Domain is used by many scientists as the largest unit of biological classification. Traditionally, kingdoms were the largest classification units. But in 1990, the American biologist Carl R. Woese and others proposed that life on Earth could be divided into three domains. They developed this proposal largely from research on the genetic (hereditary) structure of living things. Today, most biologists recognize three domains: (1) Bacteria, (2) Archaea, and (3) Eukaryota, also called Eucarya or Eukarya.
The domains Bacteria and Archaea consist of microscopic organisms with prokaryotic cells. These cells lack a nucleus. Scientists refer to such organisms as prokaryotes. All other organisms, including human beings, have eukaryotic cells that possess a nucleus. They make up the domain Eukaryota.
The domain Bacteria includes true bacteria and the algaelike cyanobacteria. Bacteria live in nearly all environments, including on or in plants and animals. These simple, one-celled organisms often create loosely organized groups known as colonies. Cyanobacteria can form slippery, dark brownish-green coatings on moist rocks and other surfaces.
The domain Archaea consists of prokaryotes that resemble bacteria, but differ from them genetically in important ways. For example, archaea produce proteins differently than do bacteria. Archaea often occur in such harsh environments as deep-sea hydrothermal vents and highly saline (salty) pools. Some can obtain energy from sulfur and other inorganic chemicals.
The domain Eukaryota contains most familiar species. Animals, fungi, plants, and protists all belong to this domain. However, bacteria and archaea make up the vast majority of living things.
See also Classification, Scientific ; Kingdom .
