Forelimb segment length proportions in extant hominoids and Australopithecus afarensis

Abstract

Forelimb proportions have been used to infer locomotor adaptation in Australopithecus afarensis. However, little is known about proportions among individual forelimb segments in extant or fossil hominoids. The partial A. afarensis skeleton A.L. 438-1 and the more complete skeleton A.L. 288-1 provide the opportunity to assess relative length of the arm, forearm, wrist, and palm. We compare scaling relationships between pairs of forelimb bones of extant hominoids and A. afarensis, and length of individual forelimb elements to a body size surrogate. Hylobatids, and to a lesser extent orangutans, have the longest forelimb bones relative to size, although the carpus varies little among taxa, perhaps due to functional constraints of the wrist. Pan species are unique in having long metacarpals relative to ulnar length, demonstrating that they probably differ from the common chimp-human ancestor, and also that developmental mechanisms can be altered to results in differential growth of individual forelimb segments. A. afarensis has no forelimb bones that are significantly longer than those of humans for its size. It falls within the range of variation seen in modern humans for all comparisons relative to size, but appears to differ from the typical human brachial index due to a slightly shorter humerus and/or slightly longer ulna. It has short metacarpals like humans only among hominoids. Thus, while Pan may have elongated its metacarpus relative to ulnar length, A. afarensis may have reduced the length of its metacarpals and possibly its humerus relative to body size from the primitive condition.