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The three-domain system, created by Carl Woese in 1990, classifies organisms into Eukaryota, Archaea, and Bacteria. It emphasizes microbial diversity and recognizes fundamental differences between archaea and bacteria. Archaea are the most recent group to be recognized and include impressive extremophiles. Bacteria are the most numerous, followed by archaea, then eukaryotes. Most eukaryotes are dependent on bacteria and archaea for food or symbiotic interactions.
In biology, a domain is the highest possible classification of organisms. According to the modern system, called the three-domain system, created by Carl Woese in 1990, there are three of them: Eukaryota (eukaryotes), Archaea and Bacteria. Older systems include the two empire system, Prokaryota and Eukaryota, and the six kingdom system, which lists Protista, Archaebacteria, Eubacteria, Fungi, Plantae, and Animalia. Based on genetic analysis and careful argumentation, most scientists agreed that the current system better classifies the diversity of life.
Woese’s system has been praised for emphasizing microbial diversity and recognizing the fundamental differences between archaea and bacteria, enough to justify placing them in different domains. The two empire system is said to ignore these differences, while the six kingdom system overemphasizes the distinction of fungi, plants, and animals, all of which are eukaryotes, made up of complex cells with organelles and nuclei. All multicellular animals are eukaryotes.
The most fundamental division in all life is that between prokaryotes, or simple cells, and eukaryotes, or complex cells. Differences in 16S rRNA genes among prokaryotes have led them to divide them into the groups archaea and bacteria, while other genetic contrasts clarify the difference between these groups and eukaryotes.
Currently, it is unknown which domain is the world’s oldest organism lineage, although recent evidence points to archaea. In terms of biomass, bacteria are the most numerous, probably followed by archaea, then by eukaryotes. If bacteria and archaea were to disappear, eukaryotes would likely become extinct, because most eukaryotes are intimately dependent on these other organisms for food or symbiotic interactions, such as nitrogen fixation. In humans, symbiotic stomach bacteria are required to break down complex carbohydrates.
Archaea are the most recent group to be recognized, identified as distinct in 1977. Originally, these organisms were found in extreme environments, such as the hot springs in Yellowstone Park, but they’ve since been found virtually everywhere. Archaea include some of the most impressive extremophiles—organisms that thrive under extreme conditions of temperature, acidity, pressure, sugar concentration, lack of oxygen, or other variables.
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