The diagram compares the skeletal structures of two ancestors of modern humans: Australopithecus afarensis and Homo erectus.
Overall, the two skeletons share a broadly similar structure but differ in proportions and specific adaptations. While Australopithecus afarensis was suited for both walking and tree climbing, Homo erectus developed physical features that made it more efficient at endurance running.
In terms of the upper body, Australopithecus afarensis had an unbalanced head with a protruding face, high narrow shoulders, and a wide chest. By contrast, Homo erectus evolved a balanced head with a shorter snout, low wide shoulders, and a narrower chest. Their arms also differed: the former had long forearms, whereas the latter developed shorter ones, reflecting reduced reliance on climbing.
The lower body also reveals significant contrasts. The waist of Australopithecus afarensis was short and wide, while Homo erectus had a tall and narrow waist. Similarly, the hip, knee, and ankle joints were small in the former but larger in the latter, enhancing stability during running. Foot structure highlights further differences: the earlier species had long toes, a small heel bone, and a partial foot arch, whereas the later species evolved short toes, an enlarged heel bone, and a fully stabilized arch. In addition, Homo erectus had a long Achilles tendon, absent in Australopithecus afarensis, providing greater efficiency in long-distance running.