Stephen Stearns, teaching Yale's Evolution and Medicine course, examines the concept of evolutionary mismatch, the idea that many modern diseases arise because human bodies evolved for environments and lifestyles very different from those we live in now. This segment focuses on evidence drawn from comparisons across time and across geography, showing how disease patterns shift when populations move into new environments or when environments change around them. Drawing on the Sinauer textbook Evolutionary Medicine, Stearns lays out the kind of data researchers use to test whether a health problem looks like a mismatch condition, tracing changes in diet, activity, and exposure over historical time and across different regions of the world. The lecture is a short, focused segment within a longer course sequence on evolutionary approaches to human health.