ORIGIN OF HABITUAL TERRESTRIAL BIPEDALISM IN THE ANCESTOR OF THE HOMINIDAE

In this paper, the evolution of habitual terrestrial bipedalism in the stem proto-hominid is reconstructed through an examination of historical transformations of shared derived morphological-behavioural complexes related to bipedalism in catarrhines. This historical reconstruction indicates that the bipedal threat display-appeasement behaviour complex may be of particular importance in understanding the origin of habitual terrestrial bipedalism because it is the most recently acquired morphological-behavioural complex shared by the African great apes and humans. We suggest that in the late middle and late Miocene of East Africa, as habitats were becoming more open and desiccated and resources more widely separated, increased intraspecific competition among prehominids for resources ensued. We propose that bipedal displays and their appeasement were the behaviours essential to the success of pre-hominids in this environment in that they allowed for the relatively peaceful resolution of intragroup and intraspecific conflicts. This theory provides a major, proximate pre-adaptive cause for the later adoption of bipedalism by proposing the use of an existing behaviour in a new environmental context, namely increased use of, and deference to bipedal displays to mitigate violence and make possible the equitable allocation of scarce resources. It is suggested that this increased the evolutionary fitness of pre-hominids by removing a major cause of morbidity and mortality observed in living African apes, i.e. intra- and intergroup aggression. This theory of the origin of habitual terrestrial bipedalism in the ancestor of the Hominidae differs from others in that it is consistent with available environmental, palaeontological, anatomical and behavioural evidence and known types of social organization in extant hominoids. Bipedal postures, once adopted for social control, would have become common elements of the prehominid locomotor repertoire. With time, this new locomotor mode would have become increasingly multifunctional. © 1993 Academic Press. All rights reserved.