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A phylogenetic tree based on rRNA data, showing the separation of bacteria, archaea, and eukaryotes.

The three-domain system is a biological classification introduced by Carl Woese in 1990^[1] 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.

[edit] Classification

The three-domain system is loosely based on the traditional five-kingdom system but divides the Monera into two "domains," leaving the remaining eukaryotic kingdoms in the third domain. The current system has the following kingdoms in the three domains:

Archaea Domain - prokaryotic, no nuclear membrane, distinct biochemistry and RNA markers from eubacteria, 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

• Kingdom Archaebacteria
  Examples:

• Methanogens - metabolize hydrogen and carbon dioxide into methane
• Halophiles - thrive in salt
• Thermoacidophiles - thrive in acid and high temperatures (up to 230 degrees F)

Bacteria Domain - prokaryotic, no nuclear membrane, traditionally classified as bacteria, contain all known pathogenic organisms, studied far more extensively than Archaea

• Kingdom Eubacteria
  Examples:

• Cyanobacteria - photosynthesizing bacteria
• Spirochaete - Gram-negative bacteria that include those causing syphilis and Lyme disease
• Firmicutes - Gram-positive bacteria including Bifidobacterium animalis which is present in the human large intestine

Eukarya Domain - eukaryotes, nuclear membrane

• Kingdom Protista or protists
  Examples:

• Rhodophyta - red algae
• Chromalveolata - includes dinoflagellates

• Kingdom Fungi or fungi
  Examples:

• Saccharomycotina - includes true yeasts
• Basidiomycota - includes shiitake mushrooms

• Kingdom Plantae or plants
  Examples:

• Bryophyta - mosses
• Magnoliophyta - flowering plants

• Kingdom Animalia or animals
  Examples:

• Arthropoda - includes insects, arachnids, and crustaceans
• Chordata - includes vertibrates and, as such, human beings

[edit] Niches

Each of the three cell types tends to fit into recurring specialties or roles. Bacteria tend to be the most prolific reproducers, at least in moderate environments. Archaeans tend to adapt quickly to extreme environments, such as high temperatures, high acids, high sulfur, etc. This includes adapting to use a wide variety of food sources. Eukaryotes are the most flexible with regard to forming cooperative colonies, such as in multi-cellular organisms, including humans. In fact, the structure of a Eukaryote is likely to have derived from a joining of different cell types, forming organelles.

[edit] See also

• Five-kingdom system
• Monera
• Phylogenetic tree
• Taxonomy
• Two-empire system

[edit] References

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