Three-domain system

The three-domain system is a biological classification introduced by Carl Woese in 1990 that emphasizes his separation of prokaryotes into two groups, originally called Eubacteria and Archaebacteria. Woese argued that, on the basis of differences in 16S rRNA genes, these two groups and the eukaryotes each arose separately from an ancestral progenote with poorly developed genetic machinery. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdoms. The groups were also renamed the Bacteria, Archaea, and Eukarya, further emphasizing the separate identity of the two prokaryote groups.
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