Limb Bone Structural Proportions and Locomotor Behavior in A.L. 288-1 ("Lucy")

Abstract

While there is broad agreement that early hominins practiced some form of terrestrial bipedality, there is also evidence that arboreal behavior remained a part of the locomotor repertoire in some taxa, and that bipedal locomotion may not have been identical to that of modern humans. It has been difficult to evaluate such evidence, however, because of the possibility that early hominins retained primitive traits (such as relatively long upper limbs) of little contemporaneous adaptive significance. Here we examine bone structural properties of the femur and humerus in the Australopithecus afarensis A.L. 288–1 ("Lucy", 3.2 Myr) that are known to be developmentally plastic, and compare them with other early hominins, modern humans, and modern chimpanzees. Cross-sectional images were obtained from micro-CT scans of the original specimens and used to derive section properties of the diaphyses, as well as superior and inferior cortical thicknesses of the femoral neck. A.L. 288–1 shows femoral/humeral diaphyseal strength proportions that are intermediate between those of modern humans and chimpanzees, indicating more mechanical loading of the forelimb than in modern humans, and by implication, a significant arboreal locomotor component. Several features of the proximal femur in A.L. 288–1 and other australopiths, including relative femoral head size, distribution of cortical bone in the femoral neck, and cross-sectional shape of the proximal shaft, support the inference of a bipedal gait pattern that differed slightly from that of modern humans, involving more lateral deviation of the body center of mass over the support limb, which would have entailed increased cost of terrestrial locomotion. There is also evidence consistent with increased muscular strength among australopiths in both the forelimb and hind limb, possibly reflecting metabolic trade-offs between muscle and brain development during hominin evolution. Together these findings imply significant differences in both locomotor behavior and ecology between australopiths and later Homo.

Relationship between body mass, lean mass, fat mass, and limb bone cross‐sectional geometry: Implications for estimating body mass and physique from the skeleton

“…Work by Ruff () suggests that the relative importance of gravitational and muscular forces varies by limb, the former being more important for the lower limb and the latter for the upper limb, particularly in males. Adjusting for body mass, there was a strong correlation ( r = 0.70) between the residuals of muscle area and humeral shaft strength in the oldest individuals (17 years) in the same dataset (Ruff, Burgess, Ketcham, & Kappelman, ).…”

Functional morphology in the pages of the AJPA

“…Developmental integration of different features is another important factor that must be considered in this context (see Richtsmeier, this volume), and the influence of non mechanical factors on skeletal form (Lieberman, ; Ruff et al, ). How one weights the various lines of evidence in this regard has an important bearing on reconstruction of past mechanical environments, i.e., behavior, including diet and mastication (Daegling et al, ; Strait et al, ) and locomotion (Haile‐Selassie et al, ; Lovejoy, Meindl, Ohman, Heiple, & White, ; Ruff, ; Ruff, Burgess, Ketcham, & Kappelman, ). Experimental studies provide insight into the limits of developmental plasticity (Lieberman et al, ; Ruff et al, ; Wallace et al, ).…”

One hundred years of paleoanthropology: An American perspective

“…Most of the features of concern were of unknown plasticity and therefore could not resolve the issue. More recently, it has been possible (Dowdeswell et al, ; Ruff, Burgess, Ketcham, & Kappelman, ) to assess the distributions of diaphyseal hypertrophy for several early hominins, a feature that responds to habitual loads (Ruff, Holt, & Trinkaus, ). It indicates varying levels and patterns of limb loading relative to more recent humans, and hence locomotor contrasts, albeit in the context of paleontological and pedographic indications of a fully erect and effective bipedality (Crompton et al, ; Ward, ).…”