An archaeal origin of eukaryotes supports only two primary domains of life

Accumulating evidence that the eukaryotic nuclear lineage originated from within the Archaea provides support for a tree containing only two primary domains of life—the Achaea and Bacteria—over the currently accepted ‘three-domains tree’. Ever since the discovery in 1977 of a group of microorganisms called the Archaea, researchers have generally assumed that all life on Earth can be arranged into three domains: the Bacteria and Archaea, both lacking nuclei but clearly different from one another, and the eukaryotes, which have nucleated cells. But stimulated by the discovery of lineages of environmental Archaea containing genes previously thought specific to eukaryotes, there has been increasing support for a two-domain model of life, in which the eukaryotes evolved from within the Archaea. In this Review, Martin Embley and colleagues conclude that increasing knowledge of archaeal diversity, together with improvements in reconstructing long-sundered phylogenies, now favour the two-domain view. The discovery of the Archaea and the proposal of the three-domains ‘universal’ tree, based on ribosomal RNA and core genes mainly involved in protein translation, catalysed new ideas for cellular evolution and eukaryotic origins. However, accumulating evidence suggests that the three-domains tree may be incorrect: evolutionary trees made using newer methods place eukaryotic core genes within the Archaea, supporting hypotheses in which an archaeon participated in eukaryotic origins by founding the host lineage for the mitochondrial endosymbiont. These results provide support for only two primary domains of life—Archaea and Bacteria—because eukaryotes arose through partnership between them.