Question

What evidence supports the model that the gill arches in fish are homologous to the moddle ear bones in mammals?

Answer 1

The evolution of mammalian auditory ossicles was an evolutionary event in which bones in the jaw of reptiles were co-opted to form part of the hearing apparatus in mammals. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution.

In reptiles, the eardrum is connected to the inner ear via a single bone, the columella, while the upper and lower jaws contain several bones not found in mammals. Over the course of the evolution of mammals, one bone from the lower and one from the upper jaw (the articular and quadrate bones) lost their purpose in the jaw joint and were put to new use in the middle ear, connecting to the existing stapes bone and forming a chain of three bones, the ossicles, which transmit sounds more efficiently and allow more acute hearing. In mammals, these three bones are known as the malleus, incus, and stapes (hammer, anvil, and stirrup respectively). Mammals and birds also differ from other vertebrates by having evolved a cochlea.

The evidence that the malleus and incus are homologous to the reptilian articular and quadrate was originally embryological, and since this discovery an abundance of transitional fossils has both supported the conclusion and given a detailed history of the transition.The evolution of the stapes (from the hyomandibula) was an earlier and distinct event.