06 – Habitual bipedalism—Laetoli and Lucy
3.6 million years ago
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.
Environmental and Climate Changes
Continued aridity, enhanced seasonality—Loss of forests and savanna expansion 4.4 Ma
Australopithecus afarensis 3.9 to 2.9 Ma