Origin of Auditory Cortex

Abstract

All mammals, including the egg-laying mono-tremes, possess an auditory cortex---that is, an auditory area in their cerebral neocortex. In contrast, no reptile or bird possesses even neocortex itself let alone an auditory area within it. Therefore, the question of the evolutionary origin of auditory cortex is entangled in the larger question of the origin of mammalian neocortex. Because there are no animals now extant that serve as an unarguable witness to the transition from the reptilian to the mammalian form of cerebral cortex the question of the origin of mammalian cortex is an old and persistent one. We now have but two snapshots of the forebrain through geological time---a reptilian stage without neocortex and a later mammalian stage with a relatively large amount of neocortex and an auditory area already within it.