Uncover the surprising ancient origins of mammals, a story that began long before the dinosaurs' reign. Surveying Earth's tumultuous past reveals how some of our earliest ancestors, the therapsids – creatures that looked like reptiles but were starting to develop mammalian traits – survived multiple global catastrophes. Through remarkable adaptations like burrowing and the emergence of warm-bloodedness, they forged a lineage that persevered against all odds. Fascinating fossil clues and cutting-edge science reveal how a small, resilient group of animals faced extinction events, outlived giants, and ultimately gave rise to the diverse world of mammals we know today, including us.