Experimental approaches to understanding the evolution of human phenotypic diversity

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Modern humans vary in observable characteristics (phenotype) like body size/breadth, limb proportions, body composition, and musculoskeletal traits, yet we are all extremely genetically similar, creating a compelling paradox of particular interest for understanding human evolutionary history. The key to unlocking this paradox is in understanding the mechanisms shaping human plasticity; plasticity is the keystone of the human adaptive response, allowing phenotype to be sensitively adjusted to environmental stresses much faster than is possible through genetic adaptation alone. Some of these stresses stem from balancing the costs of locomotion and thermoregulation in a given environment, both being energetically expensive functions that are critical for survival. Biological anthropologists traditionally use simple models to link phenotypic variation to locomotor costs when moving over flat, open terrain and thermoregulatory costs at rest. However, global patterns do not always match what would be expected based on these simple models. In reality, selection pressures shaping locomotor morphology are complex and vary in relation to multiple environmental factors like vegetation and terrain, and sex-specific factors like the costs of reproduction and load-carrying; none of these have been adequately incorporated into the biomechanical and energetic models on which biological anthropology has traditionally relied. My proposed research represents a transformative new approach to understanding how locomotor and energetic selection pressures have shaped modern human diversity and sexual dimorphism. This work combines engineering-based musculoskeletal modelling with laboratory- and field-based experimental analyses with athletes in order to: i) directly test sex-specific relationships between human phenotypic variation, locomotor costs, and gait kinematics in different environmental conditions, ii) computationally model whether or not variation in skeletal traits typically used to infer behavioural differences between past populations actually produces meaningful adaptive kinematic benefits, and iii) test the extent to which prolonged locomotion-related energetic stress may have contributed to the evolution of sex differences in body composition. In doing so, I will directly link sex-specific phenotypic variation with its kinematic and energetic consequences and offer unprecedented insight into how modern human diversity has been shaped by the environments in which we have lived, moved, and adapted.  The proposed research will also provide crucial sex-disaggregated data to fields in which females are often underrepresented in research. Further, the custom computer modelling methods developed and real-time motion data collected will enable the construction of gait simulations that better incorporate inter-individual variation and help inform the design of customized assistive solutions for individuals with physical and/or locomotor challenges.