Australian green tree frog (Litoria caerulea)
Scanning electron micrograph of S. aureus; false color added
Electron micrograph of Sulfolobus infected with Sulfolobus virus STSV1.
The three-domain system includes Eukarya (represented by the Australian green tree frog, left), Bacteria (represented by S. aureus, middle) and Archaea (represented by Sulfolobus, right).

In nucleus. All life that has a nucleus and membrane-bound organelles, and most multi-cellular life, is included in the Eukarya.

Alternative classifications

A speculatively rooted tree for rRNA genes, showing major branches Bacteria, Archaea, and Eukaryota

Alternative classifications of life so far proposed include:

Exclusion of viruses

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. [...] 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.[7]

See also


  1. ^ 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.  
  2. ^ Mayr, Ernst (1998). "Two empires or three?". Proc Natl Acad Sci USA 95 (17): 9720–9723.  
  3. ^  
  4. ^ Archibald, John M. (23 December 2008). "The eocyte hypothesis and the origin of eukaryotic cells". PNAS 105 (51): 20049–20050.  
  5. ^ Lake, James A.; Henderson, Eric; Oakes, Melanie; Clark, Michael W. (June 1984). "Eocytes: A new ribosome structure indicates a kingdom with a close relationship to eukaryotes". PNAS 81 (12): 3786–3790.  
  6. ^ Williams, Tom A.; Foster, Peter G.; Cox, Cymon J.; Embley, T. Martin (December 2013). "An archaeal origin of eukaryotes supports only two primary domains of life". Nature 504 (7479): 231–236.  
  7. ^ 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