The Human Brain: Conservation of the Subcortical Auditory System

Abstract

The results show that the human has the largest subcortical auditory system in terms of absolute size. However, humans have one of the smallest auditory systems relative to their brain size. The results suggest that the human condition is one result of an expansion of non-auditory brain parts rather than a reduction of the auditory system over geological time. We have found that the subcortical human auditory system is not remarkably different from that of other mammals. In part, this demonstrates that conservatism is a major theme of evolutionary adaptation. To reach this conclusion the volumes of ten subcortical auditory nuclei were measured individually in a human brain and in the brains of 57 other mammalian species. The nuclear sizes relative to the total of subcortical auditory tissue were normalized and analyzed for statistically reliable similarities and/or deviations in the auditory brainstem, midbrain, and thalamus. The overall form of the auditory system of human was compared to that of a selection of species chosen to be reflective of the relative proportion of those species alive today. In sum, in its overall form the subcortical auditory system is highly similar among mammals, including humans. The data suggests that the overall form of the system has been relatively stable over geological time - probably more than 135 million years. Key Words: Hearing, humans, evolution, comparative psychology, mammals Humans and other living members of the class Mammalia all share a common ancestor within the last 250 million years. Among mammals, some neural structures of the brain differ in size by a factor of more than 200 even after adjusting for differences in body size (i.e., Armstrong, 1985; Stephan and Andy, 1969; Stephan et al., 1988, 1991). By contrast, a coordinated enlargement of many independent components of a functional system, such as the auditory system, without enlargement of the rest of the brain is less likely. In other words, if one part of the brain gets larger, then usually the entire brain gets larger. That is because the probability of enlargement of a subsystem would be the vanishingly small product of the probability of each component enlarging individually. Because of this, there is a natural conservation of the brain during evolution. That is, the brain is resistant to change. The question arises as to what the significance of a conserved order might be for human evolution and for mammalian evolution generally. If a species undergoes strong selective pressure for the optimization of hearing and this depends on the size of the central auditory system, what changes take place in the organization of the brain as a whole? Because brain tissue is metabolically expensive, the need for energetic efficiency should result in the most localized possible increase in brain volume, corresponding to the behavioral adaptation. Both specific adaptations and developmental constraints play a role in determining the paths of evolution (i.e., Gould, 1977). From these, an emerging theme of evolutionary adaptation has been that of conservatism. The results of this work reflect this conservatism at brain levels below the neocortex. We have attempted to show that the study of brain specializations both of humans and other mammals and the accompanying alteration or stability of brain structures can yield a formal and quantitative understanding of the range of local adaptations permitted in our evolution and in the evolution of mammals. We present several measures of the degree of conservation of fundamental patterns in the human and in the mammalian auditory system in general. In contrast to the conservatism of the brain is the environmental pressure for enlargement or shrinkage of certain structures. If humans were strongly selected for their ability to make accurate computations for auditory localization, this would have added greatly to their ability to survive. In turn, the size of nuclei for localization would be relatively large. …