The diagram illustrates the skeletal systems of two ancestors, namely the Australopithecus afarensis and the Homo erectus.
Overall, there were various differences in the construction of the bones in adaption to different lifestyles, with the Homo erectus being broadly leaner in size. In fact, while the Australopithecus afarensis developed more extensive upper body parts, the homo erectus tended to have body parts sitting below stomachs more refined and stabilized.
The Australopithecus afarensis appeared to have a bigger head, albeit being more unbalanced. They also had higher and narrower shoulders, coupled with a wider chest that spread evenly sidewise, compared to that of the Homo erectus. The size of the forearm was also divergent, with the Australopithecus afarensis being significantly longer, benefiting their lifestyle of climbing trees. In addition, despite not being demonstrated, the Homo erectus had a bigger gluteus maximus.
Regarding lower parts, the Homo erectus had a shorter femoral neck than the Australopithecus afarensis. It is also evident that the former had leaner legs despite developing larger hip, knee, ankle joints and an achilles tendon in comparison with the latter. Considering the foot, the Homo erectus had significantly shorter toes yet a more stabilized foot arch and enlarged heel bone.