Three-domain system

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A phylogenetic tree based on rRNA data, emphasizing the separation of bacteria, archaea, and eukarya as proposed by Carl Woese et al. in 1990, with the hypothetical last universal common ancestor Phylogenetic tree of life 1990 LUCA.svg
A phylogenetic tree based on rRNA data, emphasizing the separation of bacteria, archaea, and eukarya as proposed by Carl Woese et al. in 1990, with the hypothetical last universal common ancestor

The three-domain system is a taxonomic classification system that groups all cellular life into three domains, namely Archaea, Bacteria and Eukarya, introduced by Carl Woese, Otto Kandler and Mark Wheelis in 1990. [1] The key difference from earlier classifications such as the two-empire system and the five-kingdom classification is the splitting of Archaea (previously named "archaebacteria") from Bacteria as completely different organisms. It has been challenged by the two-domain system that divides organisms into Bacteria and Archaea only, as Eukaryotes are considered as a clade of Archaea. [2] [3] [4]



Woese argued, on the basis of differences in 16S rRNA genes, that bacteria, archaea, and 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. Originally his split of the prokaryotes was into Eubacteria (now Bacteria) and Archaebacteria (now Archaea). [5] 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. [1]

Acceptance of the validity of Woese's phylogenetically valid classification was a slow process. Prominent biologists including Salvador Luria and Ernst Mayr objected to his division of the prokaryotes. [6] [7] Not all criticism of him was restricted to the scientific level. A decade of labor-intensive oligonucleotide cataloging left him with a reputation as "a crank", and Woese would go on to be dubbed "Microbiology's Scarred Revolutionary" by a news article printed in the journal Science in 1997. [8] The growing amount of supporting data led the scientific community to accept the Archaea by the mid-1980s. [9] Today, very few scientists still accept the concept of a unified Prokarya. [10]


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Staphylococcus aureus VISA 2.jpg
Australia green tree frog (Litoria caerulea) crop.jpg
The three-domain system includes the Archaea (represented by Sulfolobus , left), Bacteria (represented by Staphylococcus aureus , middle) and Eukaryotes (represented by the Australian green tree frog, right).

The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems. This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus. The three-domain system sorts the previously known kingdoms into these three domains: Archaea, Bacteria, and Eukarya. [2]

Domain Archaea

The Archaea are prokaryotic, with no nuclear membrane, but with biochemistry and RNA markers that are distinct from bacteria. The Archaeans possess unique, ancient evolutionary history for which they are considered some of the oldest species of organisms on Earth, most notably their diverse, exotic metabolisms.

Some examples of archaeal organisms are:

Domain Bacteria

The Bacteria are also prokaryotic; their domain consists of cells with bacterial rRNA, no nuclear membrane, and whose membranes possess primarily diacyl glycerol diester lipids. Traditionally classified as bacteria, many thrive in the same environments favored by humans, and were the first prokaryotes discovered; they were briefly called the Eubacteria or "true" bacteria when the Archaea were first recognized as a distinct clade.

Most known pathogenic prokaryotic organisms belong to bacteria (see [11] for exceptions). For that reason, and because the Archaea are typically difficult to grow in laboratories, Bacteria are currently studied more extensively than Archaea.

Some examples of bacteria include:

Domain Eukarya

Eukaryota are organisms whose cells contain a membrane-bound nucleus. They include many large single-celled organisms and all known non-microscopic organisms. The domain contains, for example:


Each of the three cell types tends to fit into recurring specialities 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 is likely to have derived from a joining of different cell types, forming organelles.

Parakaryon myojinensis ( incertae sedis ) is a single-celled organism known to be a unique example. "This organism appears to be a life form distinct from prokaryotes and eukaryotes", [12] with features of both.


Alternative versions of the three domains of life's phylogeny 3 domains.png
Alternative versions of the three domains of life's phylogeny

Parts of the three-domain theory have been challenged by scientists including Ernst Mayr, Thomas Cavalier-Smith, and Radhey S. Gupta. [13] [14] [15]

Recent work has proposed that Eukaryota may have actually branched off from the domain Archaea. According to Spang et al. Lokiarchaeota forms a monophyletic group with eukaryotes in phylogenomic analyses. The associated genomes also encode an expanded repertoire of eukaryotic signature proteins that are suggestive of sophisticated membrane remodelling capabilities. [16] This work suggests a two-domain system as opposed to the three-domain system. [3] [4] [2] Exactly how and when archaea, bacteria, and eucarya developed and how they are related continues to be debated. [17] [2] [18]

See also

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<span class="mw-page-title-main">Carl Woese</span> American microbiologist (1928–2012)

Carl Woese was an American microbiologist and biophysicist. Woese is famous for defining the Archaea in 1977 through a pioneering phylogenetic taxonomy of 16S ribosomal RNA, a technique that has revolutionized microbiology. He also originated the RNA world hypothesis in 1967, although not by that name. Woese held the Stanley O. Ikenberry Chair and was professor of microbiology at the University of Illinois Urbana–Champaign.

In biology, a kingdom is the second highest taxonomic rank, just below domain. Kingdoms are divided into smaller groups called phyla.

In biological taxonomy, a domain, also dominion, superkingdom, realm, or empire, is the highest taxonomic rank of all organisms taken together. It was introduced in the three-domain system of taxonomy devised by Carl Woese, Otto Kandler and Mark Wheelis in 1990.

<span class="mw-page-title-main">Thermoproteota</span> Phylum of archaea

The Thermoproteota are prokaryotes that have been classified as a phylum of the Archaea domain. Initially, the Thermoproteota were thought to be sulfur-dependent extremophiles but recent studies have identified characteristic Thermoproteota environmental rRNA indicating the organisms may be the most abundant archaea in the marine environment. Originally, they were separated from the other archaea based on rRNA sequences; other physiological features, such as lack of histones, have supported this division, although some crenarchaea were found to have histones. Until recently all cultured Thermoproteota had been thermophilic or hyperthermophilic organisms, some of which have the ability to grow at up to 113°C. These organisms stain Gram negative and are morphologically diverse, having rod, cocci, filamentous and oddly-shaped cells.

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