In the broadest sense, hearing is the detection, classification (“what”), and localization (“where”) of mechanical disturbances of the surrounding medium. Such disturbances include sound—properly defined as longitudinal pressure waves propagating in an elastic medium—and other mechanical waves. Other sensory systems detect mechanical [Page 193]disturbances such as touch, vibration, and pressure sensors, and the origin of hearing is entwined in their evolution, but dedicated sound-receiving structures have only evolved in arthropods and vertebrates. The ancestral sensory cells in all hearing organs can be traced back to mechanoreceptors in organisms at least in the Cambrian (about 500 million years ago) period. This entry describes the evolution of the auditory system.

In insects, hearing organs developed independently at least 19 times from mechanoreceptors in the cuticle coupled to tympanic ...