Limb bones confirm the earliest discovered hominin walked on two legs

American Association for the Advancement of Science (AAAS)—A re-examination of the femur and ulna of Sahelanthropus tchadensis, commonly regarded as the earliest known hominin, confirms that this species walked on two legs. The new study identifies 3 bone features associated with bipedal hominin hip and knee function that corroborate earlier findings. Classifying the features of suspected early bipeds – such as S. tchadensis, which lived roughly 7 million years ago – can help scientists understand the morphological phases of hominin evolution. Previously, analysis of a S. tchadensis cranium suggested this species carried its head like other bipeds do. Subsequent studies based on its partial ulnae and femur shafts also hinted at bipedalism. However, debate remained, because the species’ leg bones share some similarity with chimpanzee morphology. Here, Scott Williams and colleagues have re-examined those ulna and femur bones, examining their proportionality, size, and 3D morphology as compared with hominins and apes. Sizes were most similar to Pan genus apes, such as chimpanzees. However, proportions were more hominin-like. The bones further contained two features that are essential for bipedal knee and hip motion: strong femoral antetorsion and proxies for a gluteal complex. The femur had a tubercle, suggesting the existence of a ligament for bipedal gait stabilization. “Together, these features […] may represent some of the earliest adaptations to bipedalism in the hominin lineage,” Williams et al. write, noting that the species probably could still navigate arboreal landscapes with ease.

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