Stephen Stearns, teaching Yale's Evolution and Medicine course, examines how mammalian reproduction evolved and what that evolutionary history explains about modern reproductive biology and disease. Drawing on the textbook Evolutionary Medicine, Stearns traces the shift from egg-laying ancestors to live birth and lactation, and considers the trade-offs natural selection made along the way, from placental function to the timing of maternal investment. The lecture sits within a larger sequence on reproduction, part of a broader course applying evolutionary theory to clinical questions. Stearns keeps the argument grounded in mechanism: why certain reproductive strategies persisted, what constraints shaped them, and how understanding those origins helps explain present-day vulnerabilities such as pregnancy complications or reproductive cancers. The style is plain chalk-and-talk lecturing aimed at students already following the course, not a standalone popular science piece.