Archaea are a group of prokaryotes (cells that lack nuclei and other organelles) that were first discovered in extreme environments such as hot springs, acid waters and hypersaline ponds.^[1] In the past they were viewed as an unusual group of bacteria and named archaebacteria but since the Archaea have an independent evolutionary history and show many differences in their biochemistry from other forms of life, they are now classified as a separate domain.

Archaea are further divided into four recognized phyla, but many more phyla may exist. Of these groups the Crenarchaeota and the Euryarchaeota are most intensively studied. Classifying the Archaea is still difficult, since the vast majority of these organisms have never been studied in the laboratory and have only been detected by analysis of their nucleic acids in samples from the environment.

Gallery: Archaea