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:
  1. Chimpanzees are unable to extend their knee-joints to produce a straight leg in the stance phase.
  2. Muscular power has to be exerted to support the body.
  3. The constantly flexed position of the chimpanzee leg also mean there is no toe off and heel strike in the swing phase.
During each swing phase the center gravity of the body has to be shifted towards the supporting leg and the tendency for the body to collapse toward the unsupported side is countered by the contraction of the muscles (gluteal abductors) on the side o the hip that is in the stance phase.

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.
  • An extended knee joint.
  • A platform foot.
We must view the human upright posture as an expression of an ancient primate evolutionary trend. The dominant motif of that trend is an erect body. That trend went through vertical clinging and leaping. Through quadrupedalism (monkeys and apes) to brachiation (apes). The transformation from ape to hominid was not between a true quadruped (horse, dog) and a true biped, a point to calculate evolutionary constraints that might have operated in the origin of hominids.

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.

  • Women "gatherer" hypothesis
  • Challenged the male the hunter model.
  • Plant not meat was the major food items.
  • Plant, not meat were the focus for technological innovation and new social behaviors.
  • More conservative.
  • Focus on the need to carry things: specifically food for sharing within infants.
  • 2nd hypothesis that focuses on the need to carry things is "Man the provisioner".
  • Male gathers food and return to homebase to share food with female and offspring.
The system would work only if the males could be certain the infants were his. The need for pair bonding and sexualfidelity. One criticism: focused on calculations that purported to show large hominids were a reproductive disadvantage compared with humans. The most parsimonious scientific attractive explanation of bipedalism is they have evolved not as a change in the nature of diet or social structure, but as a result of a change in the distribution of existing dietary resources. In the late Miocene, hominid dietary resources become thinly dispersed in some areas, the continued exploitation of which demanded a more efficient mode of travel. Hence the evolution of bipedalism.