Archaea

Archaea are a major division of microorganisms. Like bacteria, archaea are single-celled organisms lacking nuclei and are therefore prokaryotes, classified as belonging to kingdom Monera in the traditional five-kingdom taxonomy. Although there is still uncertainty in the approximate phylogeny of the groups, Archaea, Eukaryota and Bacteria are the fundamental classifications in what is called the three-domain system. Despite being prokaryotes, archaea are more closely related to eukaryotes than to bacteria. Archaea and eukaryotes are therefore grouped together in the clade Neomura, which is thought to have arisen from Gram-positive bacteria. Archaea were originally described in extreme environments, but have since been found in all types of habitat.