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Systematics of Biological Systematics

Well before Linnaeus, plants and animals were considered separate Kingdoms.[51] Linnaeus used this as the top rank, dividing the physical world into the plant, animal and mineral kingdoms. As advances in microscopy made classification of microorganisms possible, the number of kingdoms increased, five and six-kingdom systems being the most common.

Domains are a relatively new grouping. First proposed in 1977, Carl Woese‘s three-domain system was not generally accepted until later.[52] One main characteristic of the three-domain method is the separation of Archaea and Bacteria, previously grouped into the single kingdom Bacteria (a kingdom also sometimes called Monera),[51] with the Eukaryota for all organisms whose cells contain a nucleus.[53] A small number of scientists include a sixth kingdom, Archaea, but do not accept the domain method.[51]

Thomas Cavalier-Smith, who has published extensively on the classification of protists, has recently proposed that the Neomura, the clade that groups together the Archaea and Eucarya, would have evolved from Bacteria, more precisely from Actinobacteria. His 2004 classification treated the archaeobacteria as part of a subkingdom of the kingdom Bacteria, i.e. he rejected the three-domain system entirely.[54] Stefan Luketa in 2012 proposed a five “dominion” system,