Chimpanzees and gibbons walk on 2 feet when circumstances demand it. Anthropologists sought special, essentially human explanations for the origin of bipedalism. Earliest hominid fossils from Ethiopia and Tanzania are dated a million years earlier and show significant adaptation to bipedalism in combination with a hominid dental pattern that has distinct apelike overtones. These creatures Australopithecus afarensis lived 3 million years after the origin of the hominid lineage about 6.5 million years ago. Might have been apelike in al respects, apart from an adaptation to upright walking. Bipedalism would be the primary hominid adaptation.
The striding gait of human bipedalism involves the fluid flow of a series of actions collectively, the swing phase and the stand phase. One leg alternating with the other. The leg I the swing phase pushes off with the power of the toe, swing under the body in a slightly flexed position. Finally become extended as the foot make contact with the ground first with the heel. Once heel strike occurred, the leg remained extended providing support for the body. The stance phase, while the other leg goes through the swing phase with the body moving forward.
Two key features differentiate human and chimpanzee bipedalism:
In chimpanzees the thighbone does not slope inward to the knees as much as humans do which means that the feet are normally placed well apart. The gluteal abductors are not highly developed. During bipedal walking the animal is forced to shift its upper body substantially from side to side with each step so to bring the center of gravity over the weight bearing leg.
Chimpanzee anatomy is a compromise between an adaptation to tree climbing and terrestrially (knuckle walking). Human anatomy is fully terrestrial adaptation. The suite of anatomical adaptations that underlie human bipedalism is extensive and includes a curved lower spine, a shorter, broader pelvis, and an angled femur, which are served by reorganized musculature lengthened lower limbs and enlarged joint surface areas.
Darwin essentially equated hominid origins with human origins and proposed an evolutionary package including upright walking, material culture, modified dentition and expands intelligence. Hominid diets remained predominantly vegetarian until 1.5 million years ago, the origin of Homo erectus.