Comment: Two and a half years of blogging about Asperger’s (the myth, the so-called disorder and developmental disability – and the personality type), has led to an ever-expanding multitude of topics that deal with the question of “what differentiates” Homo sapiens as a species, and “just who” qualifies to represent “our species” (the social version). After long consideration and study, I’ve come to a tentative conclusion: Bipedalism is the key to “who we are”. The “rest of our story” follows this anatomical event.
However, in contrast to our self-centered history of life, with “man” at the evolutionary pinnacle of inevitable superiority, birds constitute the overwhelming majority of bipedal animals – Homo sapiens, and our tiny group of related apes, are not at all the “typical” or sole evolutionary experiment with this form of locomotion.
Bipedal success: 9700 living species of birds vs. ONE species of bipedal ape.
Wiley Online Library / Journal of Anatomy (full article is available, but link(s) fail)
Bipedal animals, and their differences from humans
Humans, birds and (occasionally) apes walk bipedally. Humans, birds, many lizards and (at their highest speeds) cockroaches run bipedally. Kangaroos, some rodents and many birds hop bipedally, and jerboas and crows use a skipping gait. This paper deals only with walking and running bipeds. Chimpanzees walk with their knees bent and their backs sloping forward. Most birds walk and run with their backs and femurs sloping at small angles to the horizontal, and with their knees bent. These differences from humans make meaningful comparisons of stride length, duty factor, etc., difficult, even with the aid of dimensionless parameters that would take account of size differences, if dynamic similarity were preserved. Lizards and cockroaches use wide trackways. Humans exert a two-peaked pattern of force on the ground when walking, and an essentially single-peaked pattern when running. The patterns of force exerted by apes and birds are never as markedly two-peaked as in fast human walking. Comparisons with quadrupedal mammals of the same body mass show that human walking is relatively economical of metabolic energy, and human running is expensive. Bipedal locomotion is remarkably economical for wading birds, and expensive for geese and penguins.
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. (That is, the transition from tree living to ground dwelling is not “bizarre, strange, nor incomprehensible”; nor does it require “supernatural” intervention)
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.