ARCHAEA Archaea; from Greek, "ancient ones"; (singular Archaeum, Archaean, or Archaeon), also called Archaebacteria, is a major division of living organisms. 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. Like bacteria, Archaea are single-celled organisms lacking nuclei and are therefore prokaryotes, classified as belonging to kingdom Monera in the traditional six-kingdom taxonomy. They were originally described in extreme environments, but have since been found in all types of habitat. A single organism from this domain has been called an "archaean." Furthermore, this biologic term is also used as an adjective. Contents 1 History 2 Archaea, Bacteria and Eukaryotes 3 Habitats 4 Form 5 Evolution and classification 6 Biologists who have studied Archaea 7 See also 8 References 9 Further reading and external links History Archaea were identified in 1977 by Carl Woese and George E. Fox as being a separate branch based on their separation from other prokaryotes on 16S rRNA phylogenetic trees. These two groups were originally named the Archaebacteria and Eubacteria, treated as kingdoms or subkingdoms, which Woese and Fox termed Urkingdoms. Woese argued that they represented fundamentally different branches of living things. He later renamed the groups Archaea and Bacteria to emphasize this, and argued that together with Eukarya they compose three Domains of living organisms. The biological term, Archaea, should not be confused with the geologic phrase Archean eon, also known as the Archeozoic era. This latter term refers to the primordial period of earth history when Archaea and Bacteria were the only cellular organisms living on the planet. Probable fossils of these microbes have been dated to almost 3.8 billion years ago. Their remains have been found in sediment from western Greenland, the oldest sediment to be discovered. (3,800,000,000 years ago). * * * * * * * * *