In 1969, Carl Woese began using molecular sequences of RNA to study the evolutionary history of life on Earth, eventually determining the first “tree of life.” This project led, in 1977, to the discovery of a third branch of life: the archaea microorganisms distinct from bacteria, which they resemble, and eukaryotes, including plants and animals. The concepts and discoveries emanating from this work transformed biology, particularly evolution, ecology, and microbiology.