In biological taxonomy, a domain (also superregnum, superkingdom, empire, or regio) is the highest taxonomic rank of organisms in the Woese taxonomic system (there is sometimes a superkingdom Neomura), higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya. The arrangement of taxa reflects the fundamental differences in the genomes.
Alternative classifications 
Alternative classifications of life so far proposed include:
- The two-empire system or superdomain system, with top-level groupings of Prokaryota (or Monera) and Eukaryota.
- The Three-domain system (Bacteria, Archaea, Eukarya) with five supergroups in the Eukarya (Unikonta, Excavata, Chromalveolata, Rhizaria and Archaeplastida)
None of the three systems currently include non-cellular life. As of 2011 there is talk about Nucleocytoplasmic large DNA viruses possibly being a fourth branch domain of life, a view supported by researchers in 2012 who explain in their abstract, "The discovery of giant viruses with genome and physical size comparable to cellular organisms, remnants of protein translation machinery and virus-specific parasites (virophages) have raised intriguing questions about their origin. Evidence advocates for their inclusion into global phylogenomic studies and their consideration as a distinct and ancient form of life." They continue, "Results call for a change in the way viruses are perceived. They likely represent a distinct form of life that either predated or coexisted with the last universal common ancestor (LUCA) and constitute a very crucial part of our planet’s biosphere."
See also 
- Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya.". Proc Natl Acad Sci USA 87 (12): 4576–9. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744. Retrieved 11 February 2010.
- Mayr, Ernst (1998). "Two empires or three?". Proc Natl Acad Sci USA 95 (17): 9720–9723. Bibcode:1998PNAS...95.9720. doi:10.1073/pnas.95.17.9720. PMC 33883. PMID 9707542. Retrieved 5 Sept 2011.
- Cavalier-Smith, T. (2004). "Only six kingdoms of life". Proc. R. Soc. Lond. B 271 (1545): 1251–62. doi:10.1098/rspb.2004.2705. PMC 1691724. PMID 15306349. Retrieved 2010-04-29
- Campbell, N. A., et al. (2008) "Biology." 8th edition. Person International Edition, San Francisco
- Holt, Jack R. and Carlos A. Iudica, (2010) "Taxa of Life." Retrieved 09-03-2011.
- Nasir, Arshan; Kim, Kyung Mo; and Caetano-Anolles, Gustavo, "Giant viruses coexisted with the cellular ancestors and represent a distinct supergroup along with superkingdoms Archaea, Bacteria and Eukarya." BMC Evol Biol. 2012; 12: 156. Published online 2012 August 24. doi: 10.1186/1471-2148-12-156
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