Two-empire system

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.[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]

Taxonomical root node Two superdomains (controversial) Two empires Three domains Six kingdoms
Biota / Vitae
Acytota / Aphanobionta (Viruses, Viroids, Prions?, ...) non-cellular life
cellular life
Prokaryota / Procarya
Bacteria Eubacteria
Archaea Archaebacteria
Eukaryota / Eukarya Protista

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

Tree of Living Organisms 2
Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes and prokaryotes

See also


  1. ^ Cavalier-Smith, T (2002). "The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification". Int J Syst Evol Microbiol. 52 (1): 7–76. doi:10.1099/00207713-52-1-7. PMID 11837318.
  2. ^ Mayr, E. (1998). "Two empires or three?". Proc. Natl. Acad. Sci. USA. 95: 9720–9723. doi:10.1073/pnas.95.17.9720. PMC 33883. PMID 9707542.
  3. ^ Gupta, Radhey S (1998). "Life's Third Domain (Archaea)". : An Established Fact or an Endangered Paradigm?: A New Proposal for Classification of Organisms Based on Protein Sequences and Cell Structure." Theoretical Population Biology. 54 (2): 91–104. doi:10.1006/tpbi.1998.1376. PMID 9733652.
Domain (biology)

In biological taxonomy, a domain ( or ) (Latin: regio), also superkingdom or empire, is the highest taxonomic rank of organisms in the three-domain system of taxonomy designed by Carl Woese in 1990.

According to this system, the tree of life consists of three domains: Archaea, Bacteria, and Eukarya. The first two are all prokaryotic microorganisms, or single-celled organisms whose cells have no nucleus. All life that has a nucleus and membrane-bound organelles, and multicellular organisms, is included in the Eukarya.

Kingdom (biology)

In biology, kingdom (Latin: regnum, plural regna) is the second highest taxonomic rank, just below domain. Kingdoms are divided into smaller groups called phyla.

Traditionally, some textbooks from the United States used a system of six kingdoms (Animalia, Plantae, Fungi, Protista, Archaea/Archaebacteria, and Bacteria/Eubacteria) while textbooks in countries like Great Britain, India, Greece, Australia, Latin America and other countries used five kingdoms (Animalia, Plantae, Fungi, Protista and Monera).

Some recent classifications based on modern cladistics have explicitly abandoned the term "kingdom", noting that the traditional kingdoms are not monophyletic, i.e., do not consist of all the descendants of a common ancestor.

Mark Wheelis

Mark L. Wheelis is an American microbiologist. Wheelis is currently a professor in the College of Biological Sciences, University of California, Davis. Carl Woese and Otto Kandler with Wheelis wrote the important paper Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya that proposed a change from the Two-empire system of Prokaryotes and Eukaryotes to the Three-domain system of the domains Eukaryota, Bacteria and Archaea.Wheelis's research interests include the history of biological warfare. He co-authored (with Larry Gonick) The Cartoon Guide to Genetics (1983). Wheelis provided the scientific knowledge and text, while Gonick contributed the illustrations and humor.


A prokaryote is a unicellular organism that lacks a membrane-bound nucleus, mitochondria, or any other membrane-bound organelle. The word prokaryote comes from the Greek πρό (pro) "before" and κάρυον (karyon) "nut or kernel". Prokaryotes are divided into two domains, Archaea and Bacteria. Species with nuclei and organelles are placed in the third domain, Eukaryota. Prokaryotes reproduce without fusion of gametes. The first living organisms are thought to have been prokaryotes.

In the prokaryotes, all the intracellular water-soluble components (proteins, DNA and metabolites) are located together in the cytoplasm enclosed by the cell membrane, rather than in separate cellular compartments. Bacteria, however, do possess protein-based bacterial microcompartments, which are thought to act as primitive organelles enclosed in protein shells. Some prokaryotes, such as cyanobacteria, may form large colonies. Others, such as myxobacteria, have multicellular stages in their life cycles.Molecular studies have provided insight into the evolution and interrelationships of the three domains of biological species. Eukaryotes are organisms, including humans, whose cells have a well defined membrane-bound nucleus (containing chromosomal DNA) and organelles. The division between prokaryotes and eukaryotes reflects the existence of two very different levels of cellular organization. Distinctive types of prokaryotes include extremophiles and methanogens; these are common in some extreme environments.

Three-domain system

The three-domain system is a biological classification introduced by Carl Woese et al. in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria (now Bacteria) and Archaebacteria (now Archaea). Woese argued that, on the basis of differences in 16S rRNA genes, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed genetic machinery, often called a progenote. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdoms. Woese initially used the term "kingdom" to refer to the three primary phylogenic groupings, and this nomenclature was widely used until the term "domain" was adopted in 1990.Parts of the three-domain theory have been challenged by scientists such as Radhey Gupta, who argues that the primary division within prokaryotes should be between those surrounded by a single membrane, and those with two membranes.


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