Mechanisms for the acquisition of habitual bipedality: are there biomechanical reasons for the acquisition of upright bipedal posture?

Morphology and biomechanics are linked by causal morphogenesis ('Wolff's law') and the interplay of mutations and selection (Darwin's 'survival of the fittest'). Thus shape-based selective pressures can be determined. In both cases we need to know which biomechanical factors lead to skeletal adaptation, and which ones exert selective pressures on body shape. Each bone must be able to sustain the greatest regularly occurring loads. Smaller loads are unlikely to lead to adaptation of morphology. The highest loads occur primarily in posture and locomotion, simply because of the effect of body weight (or its multiple). In the skull, however, it is biting and chewing that result in the greatest loads. Body shape adapted for an arboreal lifestyle also smooths the way towards bipedality. Hindlimb dominance, length of the limbs in relation to the axial skeleton, grasping hands and feet, mass distribution (especially of the limb segments), thoracic shape, rib curvatures, and the position of the centre of gravity are the adaptations to arboreality that also pre-adapt for bipedality. Five divergent locomotor/morphological types have evolved from this base: arm-swinging in gibbons, forelimb-dominated slow climbing in orangutans, quadrupedalism/climbing in the African apes, an unknown mix of climbing and bipedal walking in australopithecines, and the remarkably endurant bipedal walking of humans. All other apes are also facultative bipeds, but it is the biomechanical characteristics of bipedalism in orangutans, the most arboreal great ape, which is closest to that in humans. If not evolutionary accident, what selective factor can explain why two forms adopted bipedality? Most authors tend to connect bipedal locomotion with some aspect of progressively increasing distance between trees because of climatic changes. More precise factors, in accordance with biomechanical requirements, include stone-throwing, thermoregulation or wading in shallow water. Once bipedality has been acquired, development of typical human morphology can readily be explained as adaptations for energy saving over long distances. A paper in this volume shows that load-carrying ability was enhanced from australopithecines to Homo ergaster (early African H. erectus), supporting an earlier proposition that load-carrying was an essential factor in human evolution.