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( Archaea)
Crenarchaeota
Euryarchaeota
Korarchaeota
Nanoarchaeota
ARMAN The Archaea (pronounced /?r'ki??/) are a group of prokaryotic, single-celled microorganisms. In this they are similar to bacteria but these two groups evolved differently, and are classified as different domains in the three-domain system. Originally these organisms were named archaebacteria. However, this term has not been favored since the three-domain system became popular. Although there is still uncertainty in the phylogeny, Archaea, Eukaryota and Bacteria were introduced as the fundamental classifications in what would later become the three-domain system by Carl Woese in 1977. As prokaryotes, archaea are also classified in kingdom Monera in the traditional five-kingdom Linnaean taxonomy. While their prokaryotic cell structure is similar to Bacteria, the genes of Archaea and several of their metabolic pathways are more closely related to those of eukaryotes. One way to account for this is to group archaeans and eukaryotes together in the clade Neomura, which might have arisen from gram-positive bacteria. On the other hand, other studies have suggested that Archaea may instead be the most ancient lineage in the world, with bacteria and eukaryotes diverging from this group.[1] Archaea were originally described in extreme environments, but have since been found in all habitats and may contribute up to 20% of total biomass.[2] These cells are particularly common in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet.[3] A single individual or species from this domain is called an archaeon (sometimes spelled "archeon"),[4] while the adjectival form is archaeal or archaean. The etymology is Ancient Greek, from ???a?a meaning "ancient things".
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