The size of the sacroiliac and lumbosacral joint surfaces are related to the amount of weight transmitted through the pelvis during locomotion. As humans walk, the pelvis must cope with the stress of weight transfer, while quadrupedal chimpanzees are able to alleviate some of this stress through the support of their upper limbs. Thus, the surface size of the sacroiliac and the lumbosacral joints in humans are much larger than that seen in chimpanzees. Lucy has comparatively larger sacroiliac and lumbosacral joint surfaces compared to chimpanzees, as expected of bipeds.

Additionally, the angle of curvature of the sacrum reflects that of the lumbar curvature, and is more pronounced in humans while the chimpanzee sacrum is straight. Lucy’s sacrum shows some curvature, but is not as pronounced as in modern humans.