The three-domain system is a
biological classification
In biology
Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...
introduced by
Carl Woese
Carl Richard Woese (; July 15, 1928 – December 30, 2012) was an American microbiologist
A microbiologist (from Greek ) is a scientist
A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an B ...

''et al.'' in 1990
that divides
cellular life
The cell (from Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
forms into ''
archaea
Archaea ( ; singular archaeon ) constitute a domain
Domain may refer to:
Mathematics
*Domain of a function, the set of input values for which the (total) function is defined
**Domain of definition of a partial function
**Natural domain of a pa ...

'', ''
bacteria
Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

'', and ''
eukaryote
Eukaryotes () are organism
In biology
Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

''
domains. The key difference from earlier classifications is the splitting of ''archaea'' from ''bacteria''.
Background
Woese argued, on the basis of differences in
16S rRNA 16S rRNA may refer to:
* 16S ribosomal RNA, the prokaryotic ribosomal subunit
* Mitochondrially encoded 16S RNA, the eukaryotic ribosomal subunit
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