A group of prokaryotic organisms that are more closely related to eukaryotes than bacteria. Based on comparative analyses of small subunit ribosomal ribonucleic acid (rRNA) sequences and selected protein sequences, the three primary lines of descent from the common ancestor are the Archaea (archaebacteria), the Bacteria, and the Eucarya (eukaryotes). Although the Archaea look like Bacteria cytologically (they are both prokaryotes), they are not closely related to them.
The Archaea can be divided into two evolutionary lineages on the basis of rRNA sequence comparisons, the Crenarchaeotae and the Euryarchaeotae. The crenarchaeotes are organisms that grow at high temperatures (thermophiles) and metabolize elemental sulfur. Most are strict anaerobes that reduce sulfur to hydrogen sulfide (sulfidogens), but a few can grow aerobically and oxidize sulfur to sulfuric acid. The euryarchaeotes have a number of different phenotypes. Thermococcus and Pyrococcus are sulfidogens like many crenarchaeotes. Archaeoglobus reduces sulfate to sulfide. Thermoplasma grows under acidic conditions aerobically or anaerobically (as a sulfidogen). Many euryarchaeotes are methane-producing anaerobes (methanogens) and some grow aerobically in the presence of very high concentrations of salt (halophiles). See Methanogenesis (bacteria)
The thermophilic archaea are found in high-temperature environments around the world. They have been isolated from soils and shallow marine sediments heated by nearby volcanoes and from deep-sea hydrothermal vents. Some are used as a source for heat-stable enzymes useful for industrial applications. The methanogenic archaea inhabit the digestive tracts of animals (especially ruminants like cows), sewage sludge digesters, swamps (where they produce marsh gas), and sediments of marine and fresh-water environments. They are of interest commercially because of their ability to produce methane from municipal garbage and some industrial wastes. Halophilic archaea live in the Great Salt Lake, the Dead Sea, alkaline salt lakes of Africa, and salt-preserved fish and animal hides. They are also commonly found in pools used to evaporate seawater to obtain salt.
The discovery of the Archaea caused a major revision in the understanding of evolutionary history. It had previously been thought that all prokaryotes belonged to one evolutionary lineage. Since their cellular organization is simpler, prokaryotes were assumed to be ancestors of eukaryotes. The discovery of the relationship of the Archaea to the Eucarya revealed that prokaryotes do not comprise a monophyletic group since they can be divided into two distinct lineages. Although the three descended from a common ancestor, modern eukaryotes may have arisen from fusions of bacterial and archaeal endosymbionts with ancestral eukaryotes. Chloroplasts and mitochondria arose from free-living bacteria which became endosymbionts. The discovery of the Archaea has also given microbiologists a better picture of the common ancestor. The deepest-branching eukaryotes (like Giardia) are strict anaerobes that lack mitochondria, and they diverged much later than the deepest-branching bacteria and archaea. The earliest archaea and bacteria (Thermotoga and Aquifex) are also anaerobes and are also extreme thermophiles. Therefore the common ancestor of these groups was probably also an extremely thermophilic anaerobe. Therefore, it is possible that life may have arisen in a relatively hot environment, perhaps like that found in deep-sea hydrothermal vents. See Bacteria, Eukaryotae, Prokaryotae