Me Interesa

Proc. Nati. Acad. Sci. USA Vol. 87, pp. 4576-4579, June 1990 Evolution

Towards a natural system of organisms: Proposal for the domains Archaea, Bacteria, and Eucarya
(Euryarchaeota/Crenarchaeota/kingdom/evolution)

CARL R. WOESE*t, OTTO KANDLERt,
Contributed by Carl R. Woese, March 26, 1990

AND

MARK L. WHEELIS§
Munchen, Menzinger Strasse 67,

*Department of Microbiology,University of Illinois, 131 Burrill Hall, Urbana, IL 61801; tBotanisches Institut der Universitat 8000 Munich 19, Federal Republic of Germany; and §Department of Microbiology, University of California, Davis, CA 95616

Molecular structures and sequences are genABSTRACT erally more revealing of evolutionary relationships than are classical phenotypes (particularly so among microorganisms). Consequently,the basis for the definition of taxa has progressively shifted from the organismal to the cellular to the molecular level. Molecular comparisons show that life on this planet divides into three primary groupings, commonly known as the eubacteria, the archaebacteria, and the eukaryotes. The three are very dissimilar, the differences that separate them being of a more profound nature than thedifferences that separate typical kingdoms, such as animals and plants. Unfortunately, neither of the conventionally accepted views of the natural relationships among living systems-i.e., the five-kingdom taxonomy or the eukaryote-prokaryote dichotomy-reflects this primary tripartite division of the living world. To remedy this situation we propose that a formal system of organisms be established inwhich above the level of kingdom there exists a new taxon called a "domain." Life on this planet would then be seen as comprising three domains, the Bacteria, the Archaea, and the Eucarya, each containing two or more kingdoms. (The Eucarya, for example, contain Animalia, Plantae, Fungi, and a number of others yet to be defined.) Although taxonomic structure within the Bacteria and Eucarya is nottreated herein, Archaea is formally subdivided into the two kingdoms Euryarchaeota (encompassing the methanogens and their phenotypically diverse relatives) and Crenarchaeota (comprising the relatively tight clustering of extremely thermophilic archaebacteria, whose general phenotype appears to resemble most the ancestral phenotype of the Archaea).
Need for Restructuring Systematics

Within the lastdecade it has become possible to trace evolutionary history back to the (most recent) common ancestor of all life, perhaps 3.5-4 billion years ago (1, 2). Prior to the mid 1970s evolutionary study had for all intents and purposes been confined to the metazoa and metaphyta, whose histories at best cover 20% of the total evolutionary
time span. A sound basis for a natural taxonomy was provided inthese cases by complex morphologies and a detailed fossil record. The evolution of the microbial world-whose history spans most of the planet's existence-was at that time beyond the biologist's purview, for, unlike their multicellular equivalents, microbial morphologies and other characteristics are too simple or uninterpretable to serve as the basis for a phylogenetically valid taxonomy (3, 4).The sequencing revolution, by making accessible the vast store of historical information contained in molecular sequences (5), has changed all that. As a result, the biologist finds that textbook descriptions of the basic organization of life have become
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outmoded and so, misleading. The time has come to bring formal taxonomy into line with the natural system emerging from molecular data. This revision, however, is not accomplished simply by emending the old system. Our present view of the basic organization of life is still largely steeped in the ancient notion that all...