The social environment shapes human health, producing strong relationships between social factors, disease risk, and survival. Over the last twenty years, studies in wild and captive mammals show that similar relationships apply outside our species, suggesting that some aspects of the social determinants of health have deep evolutionary roots. Inspired by these findings, my group has sought to understand how social experience influences plasticity in how our genes are regulated and how the “extended” genome, encoded by our microbiome, is shaped by social interaction. This work helps clarify the role of social behavior in biology, including potential mechanisms that contribute to the powerful links between social interactions and life outcomes.