The sacrum articulates with last lumbar vertebra, and also with 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. Modern chimpanzees have a relatively smaller sacroiliac joint surface. These size differences are related to the different patterns of weight transmission through the pelvis during quadrupedal and bipedal locomotion. The australopith sacrum has relatively large, but less curved sacroiliac joint than that seen in modern humans9.


The Australopithecus sacrum is broad, similar to
modern humans, and has a realtively 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.