Taking a step feels like a simple, automatic process, but in fact, our steps involve a series of mechanical shifts. The most important of these phases are the first and the last, the heel strike and the toe-off. When chimpanzees or other occasional bipeds walk upright, they do not move their weight across the foot as we do. A preserved set of footprints called the Laetoli footprints shows that hominins (most likely Australopithecus afarensis) walked using both heel strike and toe-off phases, indicating a mechanical adaptation towards permanent bipedalism. We see evidence of obligate bipedalism in the skeleton of Lucy as well: Her pelvis is broad, and the attachments for her gluteal muscles indicate that she was capable of hip abduction, a stabilizing condition that makes bipedal striding smoother and more efficient. Her knee joint also showed the increased articular surface for load bearing.

Image courtesy ASU Institute of Human Origins.