In nucleus. All life that has a nucleus and membrane-bound organelles, and most multi-cellular life, is included in the Eukarya.
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 eocyte hypothesis, first proposed by James A. Lake et al. in 1984, which posits two domains (Bacteria and Archaea, with Eukarya included in Archaea).
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.
- 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.
- Mayr, Ernst (1998). "Two empires or three?". Proc Natl Acad Sci USA 95 (17): 9720–9723.
- Archibald, John M. (23 December 2008). "The eocyte hypothesis and the origin of eukaryotic cells". PNAS 105 (51): 20049–20050.
- 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.
- 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.
- 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