Biomechanics, evolution and upright stature.

The transition from a basically quadrupedal to an upright stance must have been a critical stage in the early hominids before the appearance of Australopithecus and after a Ramaor Dryopithecine time. Two hypotheses have been postulated as to how the change occurred: 1. a gradualistic evolution from the horizontal to a more and more vertical body posture; and 2. an "either--or" position, in which our early ancestor assumed either a horizontal or a vertical posture. It is calculated that, in a static equilibrium, a semi-erect posture would be disadvantageous from the point of view of muscle forces as well as from energetic constraints. These stresses make it probable that an upright posture and carrying of objects in the hands were jointly favored by natural selection and that an intermediate stage would be short and inconclusive. The postural change would thus have occurred in a "punctuated equilibrium" manner of evolution.