The scientific community was understandably shocked in the late 1970s by the discovery of an entirely new group of organisms -- the Archaea. Dr. Carl Woese and his colleagues at the University of Illinois were studying relationships among the prokaryotes using DNA sequences, and found that there were two distinctly different groups. Those "bacteria" that lived at high temperatures or produced methane clustered together as a group well away from the usual bacteria and the eukaryotes. Because of this vast difference in genetic makeup, Woese proposed that life be divided into three domains: Eukaryota, Eubacteria, and Archaebacteria. He later decided that the term Archaebacteria was a misnomer, and shortened it to Archaea.
Archaea are a domain of single-celled microorganisms; they do not have no cell nucleus or any other organelles inside their cells. Initially, Archaea were classified as an unusual group of bacteria and named archaebacteria, but since the Archaea have an independent evolutionary history and manifest numerous differences in their biochemistry from other forms of life, they are now classified as a separate domain in the three-domain system.
Archaea replicate asexually in a process known as binary fission. Archaea achieve a swimming motility via one or more tail-like flagellae. Many archaeans are extremophiles, achieving wide environmental tolerance of temperature, salinity, and even radioactive environments. Archaea are thought to be significant in global geochemical cycling, since they comprise an estimated 20 percent of the world's biomass; however, very little is known about the domain, especially marine and deep-sea benthic varieties.
In this system the three primary branches of evolutionary descent are the Archaea, Eukarya and Bacteria. Archaea are further divided into four recognized phyla, although other phyla may exist. Of these groups the Crenarchaeota and the Euryarchaeota are most intensively studied. Classifying the archaea is somewhat challenging, since the vast majority have never been studied, and have chiefly been detected by analysis of their nucleic acids in samples from the environment.