The pelvis as a passageway. I. Evolution and adaptations.

Man has an unusual pelvis, a large fetal head, and a complicated mechanism of labour. The evolution of the pelvic girdle, like that of the hind limb, is a story with some chapters still missing. In mammals the pelvis has been modified in various interesting ways to effect compromises between its locomotor function and the requirements of parturition. Among the primates, the increasing size of the fetal brain may have necessitated a change in the attitude of the head from extension to flexion as it goes through the pelvis. The distinctive shape of the hominid pelvis is probably an adaptation to bipedal gait. The birth canal has changed from a shallow bony ring to a deep curved tube, through which the mature fetal head can only pass by rotating as it descends.