Hearing ability evolved independently in mammals, birds

The eardrum and hearing evolved independently in mammals and diapsids - a taxonomic group that includes reptiles and birds, new research has found.

The evolution of the eardrum and the middle ear is what has allowed mammals, reptiles, and birds to hear through the air, the study noted.

The findings by researchers at the RIKEN Evolutionary Morphology Laboratory and the University of Tokyo in Japan showed that the mammalian eardrum depends on lower jaw formation, while that of diapsids develops from the upper jaw.

Significantly, the researchers used techniques borrowed from developmental biology to answer a question that has intrigued paleontologists for years.

"This approach to studying middle ear evolution could help us understand other related evolutionary changes in mammals, including the ability to detect higher toned sounds and even our greater metabolic efficiency," said lead author Masaki Takechi from the RIKEN Evolutionary Morphology Laboratory.

Although scientists have suspected that the eardrum -- and thus hearing -- developed independently in mammals and diapsids, no hard evidence has been found in the fossil record because the eardrum is never fossilised.

To overcome this difficulty, the research team and their collaborators turned to evolutionary developmental biology -- or "evo-devo."

They noted that in mammals, the eardrum attaches to the tympanic ring--a bone derived from the lower jaw, but that in diapsids it attaches to the quadrate--an upper jawbone.

The scientists found that primary jaw junction shifted upwards in mammals and that the middle ear developed after this shift and must therefore have occurred independently after mammal and diapsid lineages diverged from their common ancestor.

The study was published in the journal Nature Communications.