Lateralization in Vertebrates: Its Early Evolution, General Pattern, and Development

Over the last two or three decades, a large number of examples of left-right differences in brain function and responding have been reported for a range of vertebrate species. This has provided convincing evidence against the earlier held notion of lateralization as a unique characteristic of the human species associated with tool use, language and consciousness. It is now possible to see that there is a basic pattern of lateralization common to all vertebrates, including humans. This chapter is concerned with asymmetry of the behavioral responses, in some cases dependent on structural asymmetry of either the central nervous system or of physical features but, mostly, not associated with any known structural asymmetry. It may be manifested as left- or right-side differences in responding, or in differential responsiveness to various forms of sensory stimulation according to whether those stimuli are perceived by receptors on the left or right side of the animal's midline. The evidence reported here will show that there is a common basic pattern of lateralization among vertebrates, indicating that the lateralization of birds and mammals is homologous, inherited from their common ancestor. The possible advantages and disadvantages of being lateralized will be considered.