Trabecular Evidence for a Human-Like Gait in Australopithecus africanus

Abstract

Although the earliest known hominins were apparently upright bipeds, there has been mixed evidence whether particular species of hominins including those in the genus Australopithecus walked with relatively extended hips, knees and ankles like modern humans, or with more flexed lower limb joints like apes when bipedal. Here we demonstrate in chimpanzees and humans a highly predictable and sensitive relationship between the orientation of the ankle joint during loading and the principal orientation of trabecular bone struts in the distal tibia that function to withstand compressive forces within the joint. Analyses of the orientation of these struts using microCT scans in a sample of fossil tibiae from the site of Sterkfontein, of which two are assigned to Australopithecus africanus, indicate that these hominins primarily loaded their ankles in a relatively extended posture like modern humans and unlike chimpanzees. In other respects, however, trabecular properties in Au africanus are distinctive, with values that mostly fall between those of chimpanzees and humans. These results indicate that Au. africanus, like Homo, walked with an efficient, extended lower limb.

Calcaneal robusticity in Plio-Pleistocene hominins: Implications for locomotor diversity and phylogeny

“…However, recent experimental evidence failed to provide support for the hypothesis that long bone diaphyseal properties should predictably conform to local strain magnitude (Wallace et al, 2014). Additionally, studies investigating the relationship between foot and ankle function across primates, including early hominins, and trabecular microarchitecture have presented mixed results (DeSilva and Devlin, 2012; Barak et al, 2013;Kuo et al, 2013;Su et al, 2013). Taken together, this implies that the differences between hominins in calcaneal morphology are unlikely to be the result of bony remodeling alone.…”

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

“…A key issue in such debates is the degree of developmental plasticity of different skeletal features. As noted by a number of authors, developmentally plastic traits should reflect, at least in part, the actual mechanical loading environment of the animal while it was alive, thus providing direct evidence of its behavior [4, 13, 21–26]. One such trait is the cross-sectional structure of long bone diaphyses, which is known from both experimental and observational studies to be responsive to changes in mechanical loadings during life [27–29].…”

The human iliotibial band is specialized for elastic energy storage compared with the chimp fascia lata

“…The oldest known hominin species for which we have pelvic material, Ardipithecus ramidus, probably had inferiorly oriented ischia like chimpanzees ), but in Australopithecus afarensis, the posteriorly oriented ischia (Robinson, 1972;Stern and Susman, 1983), the large bicondylar angle (Stern and Susman, 1983) and the perpendicular orientation of the tibia's distal articular surface relative to the tibial shaft (Latimer et al, 1987) suggest a more upright posture than chimpanzees. The distally flattened femoral condyles (Heiple and Lovejoy, 1971) and longitudinally oriented trabecular struts in the distal tibia (Barak et al, 2013) in Australopithecus africanus provide additional evidence that hominins by 2-to 3-million years ago had a human-like extended limb posture. Thus, fossil evidence suggests that australopiths walked with a human-like gait in terms of hip, knee and ankle angles (Latimer et al, 1987;Tardieu and Trinkaus, 1994;Crompton et al, 1998;Ward, 2002;Carey and Crompton, 2005;Barak et al, 2013), likely resulting in increased FL energy storage during walking compared with a chimp-like last common ancestor.…”

“…The distally flattened femoral condyles (Heiple and Lovejoy, 1971) and longitudinally oriented trabecular struts in the distal tibia (Barak et al, 2013) in Australopithecus africanus provide additional evidence that hominins by 2-to 3-million years ago had a human-like extended limb posture. Thus, fossil evidence suggests that australopiths walked with a human-like gait in terms of hip, knee and ankle angles (Latimer et al, 1987;Tardieu and Trinkaus, 1994;Crompton et al, 1998;Ward, 2002;Carey and Crompton, 2005;Barak et al, 2013), likely resulting in increased FL energy storage during walking compared with a chimp-like last common ancestor. However, without increases in the mass of muscles inserting in the FL, which fossil evidence indicates occurred later in the genus Homo, FL energy storage would not have reached human-like magnitudes.…”

“…Features indicating an extended lower limb posture in hominins include a posteriorly oriented ischium (Robinson, 1972), a large femoral bicondylar angle (valgus knee) (Heiple and Lovejoy, 1971;Stern and Susman, 1983), anteroposteriorly flattened femoral condyles (Heiple and Lovejoy, 1971;Ward, 2002), a more perpendicular orientation of the tibia's distal articular surface relative to the long axis of the tibial shaft (Latimer et al, 1987) and longitudinally oriented trabecular struts in the distal tibia (Barak et al, 2013). Expanded muscle attachment areas on the ilium for GMax and TFL suggest an increase in the force-generating capacity of these muscles.…”