Archaea
Archaea or Archaebacteria, common name for a group of one-celled organisms, many of which do not require oxygen or sunlight to live. Before the discovery of archaebacteria, scientists divided all living organisms into prokaryotes (organisms without a cellular nucleus), which consisted primarily of bacteria , and eukaryotes (organisms with a cellular nucleus), which consisted of fungi, plants, and animals. Archaebacteria were initially grouped with bacteria because like bacteria, they lack a well-defined nucleus. Recent evidence, however, has demonstrated that archaebacteria have a genetic makeup that more closely resembles the eukaryotes, organisms that have a well-defined nucleus. This unique structure means that archaebacteria cannot be accurately grouped with either the prokaryotes or the eukaryotes. Instead, scientists have proposed that these microorganisms be classified in a new branch of life, or domain, called archaea. Archaebacteria often live in extreme conditions that wer...