Archaea are microbes that are more closely related to Eukaryotic cells than they are to the Bacteria (http://tolweb.org/tree/home.pages/aboutoverview.html). Under a light microscope, they visually resemble Bacteria, so that it wasn't until the advent of the use of molecular methods in evolutionary biology that they were recognized as belonging to their own Domain (a phylogenetic grouping above the level of Kingdom). Archaea have ultrastructural features that are superficially similar to those in Bacteria but are usually comprised of distinctive molecules. They do, for example, have a cell wall, yet that cell wall never contains peptidoglycan. Instead, peptidoglycan is a unique molecular signature of the Bacteria. Archaea also have odd lipids in their cell membranes. They were originally discovered living in extreme environments thought to resemble conditions on early earth, but now that microbiologists have become more adept at detecting them, it is clear that the Archaea are not confined to extreme habitats and can instead be found everywhere. It is true that some Archaea are "extremophiles," found in extremely salty or hot environments, but there are also extremophile Bacteria and even some very unusual extremophile Eukarya. The best-understood groups of Archaea are:
- Methanogens use Carbon dioxide and Hydrogen to make Methane. They are found in sewage, cows, and swamps, and they do not take in oxygen.
- Extreme Halophiles live in extremely salty places (i.e.: the dead sea and great salt lake).
- Thermoacidophiles prefer extremely hot, acidic areas (i.e.: hot springs and volcanos).