Could Wading in Shallow Water Account for the Unique Shape of the Australopithecus afarensis Pelvis

Abstract

A 3D Geometric Morphometric (GM) analysis of the shape of the pelvis and femur of various extinct hominids and extant humans and apes is described. Observed differences in shape are then discussed in the context of the wading hypothesis, a model of the evolution of hominin bipedalism that has rarely been seriously considered despite some compelling arguments in its favour. The general shape of the pelvis of Australopithecus afarensis is confirmed to be fundamentally different from both Homo and extant great apes, and not intermediate between them. Although it includes some human-like traits indicating a strong propensity to bipedalism, there are also sufficient differences to indicate that australopithecines probably exhibited a different type of bipedality to the relatively efficient striding gait associated with modern humans. An analysis of putative muscle lever arm ratios is described, which generated over 135,000 ratios in all. This data was then explored using the Pivot Table feature of Microsoft Excel. Succinct species summaries of broad lever arm groups, such as those pertaining to abduction compared to those pertaining to extension were generated. The results indicate that the australopithecine hip was more adapted, than modern humans or extant great apes, to adduction, abduction and rotation of the thigh during locomotion. It is argued that this apparent lateral biomechanical advantage complements the broad platypelloid shape as a putative adaptation to side-to-side wading. This adds further evidential weight to the wading hypothesis of bipedal origins in addition to the already compelling arguments from extant ape behaviour in shallow water and the favourable evidence of the paleohabitats of the earliest bipeds.