The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

Bipedalism, obstetrics, and the evolution of constraints


Department of Anthropology, University of Illinois

Thursday 9:30-9:45, Grand Ballroom II Add to calendar

Differences in the pelvis between hominins and other apes are hypothesized to be primarily the result of selection for habitual upright locomotion. Hominins also possess an obstetric morphology that diverges from other apes and causes parturition difficulties in our own species. Because bipedalism and birth converge at the pelvis, later hominin and modern human obstetric morphology is hypothesized to be the result of a compromise between selective pressures for increased pelvic apertures, allowing for the birth of larger brained and bodied neonates, and pressures related to bipedalism. While biomechanical methods have been used previously, patterns of integration - covariances between traits - provide a new approach to this problem as these patterns reflect functional and developmental relationships and can reveal how one trait or set of traits constrains the evolution of another. The results here show that integration between obstetric and other hip traits in humans can constrain the evolution of the obstetric canal, but this relationship is likely ancestral for hominoids. Where humans differ from other apes is in possessing a significantly lower magnitude of integration within the birth canal, which allows the direction of the evolutionary response and the direction of selection to align to a significantly greater degree than possible in other apes. These findings suggest that natural selection for increased obstetric dimensions in hominins compensated for evolutionary constraints due to locomotor requirements by reducing constraints within the birth canal, allowing for morphological evolution along a trajectory that might have previously been difficult or impossible to traverse.

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