Structural analysis of the femoral diaphyses of an early modern human from Tianyuan Cave, China

The Tianyuan 1 skeleton is among the oldest fossil specimens of our species from East Asia. Important in the original assessment of the skeleton as belonging to a modern human was its femoral structure, particularly its well-developed pilasters. Also noteworthy were its large femoral diaphyseal second moments of area, which were interpreted as evidence of a physically demanding lifestyle, and pathological bone growths on the posterior femoral shafts. Here, we build on previous studies of femoral morphology in Tianyuan 1 by analyzing its diaphyseal structure using micro-computed tomography coupled with a novel method of visualizing cortical bone thickness distributions along the shaft with color maps. Additionally, we calculate diaphyseal second moments of area in Tianyuan 1 with and without its pathological bone additions in order to evaluate whether these growths are the cause of its apparently high diaphyseal robusticity. Diaphyseal color maps and second moments of area of Tianyuan 1 femora are compared to those of three recent (Holocene) human comparative samples, as well as samples of Pleistocene humans. The results show that in terms of cortical thickness distribution, Tianyuan 1 femoral diaphyses are strikingly similar to those of recent humans, as well as Pleistocene early modern humans, yet distinct from the diaphyses of Neandertals. This provides additional support for the modern human status of Tianyuan 1. Analyses of second moments of area reveal that Tianyuan 1 is best classified as a Pleistocene early modern human rather than as a member of any of the recent samples when pathological bone additions are included in area moment calculations. Interestingly, however, once pathological growths are virtually removed, Tianyuan 1 cannot be assigned with confidence to any comparative group. This suggests that the relatively high anteroposterior femoral diaphyseal strength mid-distally in Tianyuan 1 is to some degree a product of its pathological condition.