The two-empire system (two-superkingdom system) was the top-level biological classification system in general use before the establishment of the three-domain system. It classified life into Prokaryota and Eukaryota. 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.
Some prominent scientists, such as Thomas Cavalier-Smith, still hold to the two-empire system. 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." 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.
|Taxonomical root node||Two superdomains (controversial)||Two empires||Three domains||Six kingdoms|
|Biota / Vitae
|Acytota / Aphanobionta (Viruses, Viroids, Prions?, ...) non-cellular life|
|Prokaryota / Procarya
|Eukaryota / Eukarya||Protista|
- Cavalier-Smith, T (2002). "The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification". International Journal of Systematic and Evolutionary Microbiology. 52 (1): 7–76. doi:10.1099/00207713-52-1-7. PMID 11837318.
- Mayr, E. (1998). "Two empires or three?". Proceedings of the National Academy of Sciences. 95 (17): 9720–9723. Bibcode:1998PNAS...95.9720M. doi:10.1073/pnas.95.17.9720. PMC 33883. PMID 9707542.
- Gupta, Radhey S (1998). "Life's Third Domain (Archaea): An Established Fact or an Endangered Paradigm?". Theoretical Population Biology. 54 (2): 91–104. doi:10.1006/tpbi.1998.1376. PMID 9733652.