Relative robusticity of the Homo floresiensis tibia and fibula

Although previous studies of the long bones of Homo floresiensis revealed that this species retained primitive characteristics of body shape and interlimb proportions, the H. floresiensis fibula remains an unexamined part of the equation. The leg of modern humans reflects our status as habitual bipeds and is signaled by a gracile fibula relative to a more robust, weight-bearing tibia. This study investigated whether LB1, the type specimen of H. floresiensis, possesses a robust fibula or displays the gracile fibula that is the signature of our species. CT scans of the fibula and tibia of LB1 and a sample of small-bodied modern humans (N=10) were used to analyze cross-sectional geometric (CSG) properties (e.g., cortical area and polar section modulus, Zp) of the tibia and fibula at midshaft. These new data were added to the much larger sample published by Marchi (2007), which included modern humans, chimpanzees, gorillas, orangutans and gibbons. External contours of additional fossils of the genus Homo (e.g., OH 35, KNM-WT-15000) were also examined in order to evaluate when the modern condition arose during the course of human evolution. Results indicate that LB1 manifests tibial/fibular robusticity indistinguishable from that of modern humans. Analysis of external contours suggests that a gracile fibula arose relatively early in human evolution and was already in place by ~1.5 mya. Although analysis of hindlimb vs. forelimb CSG of LB1 suggested a diverse locomotor repertoire (Jungers et al., 2009), this study emphasizes the importance of habitual bipedalism in H. floresiensis.