The gait of Homo naledi

Remains of the foot, upper and lower limb, thorax and cranium of Homo naledi present a mosaic of primitive and Homo-like traits. These include curved phalanges in the hands, although a human like wrist and palm, an ape-like thorax with Homo-like vertebrae, and a shoulder girdle indicative of climbing competency. The individual bones of the foot and lower limbs largely seem to show an individual compatible with obligate bipedalism, although the pedal phalanges also show curvature [1,2]. Despite the remarkably voluminous assemblages, the H. naledi remains do not include a complete lower limb confidently ascribed to a single individual. A complete lower limb of H. naledi would be informative in what it could tell us regarding potential locomotion. The aim of this study was to reconstruct the lower limbs of the H. naledi skeleton and analyse the potential gait of H. naledi whilst also reviewing recent work on the functional morphology of H. naledi and how this pertains to inferences about bipedal locomotion. The H. naledi lower limb was constructed using estimated femoral, tibial and fibular lengths from the most complete remains of H. naledi currently available [2,3]. Pelvis remains were too fragmented to reconstruct the pelvis. All transformations were performed in ‘LhpFusionBox’, which is a musculoskeletal software primarily used to analyse gait in clinical contexts, but recently adapted for paleoanthropologists [4]. Estimated lengths were used as a reference to reconstruct the individual bones by using anatomical land-marks (ALs) to scale other H. naledi material to the size of the estimated bones. Both a modern human model and new lower limb associations of a juvenile skeleton found in the Dinaledi chamber were used as guides to reconstruct a complete lower limb. Reconstructions of individual isolated and scaled limb and foot bones were then placed together to have a complete lower limb re- construction taking into account ligaments, muscles and following the orientation of joint surfaces. The entire limb was then fused to a modern human walking motion to analyse potential locomotion. The reconstruction and biomechanical analysis of the H. naledi lower limb largely demonstrates a morphology compatible with obligate bipedalism, with a medial arch (although reduced), elongated limbs, marked bicondylar angle and joint surfaces compatible with bipedal gait. The elongation of the lower limb is generally seen as a marker of obligate bipedalism although the H. naledi limbs are exceptionally elongated relative to the diaphyseal diameter of the long bones and preserved joint proportions. Longer limbs are generally thought of as more energy efficient, but they also require a greater moment of inertia, which increases energy costs. Whilst the longer tibia (and leg) may have necessitated a longer swing phase, low limb mass (as evidenced by long bone gracility and small joints) may have offset this energetically. Other unique traits such as the flaring ilium, flattened femoral lower neck and overall general mix of primitive and Homo-like traits may not have impeded obligate bipedalism, but they may have been advantageous for climbing. Primitive traits are often thought of as vestiges of an ancient past on the way to the modern human form of obligate bipedalism, however, Homo erectus and other skeletons from the genus Homo are largely thought to be fully obligate bipedals from approximately 1.5 million years onwards, and as H. naledi has been dated between 335 and 236ka [5], it is therefore curious why this particular branch of the hominid tree would ‘hang on’ to the ‘primitive traits’ for at least a million years longer. It is therefore likely that this hominin engaged in both arboreal climbing and bipedal walking.