Project Summary Early life environments can have profound and long-lasting effects on human health, yet the mechanistic basis of these effects remain poorly understood as do the factors that explain inter-individual variation. These gaps in knowledge severely limit our ability to both predict susceptible individuals and to develop effective intervention strategies. At the molecular level, early life effects on later life health are thought to be mediated by stable changes in gene regulation. However, many gene regulatory elements are responsive to environmental stimuli throughout life, making it challenging to isolate the effects of early life conditions or to understand their stability, especially when within-lifetime environmental variation is absent or longitudinal data are not available. My research program aims to overcome these challenges by working with two subsistence-level groups: 1) the Turkana of Kenya, who are experiencing rapid lifestyle change such that early life and adult conditions are largely decoupled and 2) the Tsimane of Bolivia, who have been followed longitudinally for decades. In both groups, I will generate genome-wide gene regulatory datasets (DNA methylation, gene expression) paired with information on environmental experiences and health, allowing me to identify the mechanisms that embed early life exposures into lifelong physiology. I will also generate genome-wide genotype data for each individual, in order to ask whether genetic variation predicts sensitivity versus resilience to early life challenges at the gene regulatory level. Finally, I will complement this field-based, observational work with lab-based methods capable of testing for causal connections between 1) DNA methylation and gene expression and 2) genotype and environmentally-induced gene expression, both in high-throughput. In doing so, this project will uncover the gene regulatory mechanisms that mediate early life effects on health, as well as the degree to which these relationships are modified by genetic variation. By working “in the field” and “in the lab”, the proposed work will inform our understanding of developmental and environmental processes in natural human populations as well as causal mechanisms. More broadly, the proposed projects will link a major global phenomenon—increasing urbanization and market-integration—with gene regulatory processes and health in two understudied and underserved populations (Africans and Amerindians). It will also generate new methods and resources that will inform our understanding of the general patterns of genotype x environment interactions in human complex traits.