kb/data/en.wikipedia.org/wiki/Archaea-2.md

title	chunk	source	category	tags	date_saved	instance
Archaea	3/9	https://en.wikipedia.org/wiki/Archaea	reference	science, encyclopedia	2026-05-05T07:15:06.056512+00:00	kb-cron

Archaea were split off as a third domain because of the large differences in their ribosomal RNA structure. The particular molecule 16S rRNA is key to the production of proteins in all organisms. Because this function is so central to life, organisms with mutations in their 16S rRNA are unlikely to survive, leading to great (but not absolute) stability in the structure of this polynucleotide over generations. 16S rRNA is large enough to show organism-specific variations, but still small enough to be compared quickly. In 1977, Carl Woese, a microbiologist studying the genetic sequences of organisms, developed a new comparison method that involved splitting the RNA into fragments that could be sorted and compared with other fragments from other organisms. The more similar the patterns between species, the more closely they are related. Woese used his new rRNA comparison method to categorize and contrast different organisms. He compared a variety of species and happened upon a group of methanogens with rRNA vastly different from any known prokaryotes or eukaryotes. These methanogens were much more similar to each other than to other organisms, leading Woese to propose the new domain of Archaea. His experiments showed that the archaea were genetically more similar to eukaryotes than prokaryotes, even though they were more similar to prokaryotes in structure. This led to the conclusion that Archaea and Eukarya shared a common ancestor more recent than Eukarya and Bacteria. The development of the nucleus occurred after the split between Bacteria and this common ancestor. One property unique to archaea is the abundant use of ether-linked lipids in their cell membranes. Ether linkages are more chemically stable than the ester linkages found in bacteria and eukarya, which may be a contributing factor to the ability of many archaea to survive in extreme environments that place heavy stress on cell membranes, such as extreme heat and salinity. Comparative analysis of archaeal genomes has also identified several molecular conserved signature indels and signature proteins uniquely present in either all archaea or different main groups within archaea. Another unique feature of archaea, found in no other organisms, is methanogenesis (the metabolic production of methane). Methanogenic archaea play a pivotal role in ecosystems with organisms that derive energy from oxidation of methane, many of which are bacteria, as they are often a major source of methane in such environments and can play a role as primary producers. Methanogens also play a critical role in the carbon cycle, breaking down organic carbon into methane, which is also a major greenhouse gas. This difference in the biochemical structure of Bacteria and Archaea has been explained by researchers through evolutionary processes. It is theorized that both domains originated at deep sea alkaline hydrothermal vents. At least twice, microbes evolved lipid biosynthesis and cell wall biochemistry. It has been suggested that the last universal common ancestor was a non-free-living organism. It may have had a permeable membrane composed of bacterial simple chain amphiphiles (fatty acids), including archaeal simple chain amphiphiles (isoprenoids). These stabilize fatty acid membranes in seawater; this property may have driven the divergence of bacterial and archaeal membranes, "with the later biosynthesis of phospholipids giving rise to the unique G1P and G3P headgroups of archaea and bacteria respectively. If so, the properties conferred by membrane isoprenoids place the lipid divide as early as the origin of life".

=== Relationship to bacteria ===

The relationships among the three domains are of central importance for understanding the origin of life. Most of the metabolic pathways, which are the object of the majority of an organism's genes, are common between Archaea and Bacteria, while most genes involved in gene expression are common between Archaea and Eukarya. Within prokaryotes, archaeal cell structure is most similar to that of Gram-positive bacteria, largely because both have a single lipid bilayer and usually contain a thick sacculus (exoskeleton) of varying chemical composition. In some phylogenetic trees based upon different gene / protein sequences of prokaryotic homologs, the archaeal homologs are more closely related to those of gram-positive bacteria. Archaea and gram-positive bacteria also share conserved indels in a number of important proteins, such as Hsp70 and glutamine synthetase I; but the phylogeny of these genes was interpreted to reveal inter-domain gene transfer, and might not reflect the organismal relationship(s). It has been proposed that the archaea evolved from Gram-positive bacteria in response to antibiotic selection pressure. This is suggested by the observation that archaea are resistant to a wide variety of antibiotics that are produced primarily by Gram-positive bacteria, and that these antibiotics act primarily on the genes that distinguish archaea from bacteria. The proposal is that the selective pressure towards resistance generated by the gram-positive antibiotics was eventually sufficient to cause extensive changes in many of the antibiotics' target genes, and that these strains represented the common ancestors of present-day Archaea. The evolution of Archaea in response to antibiotic selection, or any other competitive selective pressure, could also explain their adaptation to extreme environments (such as high temperature or acidity) as the result of a search for unoccupied niches to escape from antibiotic-producing organisms; Cavalier-Smith has made a similar suggestion, the Neomura hypothesis. This proposal is also supported by other work investigating protein structural relationships and studies that suggest that gram-positive bacteria may constitute the earliest branching lineages within the prokaryotes.

=== Relation to eukaryotes ===