Are the differences between Stw 431 ( Australopithecus africanus) and A.L. 288-1 ( A. afarensis) significant?

Abstract

Recent studies of early hominin body proportions paint a complex evolutionary picture, with multiple instances of reversal in body shape. These interpretations rest heavily upon the inferred limb joint proportions of Australopithecus africanus. For example, the partial skeleton Stw 431 has been suggested to show apelike joint proportions compared to the A. afarensis specimen A.L. 288-1. This suggests an evolutionary reversal in the more recent A. africanus. However, no study has examined the probability of sampling the differences between Stw 431 and A.L. 288-1 from a single extant hominoid species. The present study compares elbow/hip and elbow/lumbosacral joint size ratios between Stw 431 and A.L. 288-1 using exact randomization, based on chimpanzee and human models of variation. Results indicate that differences in elbow/hip proportions between Stw 431 and A.L. 288-1 can be sampled from a single species. In contrast, differences in elbow/lumbosacral proportions between Stw 431 and A.L. 288-1 show a significantly low probability of being sampled from a single species. Thus, Stw 431 and A.L. 288-1 are not significantly different from each with regard to limb joint proportions, but Stw 431 has a significantly smaller lumbosacral joint. This pattern does not conform to previous interpretations of limb proportions in A. africanus. Low statistical power in the present study may account for the discrepancy. Further research is needed to illuminate the functional implications of variation in relative lumbosacral joint size in early hominins.