Relative length of the immature Homo naledi tibia U.W. 101-1070: evidence for elongation of the leg

It is largely held that the transition from Australopithecus to the genus Homo involved a shift in overall body proportions, including a relative lengthening of the lower limb, though the degree to which hominin limb proportions changed and the proximate causes thereof remain contentious topics. The overall poor preservation of most articular surfaces and predominately fragmentary diaphyses in the mature Dinaledi lower limb long bone sample preclude a definitive assessment of lower limb proportions based on this material alone. The U.W. 101-1070 immature tibia is the most complete H. naledi lower limb element recovered to date, measuring 278 mm from the eminence of the tibial spines to the distal extent of the medial malleolus, and preserving nearly the entire medial condyle. To assess proportions, we evaluated tibia medial condylar dimensions relative to tibia maximum length in U.W. 101-1070, other fossil hominin tibiae, and an ontogenetic sample of humans, chimpanzees, and gorillas. Results indicate that the U.W. 101-1070 tibia is very long relative to joint size – markedly differing from all extant species and comparative fossil hominin tibiae. The combination of an exceptionally small tibial articular surface (like many australopiths) and moderately long tibia, in H. naledi, appears autapomorphic in the hominin fossil record. Though the specific implications of this finding are partially dependent on the geological age and phylogenetic position of H. naledi, U.W. 101-1070 provides an example of bone length/joint size decoupling relative to the human pattern.