Vertebrate hearing: Origin, evolution and functions

Abstract

Using the studies of fossils and modern animals, the evolution of the sensory systems involved in hearing has been elucidated in great detail. The results trace the evolution of hearing back to our fish ancestors, through the establishment of a dedicated auditory epithelium and of independently- evolved, impedance-matching middle ears in amniotes. The resulting improvement in auditory input resulted in the expansion and specialization of auditory sensory epithelia. On the one hand these changes permitted new means of communication and other interactions with the physical environment. On the other hand the interpretation of complex sensory inputs from enlarged hearing organs necessitated the expansion and specialization of brain pathways devoted to hearing. These pathways were subdivided according to the many different aspects of stimulus input (e. g. frequency, level, timing). Humans are relatively untypical mammals, in that their hearing range is restricted to relatively low frequencies but human development and upbringing enables the brain to cope with the complex processing required for speech and music appreciation. Far from being the “culmination” of some anthropomorphic evolutionary development, many other species show equal or even more spectacular specialization according to the importance of hearing in their daily lives. In modern society, the sensitivity of auditory sensory cells to damage is proving to be an inconvenient legacy of the early evolutionary trend to maximal sensitivity of sensory detection systems.