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...