A study reveals the evolutionary connection between mammalian outer ears and the gills of ancient fish.
A recent study published in Nature by researchers from the Keck School of Medicine at the University of Southern California has linked the evolutionary origins of the mammalian outer ear to the gills of ancient fish.
The study investigates the relationship between elastic cartilage found in mammalian ears and a similar rare tissue characteristic of fish gills. To establish this connection, scientists employed gene enhancers – specific DNA sequences which are responsible for the regulation of tissue development.
In an experimental set-up, when human ear enhancers were introduced into zebrafish, these enhancers activated distinctly in the gills, indicating a direct relationship between the two structures. The enhancers from zebrafish gills expressed themselves in the outer ears of genetically modified mice, suggesting that these developmental pathways have evolved from a common ancestral origin.
The research traced the evolutionary presence of elastic cartilage across different species, noting its transition from gills to the ear canal in reptiles and then onto an evolutionary journey that resulted in the distinct outer ears observed in mammals today.
The emergence of this cartilage is believed to have occurred over 400 million years ago, originating with marine invertebrates such as horseshoe crabs.
Cartilage’s properties make it exceptionally well suited to forming the outer human ear. Its flexibility allows the ear to bend and deform without breaking, which is crucial for withstanding everyday bumps and pressures. This resilience ensures that the ear maintains its shape and functionality, even after being subjected to minor impacts.
Furthermore, cartilage’s smooth, avascular nature (lacking blood vessels) contributes to the ear’s ability to heal quickly and resist infections. Because it doesn’t rely on a direct blood supply for nutrient delivery, it can withstand minor injuries without the risk of significant bleeding or prolonged inflammation.
Beyond its resilience, cartilage’s formability allows for the intricate shaping necessary for effective sound collection. The complex curves and folds of the outer ear, known as the pinna, are essential for directing sound waves into the ear canal. Cartilage provides the structural support needed to maintain these precise shapes, ensuring that the ear can efficiently capture and localise sounds from various directions.
Gage Crump, the study’s lead author and a professor of stem cell biology and regenerative medicine at the Keck School, said: “When we started the project, the evolutionary origin of the outer ear was a complete black box.”
This evolutionary perspective on ear anatomy not only deepens the understanding of mammalian biology but also sets the stage for further exploration into the functions and health of the auditory system in humans today.
