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The Archaea are a major group of prokaryotes. They were first identified in 1977 by Carl Woese and George Fox 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. Woese argued that they represented fundamentally different branches of living things, and later renamed the groups Archaea and Bacteria to emphasize this.

Archaea are similar to other prokaryotes in most aspects of cell structure and metabolism. However, their transcription and translation - the two central process in molecular biology - do not show the typical bacterial features, but are extremely similar to those of eukaryotes. For instance, archaean translation uses eukaryotic initiation and elongation factors, and their transcription involves TATA-binding proteins and TFIIB as in eukaryotes.

Several other characteristics also set the Archaea apart. Unlike most bacteria, they have a single cell membrane that lacks a peptidoglycan wall. Further, both bacteria and eukaryotes have membranes composed mainly of glycerol- ester lipids, whereas archaea have membranes composed of glycerol- etherFor ether in physics and philosophy, see Aether. For the Book of Mormon Prophet Ether see Book of Ether. Ether is the trivial name for the compound diethyl ether CHCHOCHCH; the systematic ( IUPAC) name of the compound is ethoxyethane . Alchemist Raymundus lipids. These differences may be an adaptation on the part of Archaea to hyperthermophilyA hyperthermophile is an organism that thrives in extremely hot environments that is, hotter than around 60°C; in fact, the recently-discovered "Strain 121" has been able to double its population during 24 hours in an autoclave at 121°C (hence its name).. Archaeans also have flagellaThe flagellum (plural: flagella is a propulsive structure used by many single-celled organisms to move through a liquid medium. There are three main varieties of flagellum; the bacterial flagellum (a helical filament that rotates like a screw), archaeal f that are notably different in composition and development from the superficially similar flagella of bacteria.

Many archaeans are extremophileAn extremophile is an organism, usually unicellular, which thrives in or requires "extreme" conditions. The definition of "extreme" is anthropocentric, of course. To the organism itself its environment is completely normal. Non-extremophilic organisms ares. Some live at very high temperatures, often above 100°C, as found in geyserA geyser is a special type of hot spring that erupts periodically, ejecting a column of hot water and steam into the air. The name geyser comes from Geysir the name of the best-known geyser in Iceland; that name, in turn, comes from the word gjosa "to guss and black smokerBlack smokers are a type of hydrothermal vent found on the ocean floor. Generally hundreds of meters wide, black smokers are formed when superheated water from below the Earth's crust comes through the ocean floor. They are rich in dissolved minerals froms. Others are found in very cold habitats or in highly salineFor other meanings of the word salt see salt (disambiguation In chemistry, a salt is a composed of positively charged cations and negatively charged anions, so that the product is neutral and without a net charge. They are typically the product of a chemi, acidFor alternative meanings see acid (disambiguation). An acid (represented by the generic formula AH is typically a water-soluble, sour-tasting chemical compound. An acid always has a pH of less than 7. It is a molecule or ion that contains hydrogen or thatic, or alkaline water. However, other archaeans are mesophiles, and have been found in environments like marshland, sewage, and soil. Many methanogenic archaea are found in the digestive tracts of animals such as ruminants, termites, and humans. Archaea are usually harmless to other organisms and none is known to cause disease.

Individual archaeans range from 0.1 to over 15 μm in diameter, and some form aggregates or filaments up to 200 μm in length. They occur in various shapes, such as spherical, rod-shaped, spiral, lobed, or rectangular. They also exhibit a variety of different types of metabolism. For instance the halobacteria can use light to produce ATP, although none conduct photosynthesis in the usual sense. Archaea are divided into two main groups based on rRNA trees, the Euryarchaeota and Crenarchaeota. Two other groups have been tentatively created for certain environmental samples and the peculiar species Nanoarchaeum equitum, but their affinities are uncertain.

Woese argued that the bacteria, archaea, and eukaryotes each represent a primary line of descent that diverged early on from an ancestral progenote with poorly developed genetic machinery. This hypothesis is reflected in the name Archaea, from the Greek archae or ancient. Later he treated these groups formally as domains, each comprising several kingdoms. This division has become very popular, although the idea of the progenote itself is not generally supported. Some biologists, however, have argued that the archaebacteria and eukaryotes arose from specialized eubacteria.

The relationship between archaeans and eukaryotes remains an important problem. Aside from the similarities noted above, many genetic trees group the two together, and in some the eukaryotes are placed closer to the Eurarchaeota than the Crenarchaeota are, although the membrane chemistry suggests otherwise. However, the discovery of archaean-like genes in certain bacteria, such as Thermotoga, makes their relationship difficult to determine. Some have suggested that eukaryotes arose through fusion of an archaean and eubacterium, which became the nucleus and cytoplasm, which accounts for various genetic similarities but runs into difficulties explaining cell structure.