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The '''three-domain system''' is a [[biological classification]] introduced by [[Carl Woese]] in 1990<ref>{{cite journal | author = Woese C, Kandler O, Wheelis M | title = Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. | url=http://www.pnas.org/cgi/reprint/87/12/4576 | journal = Proc Natl Acad Sci USA | volume = 87 | issue = 12 | pages = 4576–9 | year = 1990 | pmid = 2112744 | doi = 10.1073/pnas.87.12.4576}}</ref> that divides cellular life forms into '''[[archaea]]''', '''[[bacteria]]''', and '''[[eukaryote]]''' [[domain (biology)|domain]]s.  In particular, it emphasizes the separation of [[prokaryote]]s into two groups, originally called ''Eubacteria'' and ''Archaebacteria''.  Woese argued that, on the basis of differences in [[16S rRNA]] [[gene]]s, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed [[genetics|genetic]] machinery, often called a  [[progenote]]. To reflect these primary lines of descent, he treated each as a domain, divided into several different [[kingdom (biology)|kingdom]]s.  

[[Image:Phylogenetic tree.png|thumb|right|320px|A phylogenetic tree based on [[rRNA]] data, showing the separation of bacteria, archaea, and eukaryotes.]]
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==Classification==

== Niches ==

Each of the three cell types tends to fit into recurring specialties or roles. Bacteria tend to be the most prolific reproducers, at least in moderate environments. Archaeans tend to adapt quickly to extreme environments, such as high temperatures, high acids, high sulfur, etc. This includes adapting to use a wide variety of food sources. Eukaryotes are the most flexible with regard to forming cooperative colonies, such as in multi-cellular organisms, including humans. In fact, the structure of a Eukaryote itself may have derived from a joining of different cell types, forming [[organelle]]s.

== See also ==

* [[Phylogenetic tree]]
* [[Taxonomy]]
* [[Two-empire system]]
* [[Five-kingdom system]]
* [[Monera]]

==References==
{{Reflist}}

[[Category:Scientific classification]]
[[Category:Classification systems]]

[[es:Sistema de tres dominios]]
[[fr:Classification phylogénétique]]
[[pt:Sistema dos Três Domínios]]
[[zh:三域系統]]
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