At the close of the 1970s, the two-domain classification scheme long used by most biologists—prokaryotes versus eukaryotes—was upended by the discovery of an entirely new group of organisms: archaea. Initially thought to be bacteria, these single-celled microbes—many of which were first found in seemingly unlivable habitats like the volcanic hot springs of Yellowstone National Park—were in fact so different at molecular and genetic levels as to constitute a separate, third domain beside bacteria and eukaryotes. Their discovery sparked a conceptual revolution in our understanding of the evolution of life, and Patrick Forterre was—and still is—at the vanguard of this revolution.