Locomotion, Obstetrics, and the Evolution of the Homo Pelvis

The human pelvis plays important functional roles in locomotion and in the birthing of babies. The pelvis acts as a rigid lever during two-legged (bipedal) locomotion, which allows us to stabilize our hip and trunk when body weight is supported on a single leg, and it also operates to effectively transmit body weight from the trunk to the support leg. The pelvis is also important to human birthing (parturition) in that it presents a rigid, bony ring through which the head and body of our large-brained babies must pass. The evolution of this structure over the last six to seven million years (since the time of our last common ancestor with chimpanzees) has been strongly shaped by natural selection operating on both of these roles —that is, selection has favored pelvic morphology that allows for both more efficient forms of bipedal locomotion and for the birthing of large-brained babies. While most paleoanthropologists would agree that these two factors have played a key role in the evolution of the human pelvis, disagreement exists as to which features in the pelvis can be explained by each factor.