Primate Brain Evolution

pp 85-95

Encephalization and Obstetrics in Primates with Particular Reference to Human Evolution

  • Walter LeuteneggerAffiliated withDepartment of Anthropology, University of Wisconsin

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The existence of differences in pelvic morphology between Plio-Pleistocene hominids and modern humans is well documented (for a recent review see McHenry and Temerin, 1979). Pelvic remodeling during the evolution of the genus Homo has elicited two different interpretations: (1) it has been viewed as a reflection of increased efficiency in bipedalism (McHenry, 1975a; Zihlman, 1978), and (2) it has been interpreted in terms of rapid encephalization that would have generated selection pressure to increase birth canal dimensions (Lovejoy et al., 1973; Lovejoy, 1974, 1975, 1978). When the second hypothesis is scrutinized, the fossil evidence unambiguously indicates that the increase in brain size during human evolution was extraordinary both in magnitude and in rate (Pilbeam and Gould, 1974; Passingham, 1975; Sacher, 1975). This picture of encephalization from H. habilis to H. erectus to H. sapiens is based on estimated cranial capacities of a static series of adults. Obstetrical constraints and thus selection pressures on the pelvis are not, however, dependent on adult cranial dimensions but on those of the fetal cranium at term relative to birth canal dimensions. Moreover, evidence of differences between primate species in the growth rate of the brain and/or in the length of development during ontogeny (Schultz, 1941, 1956; Sacher and Staffeldt, 1974; Passingham, 1975) suggests that encephalization of adults cannot be expected to be proportionate to that of the neonate or to that of individuals at any particular developmental stage.