The current understanding of the extent of human biological variation is largely limited in scope. Up until now, most of the research has been conducted in largely sedentary, industrialized populations. Recently however, several foundational studies have demonstrated that these populations only represent one notch on the complete spectrum of human variation. As chronic diseases continue to rise worldwide, it is necessary for research to pivot its lens towards understanding the complete extent of human biological variation and the ecological, cultural, and psychological factors that influence its expression. This dissertation expands knowledge on human variation by examining the relationships between two metabolic hormones (leptin and adiponectin) and several health conditions among the Tsimane, a physically active population of forager-horticulturalists in lowland Bolivia that also face significant parasitic and pathogenic stressors. Leptin and adiponectin are key indicators of energy availability and have well-documented associations with certain health conditions in the literature. However, they have been virtually unstudied outside of the typical urban center research contexts. First, I examined the relationship between leptin and adiponectin and their association with both food insecurity and depression. Secondly, I examined the associations between leptin and adiponectin and several indicators of cardiovascular disease. Lastly, I performed laboratory validations to assess the potential limitations of using a relatively new and considerably cheaper option for biomarker analysis (dried blood spot sampling). These studies found associations between the metabolic hormones and food insecurity, depression, and several cardiovascular health indicators. However, several associations deviated from what had been reported in urban settings, demonstrating the value of exploring human variation outside of typical research contexts. Additionally, this study found that dried blood spot sampling is a very stable alternative to the more expensive and more cumbersome methods of blood collection, even despite location or equipment accessibility for non-local researchers. This opens up an avenue for future researchers to conduct studies that appreciate the extent of human variation without being hindered by cost, travel, and infrastructure.

As the junction between the head and the trunk, the neck functions in providing head stability during behaviors like feeding to facilitating head mobility during behavior like grooming and predator vigilance. Despite its importance to these vital behaviors, its form and function remain poorly understood. Fossil hominin cervical vertebrae preserve a striking diversity in form despite the commitment to orthograde bipedality. Do these differences in cervical vertebral form correspond to functional variations among our recent ancestors? This dissertation attempts to understand 1) how does the neck function in head stability and mobility 2) how do these functions relate to cervical vertebral form. Kinematic and passive range of motion studies were conducted in several species of primate to obtain measures of function which were subsequently related to skeletal form.

Results show that cervical vertebral morphology does not significantly covary with differences in joint mobility. Rather, they implicate the critical role of ligaments and muscles in facilitating head mobility. Results of the kinematics study show that the neck plays a role in maintaining head stability during locomotion. However, the kinematic data do not significantly correlate with morphological variation among primate species. Given the negative results of the extant morphological analyses, it is difficult to apply them to the fossil record. As such, the functional significance of the disparate morphologies found in the hominin fossil record remain ambiguous.