They show many differences in their biochemistry from other forms of life and have an independent evolutionary history.
In the three-domain system, they are classified as a separate domain from the phylogenetically distinct
Bacteria and Eukaryota.
Archaea are divided into four recognized phyla,but many more phyla may exist. Of these groups the Crenarchaeota and the Euryarchaeota are most intensively studied. Classification is still difficult, since the vast majority have never been studied in the laboratory. Archaea and bacteria are quite similar in size and shape,
but a few archaea have very unusual shapes. Despite this visual similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes: notably the enzymes involved in transcription and translation. Initially,archaea were seen as extremophiles that lived in harsh environments,such as hot springs and salt lakes, but they have since been found in a broad range of habitats, including soils, oceans, and marshlands.
Archaea are now recognized as a major part of Earth's life and may play roles in both the carbon cycle and the nitrogen cycle.
Archaea have, in the past, been classed with bacteria as prokaryotes (or Kingdom Monera), this classification is regarded by some as outdated.
Other aspects of archaean biochemistry are unique, such as their reliance on ether lipids in their cell membranes. Archaea reproduce asexually and divide by binary fission, fragmentation, or budding; in contrast to bacteria and eukaryotes, no known species form spores.
Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. No clear examples of archaeal pathogens or parasites are known, but they are often mutualists or commensals. One example are the methanogens that inhabit the gut of humans and ruminants, where their vast numbers aid digestion. Methanogens are used in biogas production and sewage treatment, and enzymes from extremophile archaea that can endure high temperatures and organic solvents are exploited in biotechnology.
Although probable prokaryotic cell fossils date to almost 3.5 billion years ago, most prokaryotes do not have distinctive morphologies and fossil shapes cannot be used to identify them as Archaea.Instead, chemical fossils of unique lipids are more informative because such compounds do not occur in other organisms.Some publications suggest that archaean or eukaryotic lipid remains are present in shales dating from 2.7 billion years ago;such data have since been questioned.Such lipids have also been detected in Precambrian formations. The oldest such traces come from the Isua district of west Greenland, which include Earth's oldest sediments, formed 3.8 billion years ago.The archaeal lineage may be the most ancient that exists on earth.Eukaryotes are colored red, archaea green and bacteria blue. Adapted from Ciccarelli et al.
Woese argued that the bacteria, archaea, and eukaryotes represent separate lines of descent that diverged early on from an ancestral colony of organisms.A few biologists, however, argue that the Archaea and Eukaryota arose from a group of bacteria. In any case it is thought that viruses and archaea began relationships approximately two billion years ago, and that co-evolution may have been occurring between members of these groups.It is possible that the last common ancestor of the bacteria and archaea was a thermophile, which raises the possibility that lower temperatures are "extreme environments" in archaeal terms, and organisms that live in cooler environments appeared only later.Since the Archaea and Bacteria are no more related to each other than they are to eukaryotes, the term prokaryote's only surviving meaning is "not a eukaryote", limiting its value.