Organisms on planet earth are broadly divided into prokaryotes and eukaryotes. The former are unicellular, do not have any organelles such as mitochondria, and their DNA is not packaged into a nucleus. Eukaryotes have mitochondria, their DNA is packaged into a nucleus, and most eukaryotes are complex, multicellular beings.
About 50 years ago, a subset of unicellular organisms, the Archaea, were shown to have a different line of descent as compared to bacteria. The two differ in the composition of their cell walls, and in the sequence of some of their genes. The term Archaea, suggesting ancient, was used because the first members of this domain were found living in extreme environments of very high temperatures or very high salt.
One group of archaea were shown to have proteins that closely resembled eukaryotic proteins. These organisms are found in a geological formation where geothermally heated water is forced out of a ridge in the Atlantic Ocean floor at a depth of 2400 meters below sea level. Many other related members were later found in unusual ecosystems, and came to be collectively called the Asgard, which is the home of the Gods in Norse mythology.
