Relative fibular strength and locomotor behavior in OH 35 and KNM-WT 15000

Relative fibular/tibial strength has been demonstrated to be related to the degree of arboreality/ terrestriality in anthropoid primates. In this study fibular/tibial strength was determined in OH 35, a Homo habilis (or possibly Paranthropus boisei), (1.8 myr) and KNM-WT 15000, a juvenile Homo erectus, (1.5 myr), and was compared to modern humans (n=79), chimpanzees (n=16), gorillas (n=16) and orangutans (n=11). Ontogenetic changes in fibular/tibial strength were also analyzed due to KNM-WT 15000’s juvenile status. Cross-sectional properties were derived from multi-plane radiography and either CT sections of casts (fossils) or external molds (extant). RMA regressions were run on polar second moment of area (J), a measure of torsional and average bending rigidity, of the fibula against that of the tibia for all extant species. Fossils were analyzed using their relative deviations from each regression line, expressed in SEE units. Great apes differed significantly from humans in regression line elevation, with relatively stronger fibulae. OH 35 fell in the center of the great ape distribution, within 1 SEE of each great ape taxon, but 1.9 SEE from humans. KNM-WT 15000 was more than 2 SEE from all great apes and within 0.6 SEE of humans. This was not a result of his age, as fibular/tibial strength slightly decreases with age in humans. OH 35 has some human-like features; however, the relative strength of the two bones aligns the specimen with great apes, suggesting a significant degree of arboreality. KNM-WT 15000 is demonstrated to be fully modern, complimenting other evidence for complete terrestrial bipedality.