Two-empire system

The two-empire system (two-superkingdom system) was the top-level biological classification system in general use from the early 20th century until the establishment of the three-domain system (which itself is currently being challenged by the two-domain system). It classified cellular life into Prokaryota and Eukaryota as either "empires" or "superkingdoms". When the three-domain system was introduced, some biologists preferred the two-superkingdom system, claiming that the three-domain system overemphasized the division between Archaea and Bacteria. However, given the current state of knowledge and the rapid progress in biological scientific advancement, especially due to genetic analyses, that view has all but vanished.

Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes and prokaryotes

Some prominent scientists, such as the late Thomas Cavalier-Smith, still hold and held to the two-empire system.^[1] The late Ernst Mayr, one of the 20th century's leading evolutionary biologists, wrote dismissively of the three-domain system, "I cannot see any merit at all in a three empire cladification."^[2] Additionally, the scientist Radhey Gupta argues for a return to the two-empire system, claiming that the primary division within prokaryotes should be among those surrounded by a single membrane (monoderm), including gram-positive bacteria and archaebacteria, and those with an inner and outer cell membrane (diderm), including gram-negative bacteria.^[3]

This system was preceded by Haeckel's three-kingdom system: Animalia, Plantae and Protista.

Linnaeus 1735[4]	Haeckel 1866[5]	Chatton 1925[6][7]	Copeland 1938[8][9]	Whittaker 1969[10]	Woese et al. 1977[11][12]	Woese et al. 1990[13]	Cavalier-Smith 1993[14][15][16]	Cavalier-Smith 1998[17][18][19]	Ruggiero et al. 2015[20]
—	—	2 empires	2 empires	2 empires	2 empires	3 domains	3 superkingdoms	2 empires	2 superkingdoms
2 kingdoms	3 kingdoms	—	4 kingdoms	5 kingdoms	6 kingdoms	—	8 kingdoms	6 kingdoms	7 kingdoms
—	Protista	Prokaryota	Monera	Monera	Eubacteria	Bacteria	Eubacteria	Bacteria	Bacteria
					Archaebacteria	Archaea	Archaebacteria		Archaea
		Eukaryota	Protista	Protista	Protista	Eucarya	Archezoa	Protozoa	Protozoa
							Protozoa
							Chromista	Chromista	Chromista
Vegetabilia	Plantae		Plantae	Plantae	Plantae		Plantae	Plantae	Plantae
				Fungi	Fungi		Fungi	Fungi	Fungi
Animalia	Animalia		Animalia	Animalia	Animalia		Animalia	Animalia	Animalia

See also

• Domain (biology)
• Kingdom (biology)
• Three-domain system