The
three-domain system is a
biological classification introduced by
Carl Woese et al. in 1977 that divides
cellular life forms into
archaea,
bacteria, and
eukaryote domains. In particular, it emphasizes the separation of
prokaryotes into two groups, originally called
Eubacteria (now
Bacteria) and
Archaebacteria (now
Archaea). Woese argued that, on the basis of differences in
16S rRNA genes, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed
genetic machinery, often called a
progenote. To reflect these primary lines of descent, he treated each as a domain, divided into several different
kingdoms. Woese initially used the term "kingdom" to refer to the three primary phylogenic groupings, and this nomenclature was widely used until the term "domains" was adopted in 1990.