, showing that it has ties to the gills of ancient fish and marine invertebrates. The research indicates that both the outer ear and gills consist of elastic cartilage and share gene control elements, pointing to their evolutionary link. This discovery offers fresh insights into how the mammalian ear has evolved, demonstrating how structures can change over time to fulfill different roles.
The outer ear is a distinctive feature of mammals, but its evolutionary background has long been unclear. A recent study published in Nature by Gage Crump’s USC Stem Cell lab has revealed that this complex cartilage structure can be traced back to the gills of fish and marine invertebrates.
“At the start of our project, the evolutionary beginnings of the outer ear were completely unknown,” stated Crump, a professor specializing in stem cell biology and regenerative medicine at the Keck School of Medicine of USC. “We initially focused on fish jawbone development and regeneration, which was inspired by Stephen Jay Gould’s well-known essay ‘An earful of jaw,’ detailing how fish jawbones evolved into the middle ear bones of mammals. This prompted us to question whether the cartilaginous outer ear could also have evolved from some structure in ancestral fish.”
The team’s first breakthrough came when they found that both gills and outer ears are made of a rare type of tissue known as elastic cartilage. “Before we started our study, knowledge about the existence of elastic cartilage outside of mammals was scarce,” Crump explained. “It was uncertain if fish even had elastic cartilage. We discovered that they do indeed possess it.”
Even though gills and outer ears differ in appearance and function, they both share the characteristic of not mineralizing, which makes recovery in the fossil record rare. Consequently, a novel approach was required to investigate their evolutionary relationship. The study’s lead author, Mathi Thiruppathy, a PhD student in Crump’s lab, concentrated on gene control elements known as enhancers. While the genes influenced by these enhancers typically play roles in the development of various unrelated tissues and organs, enhancers are usually more specific to particular tissues.
The researchers managed to insert enhancers responsible for producing elastic cartilage in the human outer ear into zebrafish genomes. Interestingly, these human ear enhancers were activated specifically within the gills of the modified zebrafish. They also reversed the experiment by integrating zebrafish enhancers associated with gill formation into the genomes of mice, finding that these enhancers were active in the outer ears of the mice. This connection was key in linking seemingly unrelated structures.
Alongside their collaborators, the researchers then explored whether the enhancers from human outer ears and fish gills could trace the evolutionary path from gills to outer ears across intermediate species such as amphibians and reptiles. They discovered that when either the human ear or fish gill enhancers were introduced into tadpoles’ genomes, these enhancers were active in their gills. However, as reptiles appeared, the elastic cartilage of gills shifted to the ear canal, which the scientists demonstrated in a series of experiments with green anole lizards. This cartilage ultimately evolved into the prominent outer ears seen in early mammals.
Another unexpected finding was that elastic cartilage in gills might have emerged much earlier than previously believed. Earlier studies had identified cartilage-like tissues in the gills and tentacles of marine invertebrates, including horseshoe crabs, which have remained virtually unchanged for around 400 million years. The researchers conducted DNA sequencing on individual cells from horseshoe crab gills and identified a crab enhancer that, when introduced into zebrafish genomes, exhibited gill activity. This suggests that the first forms of elastic cartilage, akin to that in our outer ears, may have originated in ancient marine invertebrates.
“This study adds a new chapter to the understanding of the evolution of the mammalian ear,” remarked Crump. “While the middle ear evolved from fish jawbones, the outer ear originated from cartilaginous gills. By examining how the same gene control elements can drive the development of both gills and outer ears, the scientists unveil a novel approach to understanding how structures can evolve dramatically to take on new, unanticipated roles.”
About the study
Other contributors include Lauren Teubner, Ryan R. Roberts, Seth Ruffins, Arijita Sarkar, Jade Tassey, Denis Evseenko, and Thomas P. Lozito from USC; Micaela Lasser and Helen Rankin Willsey from the University of California, San Francisco; Alessandra Moscatello and Ya-Wen Chen from the Icahn School of Medicine at Mount Sinai; Christian Hochstim from Children’s Hospital Los Angeles and USC; and J. Andrew Gillis from the Marine Biological Laboratory at Woods Hole.
This research was supported by NIDCR (grant numbers R35DE027550 and F31DE030706). The lizard experiments were backed by NIH/NIGMS (grant number R01GM115444). Funding for human tissue experiments was provided through USC Stem Cell Challenge Grants. Helen Rankin Willsey is recognized as a Chan Zuckerberg Biohub Investigator.
