Hominid brain evolution: The approach from paleoneurology

The recent literature on hominid paleoneurology is reviewed and critically assessed. Two theoretical approaches for interpreting hominid paleoneurological data are discussed: (1) identifying cerebral “rubicons” that theoretically distinguish hominids from other primates and (2) analyzing brain evolution in terms of “residual” encephalization factors that remain after body size factors have been accounted for. Although these two approaches are scientific, much of the literature on human brain evolution is speculative—i.e., based on efforts to identify “prime movers,” such as hunting or warfare, that theoretically were responsible for the marked increase in both absolute and relative brain size that occurred during hominid evolution. The search for rubicons has been unfruitful. Features that distinguish hominid from pongid brains seem to be the result of allometry (i.e., determined by brain size) rather than the result of selection for qualitatively different neurological features. However, this finding may simply reflect the crude nature of the paleoneurological evidence. Thus, the field of comparative neurology is more likely than paleoneurology to contribute to our knowledge of the details of human brain evolution.