On the role played by ontogenetic remodeling and functional transformation in the evolution of terrestrial hearing

Using examples from the octavolateral system, evidence is reviewed suggesting a relationship between regressive events, such as loss of one function, or loss of one sensory subsystem, and progressive evolutionary changes in topologically associated systems. While none of the neuronal examples in the evolutionary reorganization of the otic region are as clear-cut as the initial example of non-neuronal reorganization on which the correlation of regressive with progressive changes is based (the functional transformation of the hyomandibular bone into the stapes), the general principle that a chance correlation of two insignificant events may lead to a novel function may be valid for more aspects of the evolution of the ear, in particular the auditory system, than is currently appreciated. It is suggested that regressive events may not only be an evolutionary dead end but that they may provide, through the relaxation of constraints imposed on the respective structure, a source for innovations. However, transformations of functionally uncoupled structures into a novel adaptive function will occur only when topologically adjacent structures require these transformations to improve their own function.