Archaebacteria Eubacteria Kingdom

              In the past Archaea had been classed with bacteria as prokaryotes (or Kingdom Monera) and named archaebacteria, but this classification is regarded as outdated. In fact, the Archaea have an independent evolutionary history and show many differences in their biochemistry from other forms of life, and so they are now classified as a separate
domain in the three-domain system. In this system, the phylogenetically distinct branches of evolutionary descent are the Archaea, Bacteria and Eukaryota. So far, the Archaea have been further divided into four recognized phyla; more phyla may be established in the course of future research. Of these groups, the Crenarchaeota and the Euryarchaeota are the most intensively studied. Classification is still difficult, because the vast majority have never been studied in the laboratory and have only been detected by analysis of their nucleic acids in samples from the environment.

Halobacteria sp. strain NRC-1,
each cell about 5 μm long  

New Domain
 Archaea were first classified as a separate group of prokaryotes in 1977 by Carl Woese and George E. Fox in phylogenetic trees based on the sequences of ribosomal RNA (rRNA) genes. These two groups were originally named the Archaebacteria and Eubacteria and treated as kingdoms or subkingdoms, which Woese and Fox termed Urkingdoms. Woese argued that this group of prokaryotes is a fundamentally different sort of life. To emphasize this difference, these two domains were later renamed Archaea and Bacteria. The word archaea comes from the Ancient Greek ἀρχαῖα, meaning "ancient things".

  Archaea were first found in extreme environments, such as volcanic hot springs. Pictured here is Grand Prismatic Spring of Yellowstone National Park.

Species

Biology defines a species as a group of related organisms. The familiar exclusive breeding criterion (organisms that can breed with each other but not with others) is of no help because archaea reproduce asexually.
Archaea show high levels of horizontal gene transfer between lineages. Some researchers suggest that individuals can be grouped into species-like populations given highly similar genomes and infrequent gene transfer to/from cells with less-related genomes, as in the genus Ferroplasma On the other hand, studies in Halorubrum found significant genetic transfer to/from less-related populations, limiting the criterion's applicability. A second concern is to what extent such species designations have practical meaning.

The ARMAN are a new group of archaea recently discovered in acid mine drainage