When And Why Did Low Trabecular Bone Density Evolve In Humans? A Comparative And Experimental Study Of Trabecular Density

Modern humans are unique in many respects compared to our closest living relatives, the great apes, including anatomy related to bipedal gait such as having large lower limb joint surfaces, narrow pelvis, elongated lower limbs and novel bipedal energetics that allow us to walk and run bipedally for long distances. This dissertation focuses on investigating trabecular bone morphology in the limb joints of modern humans in comparison to other extant primates, fossil hominins and other mammals that share cursorial adaptations with modern humans with the goal of identifying whether low trabecular bone density in limb joints is a uniquely modern human trait. Additionally, it focuses on applying biomechanical theory to understand structural changes that cursors have evolved as a means of minimizing locomotor cost as well as empirically testing the influence of activity on trabecular morphology.;This dissertation consists of seven chapters, with an introduction and conclusion at the beginning and at the end, respectively. The five results focused chapters (chapters 2-6) broaden our knowledge of trabecular bone morphology evolutionarily and functionally.;The introduction (chapter 1) provides a justification for investigating trabecular bone morphology and presents the aims of the dissertation. Chapter two compares trabecular bone density among higher primates and monkeys throughout the limb joints with the aim of identifying trabecular density patterns among these primate taxa. The results reported in the chapter demonstrate that modern humans are unique in having low-density joints throughout the skeleton compared to other primates. Results from this chapter form the basis for chapters three, four and six in that it lays the foundation to further hypothesize on the nature of trabecular bone morphology among modern humans compared to fossil hominins and other mammals.;Chapter three proposes that following biomechanical theory cursorial mammals, including modern humans, do not decrease trabecular bone density in a proximal to distal fashion in order to minimize energetic cost when running. The results demonstrate that both cursorial and non-cursorial mammals have varied trabecular bone density in their limbs and do not fit a predictable biomechanical hypothesis.;Chapter four investigates the relationship of trabecular bone density, joint size and midshaft cortical area. The goal of this chapter is to see if trabecular bone density varies with joint size and midshaft cross-sectional strength (using cortical area as a proxy for midshaft cortical strength). This chapter demonstrates that trabecular density does not vary with joint size or with cross-sectional strength. The fifth chapter experimentally tests the influence of elevated running activity on trabecular density among mice. The outcome of this study raises questions about the effect of exercise on trabecular morphology, contrary to the prediction that exercise increases trabecular density and cross-sectional strength, the study shows that results differ according to skeletal element.;The sixth chapter is a comparison of trabecular density among Pleistocene hominins, modern humans and chimpanzees with the goal of understanding when in the evolutionary record low trabecular density evolved. Results suggest that 'lightly' built skeletons is a recent evolutionary phenomenon even compared to early modern humans.;In conclusion the results of this dissertation show that modern humans throughout the skeleton have low trabecular density than expected for a mammal of the same size, as one would predict because modern humans do not use their upper limbs for weight support and locomotion. Secondly, cursorial mammals including modern humans do not have lower trabecular density than non-cursorial mammals. Thirdly, trabecular bone density comparisons between fossil hominins show that low trabecular bone density pattern must have evolved later in our evolutionary record.