Three-Domain System

The three-domain system is a classification of all cellular life into three highest-level groups, called domains: Bacteria, Archaea, and Eukarya. American microbiologist Carl Woese, together with George Fox, proposed the system in a 1990 paper based on comparisons of ribosomal RNA sequences across a wide range of organisms, which revealed that the previously unified group of prokaryotes, single-celled organisms lacking a cell nucleus, in fact comprised two lineages, Bacteria and Archaea, as genetically distinct from one another as either is from Eukarya, the domain comprising all organisms whose cells contain a nucleus. Woese had first identified Archaea as a distinct group in 1977, initially calling them archaebacteria, but the full three-domain framework replacing the older five-kingdom and two-empire classification systems was not proposed until 1990. The system represented a significant revision of the tree of life, since it placed the split between Bacteria and the Archaea-Eukarya lineage as the deepest branching point among all known life, and it is now the standard framework used in molecular phylogenetics and microbiology, though the precise relationships among the three domains remain an area of active research.