The Archaea, a name that means ancient, are one of the three major groups that comprise life, consisting of unicellular microorganisms with highly varied biochemical pathways and frequently adapted to environments with extreme conditions.

They are microscopic, unicellular organisms, similar to bacteria but with different type of cell membranes, cell walls, and differences in some of the main biochemical pathways. They lack a nucleus, and typically the DNA forms a circular chromosome. The cell membrane has lipids with ether bonds rather than ester bonds like occurs in bacteria, and discarding few exceptions, the membrane is covered by a wall chemically different from the bacterial cellular wall, with pseudopeptidoglycan, glycoproteins, or polysaccharides. The cytoplasm lacks membranous organelles, and the ribosomes are of the 70 S type. They may have flagella for swimming, although some species can move by forming cellular extensions. The metabolism of Archaea is extremely varied, being able to obtain energy from sunlight, organic compounds, or mineral compounds, as well as to respire oxygen or other compounds or to produce fermentations. The group has been found in places as varied as anoxic sludge, deep sea waters, inside animals, extremely acidic waters, volcanic geysers above the boiling point, or hypersaline lakes. Reproduction is by cell division, although they have mechanisms for exchanging DNA from one cell to another.

A large number of Archaea are adapted to environments that were common just a few million years after the formation of the Earth, remaining as remnants of a period when the planet was very different from today, without oxygen and with a different atmospheric composition.

The Archaea domain diverged from the Bacteria domain long before the Eukaryota domain appeared. This latter domain was formed by the fusion of an Archaea from the Asgardarchaeota group and a bacterium from the Alphaproteobacteria group, originating a cell with completely new biochemical properties that ended up forming the Eukaryota cell, with mitochondria and nucleus. Because of this, in the Eukaryota, the genes responsible for maintaining the DNA are more similar to the Archaea domain, while many responsible for metabolism resemble more to Bacteria.

The evolutionary relationships of the main Archaea lineages can be seen in the image below: