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Heterochrony has been invoked to explain differences in the morphology of modern humans as compared to other great apes. The distal femur is one area where heterochrony has been hypothesized to explain morphological differentiation among Plio-Pleistocene hominins. This hypothesis is evaluated here using geometric morphometric data to describe the ontogenetic shape trajectories of extant hominine distal femora and place Plio-Pleistocene hominins within that context. Results of multivariate statistical analyses showed that in both  Homo  and  Gorilla , the shape of the distal femur changes significantly over the course of development, whereas that of  Pan  changes very little. Development of the distal femur of  Homo  is characterized by an elongation of the condyles, and a greater degree of enlargement of the medial condyle relative to the lateral condyle, whereas  Gorilla  are characterized by a greater degree of enlargement of the lateral condyle, relative to the medial. Early  Homo  and  Australopithecus africanus  fossils fell on the modern human ontogenetic shape trajectory and were most similar to either adult or adolescent modern humans while specimens of  Australopithecus afarensis  were more similar to  Gorilla/Pan . These results indicate that shape differences among the distal femora of Plio-Pleistocene hominins and humans cannot be accounted for by heterochrony alone; heterochrony could explain a transition from the distal femoral shape of early  Homo/A .  africanus  to modern  Homo , but not a transition from  A .  afarensis  to  Homo . That change could be the result of genetic or epigenetic factors.

Shape Ontogeny of the Distal Femur in the Hominidae with Implications for the Evolution of Bipedality