Before the discovery of Australopithecus afarensis in the 1970s, most anthropologists believed that an increase in brain capacity had preceded bipedal locomotion. However, this reconstruction of the Lucy skeleton shows that the A. afarensis was bipedal even with a brain size of around 400 cc. But the actual Lucy skeleton does not include feet, and only has two partial leg bones. How do anthropologists determine, based on the bones preserved, that A. afarensis was bipedal?