The diagram compares the skeletal systems of two early human ancestors, namely Australopithecus afarensis and Homo erectus.
Overall, Homo erectus possessed a more balanced skeletal structure, which appears to have been better adapted for upright walking and endurance running. By contrast, Australopithecus afarensis retained several more primitive features that were better suited to both bipedal movement and tree climbing.
Regarding the upper body, Australopithecus afarensis had an unbalanced head with a long snout, high and narrow shoulders, and a relatively wide chest. It also featured a short, broad waist and long forearms. In comparison, Homo erectus exhibited a balanced head with a shorter snout, lower and wider shoulders, and a narrower chest. In addition, its waist was taller and slimmer, while its forearms were noticeably shorter.
As for the lower body, Australopithecus afarensis had a small gluteus maximus, a long femoral neck, and relatively small hip, knee and ankle joints. Furthermore, it possessed a short Achilles tendon, a small heel bone, long toes, and only a partially developed foot arch. In contrast, Homo erectus had a much larger gluteus maximus, a shorter femoral neck, and enlarged hip, knee and ankle joints. It also featured a longer Achilles tendon, an enlarged heel bone, shorter toes, and a fully stabilised foot arch.