Definitive mammalian middle ear

The evolution of mammalian auditory ossicles is one of the most well-documented and importantevolutionary events, demonstrating both numerous transitional forms as well as an excellent example of 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 lower and one upper jaw bone (the articular and quadrate) 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 (collectively called 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.

Following on the ideas of Étienne Geoffroy Saint-Hilaire (1818), and studies by Johann Friedrich Meckel the Younger (1820), Carl Gustav Carus (1818), Martin Rathke (1825), and Karl Ernst von Baer (1828), the relationship between the reptilian jaw bones and mammalian middle-ear bones was first established on the basis of embryology and comparative anatomy by Karl Bogislaus Reichert (in 1837, before the publication of On the Origin of Species in 1859) and advanced by Ernst Gaupp and this is known as the Reichert–Gaupp theory.

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