Hemispheric asymmetries, paleoneurology, and the evolution of the human genus

Brain asymmetries are a distinctive feature of Homo sapiens and are associated with key evolutionary functions including language and handedness. Nonetheless, differences between humans and apes could be just a matter of degree and size and not the expression of unique traits of our species. In this chapter, I introduce paleoneurology and the study of brain morphology in fossil hominids, reviewing the anatomic factors that can influence the main asymmetries of the endocranial cavity (cortical volumes, sulcal patterns, and craniovascular features). The paleoneurological evidence suggests that most extinct human species displayed a pattern of gross endocranial asymmetries similar to modern humans. In addition, the behavioral information on handedness also points to a similar degree of laterality in archaic species of the human genus and in Neandertals. At present, there is therefore no evidence suggesting that the brain asymmetries in H. sapiens are part of a derived set of features. Of course, even a simple proportional change due to brain size increase can anyway prompt crucial cognitive changes, mostly if threshold effects are considered. Nonetheless, we still lack much information in basic anatomy to support consistent hypotheses on the biologic factors involved in endocranial asymmetries in fossil hominids. This missing information concerns endocranial morphogenesis and topology, spatial conflicts and constraints, the biomechanical balance between cerebral tissues, and the actual histologic changes associated with encephalization.