Search

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. KEY WORDS humerus; ulna; capitate; metacarpal; forearm

Limb bones are designed to be strong enough to support the body and yet be energetically conservative during locomotion. Bones of the distal segment, which are relatively costly to move, are often more slender than bones of the proximal segments, even though they must sustain proportionally greater loads. As a result, they are expected to experience a higher incidence of microdamage. With this constraint in mind, Lieberman and Crompton ([1998] Principles of Animal Design, Cambridge: Cambridge University Press, p. 7886) proposed that bones response to strain varies along the proximo-distal axis of the limb. In order to avoid fatigue fractures due to the accumulation of microdamage, the distal segment, in comparison to the proximal segment, will have an increase in remodeling events to replace damaged bone. In this paper, we test the hypothesis of Lieberman and Crompton (1998) with respect to the human lower limb. With a sample of adult individuals, we compare tibiae and femora for mid-diaphyseal cross-sectional geometry and Haversian remodeling differences. Our results indicate that the human limb is not designed like that of quadrupedal cursorial animals. The tibia is not less resistant in bending and torsion, and does not remodel more than the femur. Our findings fail to support the hypothesis of Lieberman and Crompton (1998) and suggest, instead, that the human lower limb is not designed like a cursorial animal limb. In addition, our results support previous observations that remodeling is not uniform within the cross section of a bone, probably a reflection of different loading histories within the different regions of the cross section. KEY WORDS histomorphometry; cross-sectional geometry; tibia; femur; microdamage

This study examines the functional morphology of the olecranon process in hominoids and fossil hominins. The length of the bony lever of the triceps brachii muscle (TBM) is measured as the distance between the trochlear articular center and the most distant insertion site of the TBM, and olecranon orientation is measured as the angle that this bony lever makes with the long axis of the ulna. Results show that Homo, Pan, Gorilla, most monkeys, and the Australopithecus fossils studied have similar relative olecranon lengths. Suspensory hominoids and Ateles have shorter olecranons, suggesting, in some instances, selection for greater speed in extension. The orientation that the lever arm of the TBM makes with the long axis of the ulna varies with preferred locomotor mode. Terrestrial primates have olecranons that are more posteriorly oriented as body size increases, fitting general models of terrestrial mammalian posture. Arboreal quadrupeds have more proximally oriented lever arms than any terrestrial quadrupeds, which suggests use of the TBM with the elbow in a more flexed position. Olecranon orientation is not consistent in suspensory hominoids, although they are all characterized by orientations that are either similar or more posterior than those observed in quadrupeds. Homo and the fossils have olecranons that are clearly more proximally oriented than expected for a quadruped of their size. This suggests that Homo and Australopithecus used their TBM in a flexed position, a position most consistent with manipulatory activities. KEY WORDS triceps brachii muscle; bony lever; load arm; body size; Australopithecus