Three-domain system

The three-domain system is a biological classification introduced by Carl Woese et al. in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria (now Bacteria) and Archaebacteria (now Archaea). Woese argued that, on the basis of differences in 16S rRNA genes, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed genetic machinery, often called a progenote. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdoms. Woese initially used the term "kingdom" to refer to the three primary phylogenic groupings, and this nomenclature was widely used until the term "domain" was adopted in 1990.

The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems. This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as archaea appear to be more closely related to eukaryotes than they are to other prokaryotic bacteria. The current system has the following listed kingdoms in the three domains:

Domain Archaea – prokaryotic, no nuclear membrane, distinct biochemistry and RNA markers from bacteria, possess unique ancient evolutionary history for which they are considered some of the oldest species of organisms on Earth; traditionally classified as archaebacteria; often characterized by living in extreme environments. Some examples of archaeal organisms are methanogens which produce the gas methane, halophiles which live in very salty water, and thermoacidophiles which thrive in acidic high temperature water.

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