Final answer:
The shape of the pelvis and the length of the legs are two features that differ between the skeletons of Australopithecus afarensis and Homo ergaster, indicating that Homo ergaster was a more efficient biped.
Step-by-step explanation:
Two features that differ between the skeletons of Australopithecus afarensis (Lucy) and Homo ergaster (The Nariokotome boy) that indicate that Homo ergaster was a more efficient biped are the shape of their pelvis and the length of their legs.
Australopithecus afarensis had a pelvis that was shorter and wider, similar to that of modern humans. This pelvic shape is associated with efficient bipedal locomotion. On the other hand, Homo ergaster had a relatively narrow and long pelvis, which is less conducive to efficient bipedal walking.
In addition, Homo ergaster had longer legs compared to Australopithecus afarensis. Longer legs provide a longer stride length and increased efficiency in bipedal walking. Australopithecus afarensis, on the other hand, had relatively shorter legs.