Earliest Known Hominin Calcar Femorale in Orrorin tugenensis Provides Further Internal Anatomical Evidence for Origin of Human Bipedal Locomotion

The calcar femorale (CF), a plate of dense bone internal to the lesser trochanter, is visible on computed tomographic images of the 6 million-year-old femoral fragment BAR 1003′00 (from the taxon Orrorin tugenensis), among the oldest specimens relevant to reconstructing the evolution of human bipedal locomotion. A strongly expressed CF has been used previously as an indicator of bipedality. If true, then there should be a quantifiable difference in the CF among hominoids. Absolute and normalized CF lengths were measured from computed tomographic images at five anatomical locations along the proximal portion of BAR 1003′00 in addition to samples of nine H. sapiens and ten P. troglodytes femora. The span of the CF superiorly to inferiorly within the proximal femur was measured by counting the number of cross-sections on which the CF occurred. A Bayesian approach was used to classify the BAR 1003′00 sample based on normalized lengths. The P. troglodytes femora were more variable both in the occurrence of the trait and, where present, its span in the proximal femur. The H. sapiens sample exhibited CF lengths that were consistently larger at each location than the P. troglodytes in absolute terms, but the normalized lengths overlap substantially. The Bayesian posterior probability test classifies the CF of BAR 1003′00 with H. sapiens. The BAR 1003′00’s calcar femorale has a strong anatomical similarity to the H. sapiens sample, supporting the conclusion that O. tugenensis is an early bipedal hominin. Anat Rec, 301:1834–1839, 2018.