Trabecular bone fraction variation in modern humans, fossil hominins and other primates

Evidence suggests that recent modern humans (Holocene) have low trabecular bone density (i.e., trabecular bone fraction, TBF) compared with other extant primates and fossil hominins. However, the extent to which TBF in recent humans with varying subsistence strategies differs from that of fossil hominins, and in turn, how hominins differ from various extant catarrhines is unclear. This study tests the hypotheses that first, populations with subsistence strategies demanding high physical activity exhibit greater TBF than sedentary populations and are more similar to fossil Homo. Secondly, that, australopiths have TBF that is more similar to nonhuman primates because of the greater mechanical loading on their skeletons. The study quantifies TBF in the limb epiphyses of recent humans, hominoids, cercopithecines, and fossil hominins. The results show overall a significant decrease in TBF among recent humans, whereas hominins, hominoids, and cercopithecines have similar, high TBF values. In addition, active human populations display TBF that is more similar to fossil Homo. The results suggest that this TBF decline reflects a reduction in activity levels among sedentary populations, although a systemic decline cannot be ruled out. These findings support the recent evolution of low trabecular density because of a decline in activity levels and underscore the utility of comparing multiple skeletal elements across a diverse set of recent modern humans when drawing conclusions about changes in trabecular bone in the human skeleton