Development of hearing. Part I: Phylogeny.

We humans hear the way we do because of at least three major forces. The first is phylogeny, the evolutionary changes in the auditory system since its beginnings. Another is embryology, the development of the system in each individual before birth. Finally, there is the biologically determined auditory mechanism we are born with and our interaction with the environment in early postnatal life. This series of three articles reviews each aspect in turn, so we may have a fuller appreciation of how it is we come to hear the way we do. Part I reviews the evolution of human hearing through the vertebrates. Ancestral bony fish had an equilibrial organ, which was passed to subsequent classes. Both amphibians and reptiles evolved their own auditory systems based on a balance organ inherited from fish. Amphibians independently evolved an impedance-matching apparatus through a combination of eardrum and ossicle. Reptiles evolved a flexible basilar membrane for the receptor cells. Mammals added movable, external ears and expanded the frequency response range. Finally, large-brained Homo acquired the use of language. Part II examines embryologic development and Part III considers postnatal aspects of auditory development.