The given diagram compares two different skeletal structures of modern human ancestors.
From an overall perspective, it is apparent that Australopithecus afarensis, a walker and tree climber, had bigger and wider bones than Homo erectus, a walker and endurance runner. Additionally, their skeletons were adjusted to the type of environment they lived in.
With regard to the upper parts of their skeletal systems, the two had contrasting skeleton sizes: Homo erectus had a balanced head, low and wide shoulders, a narrow chest, a tall and narrow waist, short forearms, and a large gluteus maximus, whereas Australopithecus afarensis exhibited the reverse characteristics.
As for the lower parts of their skeletal structures, the two ancestors of modern human beings had substantial differences in their skeleton forms. While Australopithecus afarensis possessed a long femoral neck, small hips, knees, and ankle joints, a short Achilles tendon, and a small heel bone, Homo erectus exhibited the opposite traits. Moreover, since Homo erectus was an endurance runner, it had a stabilized foot arch, whereas Australopithecus afarensis, adapted to climbing trees, had only a partial one.