Anatomical Adaptations for Bipedalism: Sacrum

The Australopithecus sacrum is broad, similar to modern human, and has a relatively larger sacroiliac joint surface than a modern chimpanzee.

The Australopithecus sacrum is broad, similar to modern human, and has a relatively larger sacroiliac joint surface than a modern chimpanzee.

The australopithecine sacrum is more curved than an extant chimpanzee, but not as curved as a modern human.

The sacrum articulates with both the last lumbar vertebra and the pelvis at the sacroiliac joint. The shape of the sacroiliac joint is a reflection of the lumbar curve. The sacrum is relatively broad in modern humans with large sacroiliac joint surfaces. Extant (i.e., a living species) chimpanzees have a comparatively smaller joint surface. These size differences are related to the different pattern of weight transmission through the pelvis seen in quadrupedal and bipedal locomotion. The australopithecine sacrum has relatively large, but less curved sacroiliac joint than that seen in modern humans.⁹