Final answer:
Australopithecus afarensis displayed several features indicative of bipedalism, such as a forward-placed foramen magnum, a wider and shorter pelvis, and femora with a valgus angle. However, A. afarensis had longer arms relative to body size compared to modern humans, suggesting a combination of arboreal and bipedal locomotion.
Step-by-step explanation:
The features found in Australopithecus afarensis that suggest bipedalism include several anatomical changes. The foramen magnum in A. afarensis is positioned more anteriorly, beneath the skull, which is indicative of an upright, bipedal stance. The pelvis is shorter and wider, resembling that of modern humans, which supports the body's weight above the legs. Furthermore, the femur shows a valgus angle, positioning the knees and feet under the center of gravity. These adaptations are critical for bipedal locomotion.
Comparing this to modern humans, there is a notable difference in the arms to body length ratio. A. afarensis had substantially longer arms relative to their body size, similar to that of great apes, which is typical of species that engage in tree-climbing behaviors. Modern humans, on the other hand, have shorter arms in proportion to their body size, reflecting our fully upright, bipedal mode of life.