Three-domain system

The three-domain system classifies cellular life into Bacteria, Archaea, and Eukarya, chiefly reflecting deep evolutionary relationships inferred from molecular evidence.

Bacteria: A domain of prokaryotic organisms whose cells lack a nucleus and whose membranes typically contain ester-linked fatty acids. One of the three primary lineages recognized by this classification.
Carl Woese: An American microbiologist who used ribosomal RNA comparisons to reveal the distinct lineage of Archaea. His molecular analyses established the evidence behind the three-domain proposal.
Universal common ancestor: The ancestral population from which all known cellular life ultimately descends. The three domains are often used to frame hypotheses about life’s earliest shared ancestry.
Two-domain system: A classification that groups cellular life into Bacteria and Archaea, placing eukaryotes within the archaeal lineage. It challenges the three-domain system’s treatment of Eukarya as a separate primary domain.
Archaea: A domain of prokaryotic organisms distinct from bacteria in molecular machinery, membrane chemistry, and evolutionary history. Its recognition as a separate domain is the system’s defining departure from older classifications.
George E. Fox: An American microbiologist who collaborated with Carl Woese on ribosomal RNA studies of early cellular evolution. His work helped identify Archaea as a lineage separate from bacteria.
Eukaryogenesis: The evolutionary process by which the first eukaryotic cells arose from ancestral lineages. Evidence that eukaryotes emerged from archaeal ancestry complicates a simple three-branch picture.
Three-kingdom system: A biological classification scheme that recognizes three kingdoms, with membership varying by proposed system. Earlier three-kingdom schemes divide life differently and do not match the molecular basis of domains.
Eukaryota: A domain of organisms whose cells contain nuclei and membrane-bound organelles. The third domain groups organisms with cells organized around a nucleus.
16S ribosomal RNA: A small-subunit ribosomal RNA found in bacteria and archaea, widely used to compare their evolutionary relationships. Its sequence comparisons made deep differences among prokaryotes measurable.

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