Hominin Obstetrics and the Evolution of Constraints

Abstract

Birth is significantly more complicated and dangerous in modern humans than in other great apes. This disparity is often hypothesized to be the result of evolutionary constraints on obstetric dimensions related to bipedalism and/or thermoregulation in later hominins. Previous attempts to test such hypotheses have used biomechanical methods and results have been mixed. But evolutionary constraints, restrictions or limitations on the course or outcome of evolution, are the result of an interaction between selective pressures and genetic constraints—the latter revealed in patterns of integration. Integration between traits can result in directional or stabilizing selection on one trait leading to correlated responses in other traits, which can bias and constrain evolutionary trajectories. Therefore, trait evolution may be constrained for reasons separate from those that can be estimated using biomechanical models, and to study evolutionary constraints it is necessary to understand the role genetic constraints play in morphological change. The results presented here show that genetic constraints can significantly reduce the evolutionary potential of the birth canal to evolve in humans, apes, and likely earlier hominins, but also point to an overall reduction in the level of constraints during hominin evolution. These findings suggest that divergent selection pressures for obstetric requirements and other pelvic functions in hominins reduced levels of genetic constraint on birth canal evolution, likely lowering the amount of time needed for evolutionary change, and permitting morphological evolution along a trajectory that might have previously been difficult or impossible to traverse.