A new study may offer answers: the first comprehensive look at how the mammalian inner ear has developed over evolution reveals a startling pattern of parallel evolution in eight major groups of mammals, according to researchers who have also made significant revisions to the list of known inner-ear genes.
Unlocking the Secrets of the Mammal Ear
The inner ear of the vertebrate is available in a bony shell (the petrous pyramid) set inside the skull and has more to do with hearing and balance. Until recently, scientists generally thought that the enormous array of inner-ear shapes across the animal kingdom reflected adaptation to diverse environments and modes of locomotion.
But a new study, led by researchers from University of Vienna and the Konrad Lorenz Institute for Evolution and Cognition Research, is challenging that idea. The paper, published in the high-ranking journal Nature Communications on August 31st, unveils an unexpected turn on the odyssey of the evolution of inner ear development seen in mammals.
Parallel Evolutionary Convergence
After narrowing their focus to one mammalian group, the Afrotheria (a clade that unites aardvarks and elephants with golden moles, hyraxes, elephant shrews, and sea cows), the researchers found similar results. Despite their starkly different anatomy and habitats, the shape of the inner ears of these mammals were found to be strikingly similar to other, more distantly related mammal groupings that fulfil similar ecological and behavioural niches.
For example, sea cow ears are more similar to dolphins (which are more distant relatives) than their close Afrothere relatives elephants or hyraxes in the shape of the ear. This similarity of the shape of its ears is due to having been shaped by selection pressures associated with an entirely aquatic lifestyle.
Similar processes were seen for different classes of mammals, including burrowing rodents and arboreal species that display convergence even between distantly related species occupying the same ecological niche or performing a specific type of movement.
Conclusion
Our findings establish a new paradigm for convergence during mammalian inner ear evolution. This implies that the pressure of selection can significantly mold the almost baroque structures of the ear, perhaps outweighing the effect of evolutionary history.
The results also reveal the striking ‘evolvability’ of the mammalian ear, which has evolved new components by repurposing former jaw bones over >300 million years. Our findings have unveiled this eye-opening “incredibly pompous solution” of the complicated salamander ear structure, which may be why such an elaborate anatomical, genetic and developmental complexity allowed the ear so much flexibility in adaptation to different environments and locomotion behaviors in mammalian evolution, according to Prof. San Mauro
