One of the three domains of life (together with Bacteria and Eukaryotes). Their morphology is similar to bacteria but many cellular mechanisms are actually closer to eukaryotic than bacterial. They form a distinct clade in the phylogenetic analysis based on the 16S rRNA sequence. Archaea are at least as far from bacteria as from eukaryotes (there is a lot of discussion about exactly what the "tree of life" looks like) but they are often placed with bacteria into the common grouping called prokaryotes. Originally they were discovered in extreme environments and thus are still often thought of as extremophiles. Now archaea are known to be very common in nature, being a dominating group of microorganisms for example in oceans below the photic zone.
--Katarzyna Zaremba 15:00, 27 February 2008 (CET)