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In this review, we present on the evolution of the locomotor adaptation of hominins in the Late Miocene to Late Pliocene, with emphasis on some of the prominent advances and debates that have occurred over the past fifty years. We start with the challenging issue of defining hominin locomotor grades that are currently used liberally and offer our own working definitions of facultative, habitual, and obligate bipedalism. We then discuss the nature of the Pan-Homo last common ancestor and characterize the locomotor adaptation of Sahelanthropus, Orrorin, and Ardipithecus-often referred to as facultative bipeds-and examine the debates on the extent of bipedality and arboreality in these taxa. Moreover, the question of Middle Pliocene hominin locomotor diversity is addressed based on information derived from the 'Little Foot' specimen from Sterkfontein, footprints from Laetoli, and the Burtele Foot in Ethiopia. Our review suggests that the most convincing evidence for locomotor diversity comes from Burtele, whereas the evidence from Sterkfontein and Laetoli is unconvincing and equivocal, respectively. Finally, we address the decades old issue of the significance of arboreality in the otherwise habitual biped, Australopithecus, with emphasis on Australopithecus afarensis and its implications for the paleobiology of these creatures. We conclude that many of the apelike features encountered, mostly in the upper part of the Australopithecus skeleton, were retained for their significance in climbing. Approaches that have investigated character plasticity and those exploring internal bone structure have shown that the shoulder and limbs in Au. afarensis and Australopithecus africanus were involved in arboreal activities that are thought to be key for feeding, nesting, and predator avoidance. We conclude that many of the so-called retained ape-like features persisted due to stabilizing selection, that early hominins engaged in a considerable amount of arboreality even after Australopithecus had become a habitual biped, and arboreality only ceased to be an important component of hominin locomotor behavior after the emergence of Homo erectus.

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The distal femoral metaphyseal surface presents dramatically different morphologies in juvenile extant hominoids-humans have relatively flat metaphyseal surfaces when compared with the more complex metaphyseal surfaces of apes. It has long been speculated that these different morphologies reflect different biomechanical demands placed on the growth plate during locomotor behaviour, with the more complex metaphyseal surfaces of apes acting to protect the growth plate during flexed-knee behaviours like squatting and climbing. To test this hypothesis, we built subject-specific parametric finite-element models from the surface scans of the femora of five Pan and six Homo juveniles. We then simulated the loading conditions of either a straight-leg or flexed-knee gait and measured the resulting stresses at the growth plate. When subjected to the simulated flexed-knee loading conditions, both the maximum and mean von Mises stresses were significantly lower in the Pan models than in the Homo models. Further, during these loading conditions, von Mises stresses were strongly negatively correlated with ariaDNE, a measure of complexity of the metaphyseal surface. These results indicate that metaphyseal surface morphology has a robust effect on growth plate mechanics.

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The paucity of well-preserved pelvises in the hominin fossil record has hindered robust analyses of shifts in critical biological processes throughout human evolution. The Kebara 2 pelvis remains one of the best preserved hominin pelvises, providing a rare opportunity to assess Neanderthal pelvic morphology and function. Here, we present two new reconstructions of the Kebara 2 pelvis created from CT scans of the right hip bone and sacrum. For both reconstructions, we proceeded as follows. First, we virtually reconstructed the right hip bone and the sacrum by repositioning the fragments of the hip bone and sacrum. Then, we created a mirrored copy of the right hip bone to act as the left hip bone. Next, we 3D printed the three bones and physically articulated them. Finally, we used fiducial points collected from the physically articulated models to articulate the hip bones and sacrum in virtual space. Our objectives were to (1) reposition misaligned fragments, particularly the ischiopubic ramus; (2) create a 3D model of a complete pelvis; and (3) assess interobserver reconstruction variation. These new reconstructions show that, in comparison with previous measurements, Kebara 2 possessed a higher shape index (maximum anteroposterior length/maximum mediolateral width) for the pelvic inlet and perhaps the outlet and a more anteriorly positioned sacral promontory and pubic symphysis relative to the acetabula. The latter differences result in a lower ratio between the distances anterior and posterior to the anterior margins of the acetabula. Generally, the new reconstructions tend to accentuate features of the Kebara 2 pelvis--the long superior pubic ramus and anteriorly positioned pelvic inlet--that have already been discussed for Kebara 2 and other Neanderthals.

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OBJECTIVE: Distal femoral metaphyseal surface morphology is highly variable in extant mammals. This variation has previously been linked to differences in locomotor behavior. We perform the first systematic survey and description of the development of this morphology in extant hominoids. MATERIALS AND METHODS: We collected 3D surface laser scans of the femora of 179 human and great ape individuals throughout all subadult stages of development. We qualitatively and quantitatively describe metaphyseal surface morphology. RESULTS: We find that the metaphysis is topographically simple in all hominoids during the fetal and infant periods relative to later developmental periods, and in apes it develops significant complexity throughout development. Humans, by contrast, retain relatively flat metaphyseal surfaces throughout ontogeny. DISCUSSION: Major shifts in morphology appear to coincide with major shifts in locomotor behavior, suggesting that metaphyseal morphology is developmentally plastic and highly dependent on the biomechanical loadings at the knee joint. This is consistent with a large body of biomedical research, which demonstrates the primacy of mechanical forces in determining growth plate ossification patterns. Additionally, specific metaphyseal morphology appears highly correlated with specific locomotor modes, suggesting that metaphyseal surface morphology will be useful for reconstructing the locomotor behavior of fossil primate taxa.

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