The proximal femur has played a prominent role in our understanding of the origin and evolution of human gait because of its functional importance and relatively good representation in the fossil record. This study examines the morphology of femora from the fossil record, including those attributed to Orrorin tugenensis (BAR 1002’00) and Homo floresiensis (LB1/9). Considerable debate surrounds both of these taxa, focusing primarily on the evidence that the former is a hominin and shows convincing adaptations for bipedalism, and over whether or not the latter is a pathological diminutive modern human or a distinct species (and what the anatomy suggests regarding the evolutionary history of H. floresiensis). This study addresses the questions of whether Orrorin femoral morphology more closely resembles femora of humans and fossil hominins than apes, and whether it is more similar to the femora of Homo among the hominins. Our study also tests the hypothesis that the femoral morphology of LB1/9 is consistent with that of a small-bodied modern human, or more closely resembles fossil hominins. To test these questions, we compare the proximal femoral morphology of BAR 1002’00 and LB1/9 to a large sample of adult humans, chimpanzees, bonobos, gorillas, orang-utans and most available early hominin taxa. Importantly, the human sample includes individuals from large- and small-bodied populations that overlap with the small sizes of BAR 1002’00 and LB1/9.

The results show that the external morphology of the Orrorin femur more closely resembles the femora of Australopithecus and Paranthropus than those of great apes, or fossil and modern Homo. Its morphology is not consistent with it being characterised as more like human than australopith femora, or a phylogenetic hypothesis that Orrorin is ancestral to Homo to the exclusion of the australopithecines. However, its morphology is consistent with its 6 million-year-old age and its taxonomic assignment as a basal, bipedal hominin. The external morphology of the Orrorin femora shares similarities with those of gracile and robust australopiths despite some evidence suggesting more primitive cortical structure, raising potential questions about hip function in Orrorin. The proximal femoral anatomy of LB1/9 shows that it is not a small modern human, and instead supports evidence from other anatomical regions that it represents a distinct species, Homo floresiensis, with strikingly primitive femoral morphology resembling early hominins.