After including the kingdom category intoICNP, the onlyvalidly published names of this group are kingdomPromethearchaeati and phylumPromethearchaeota. All formerly proposed "phyla" would be de-ranked to classes in this framework.[2] It appears that the eukaryotes, thedomain that contains theanimals,plants,fungi andprotists, emerged within the Promethearchaeati,[11] in a branch containing the "Heimdallarchaeia".[12] This supports thetwo-domain system of classification over thethree-domain system.[13]
In the summer of 2010, sediments were analysed from a gravitycore taken in the rift valley on the Knipovich ridge in the Arctic Ocean, near theLoki's Castlehydrothermal vent site. Specific sediment horizons previously shown to contain high abundances of novel archaeal lineages were subjected tometagenomic analysis.[14][15] In 2015, anUppsala University-led team proposed the "Lokiarchaeota" phylum based onphylogenetic analyses using a set ofhighly conserved protein-coding genes.[16] The group was named for the shape-shifting Norse godLoki, in an allusion to the hydrothermal vent complex from which the first genome sample originated.[17] The Loki of mythology has been described as "a staggeringly complex, confusing, and ambivalent figure who has been the catalyst of countless unresolved scholarly controversies",[18] analogous to the role of "Lokiarchaeota" in the debates about the origin of eukaryotes.[16][19]
In 2016, aUniversity of Texas-led team discovered "Thorarchaeia" from samples taken from theWhite Oak River in North Carolina, named in reference toThor, another Norse god.[20] Samples from Loki's Castle,Yellowstone National Park,Aarhus Bay, an aquifer near theColorado River, New Zealand'sRadiata Pool, hydrothermal vents nearTaketomi Island, Japan, and theWhite Oak River estuary in the United States contained "Odinarchaeia" and "Heimdallarchaeia";[10] following the Norse deity naming convention, these groups were named forOdin andHeimdall respectively. Researchers therefore named the group containing these microbes "Asgard", after the home of the gods in Norse mythology.[10] Two "Lokiarchaeota" specimens have been cultured, enabling a detailed insight into their morphology.[21] Superphylum "Asgard" was renamed to kingdom Promethearchaeati and phylum "Lokiarchaeota" was renamed to familyPromethearchaeaceae.[2]
Asgard archaea encode many eukaryotic signature proteins, including novelGTPases, membrane-remodelling proteins likeESCRT andSNF7, aubiquitin modifier system, andN-glycosylation pathway homologs.[10]
Asgard archaea have a regulatedactincytoskeleton, and theprofilins andgelsolins they use can interact with eukaryotic actins.[22][23] In addition, Asgard archaeatubulin from hydrothermal-living "Odinarchaeia" (OdinTubulin) was identified as a genuine tubulin. OdinTubulin forms protomers and protofilaments most similar to eukaryotic microtubules, yet assembles into ring systems more similar toFtsZ, indicating that OdinTubulin may represent an evolution intermediate between FtsZ andmicrotubule-forming tubulins.[24] They also seem to form vesicles undercryogenic electron microscopy. Some may have aPKD domainS-layer.[25] They also share the three-way ES39 expansion inLSU rRNA with eukaryotes.[26] Gene clusters or operons encoding ribosomal proteins are often less conserved in their organization in the Asgard archaea than in other archaea, suggesting that the order of ribosomal protein coding genes may follow thephylogeny.[27]
Asgard archaea are widely distributed around the world, both geographically and by habitat. Many of the known clades are restricted to sediments, whereas Promethearchaeia, "Thorarchaeia" and another clade occupy many different habitats. Salinity and depth are important ecological drivers for most Asgard archaea. Other habitats include the bodies of animals, the rhizosphere of plants, non-saline sediments and soils, the sea surface, and freshwater. In addition, Asgard archaea are associated with several other microorganisms.[30]
The class "Heimdallarchaeia" was found in 2017 to have N-terminal corehistone tails, a feature previously thought to be exclusively eukaryotic. Two other archaeal phyla, both non-Asgard archaea, were found to also have tails in 2018.[31]
In January 2020, scientists foundPromethearchaeum syntrophicum, a member of the Promethearchaeia, engaging incross-feeding with two bacterial species. Drawing an analogy tosymbiogenesis, they consider this relationship a possible link between the simpleprokaryotic microorganisms and the complexeukaryotic microorganisms occurring approximately two billion years ago.[32][25]
The phylogenetic relationships of the Asgard archaea have been studied by several teams in the 21st century.[12][11][33][29] Varying results have been obtained, for instance using 53 marker proteins from theGenome Taxonomy Database.[34][35][36] In 2023, Eme, Tamarit, Caceres and colleagues reported that the Eukaryota[37] are deep within Asgard archaea, as sister of "Hodarchaeales" within the "Heimdallarchaeia".[38] "Njordarchaeaceae" has recently been suggested as the closest branch to eukaryotes, but this result is debated.[39]
Valentin-Alvaradoet al. 2024,[40] Kioukiset al.[41] and Zhanget al. 2025[1]
In the depicted scenario, the Eukaryota are deep in the Asgard archaeal tree. A favored scenario is syntrophy, where one organism depends on the feeding of the other. Anα-proteobacterium was incorporated to become themitochondrion.[45] In culture, extant Asgard archaea form various syntrophic dependencies.[46] Gregory Fournier and Anthony Poole have proposed that Asgard archaea is part of "the Eukaryote tree", forming ataxon they call "Eukaryomorpha" defined by "shared derived characters" (eukaryote signature proteins).[47] The kingdom contains one phylum,Promethearchaeota.[48]
Severalfamily-level groups ofviruses associated with Asgard archaea have been discovered using metagenomics.[49][50][51] The viruses were assigned to Promethearchaeia, "Thorarchaeia", "Odinarchaeia" and "Helarchaeia" hosts using CRISPR spacer matching to the corresponding protospacers within the viral genomes. Two groups of viruses (called 'verdandiviruses') are related to archaeal and bacterial viruses of the classCaudoviricetes, i.e., viruses with icosahedral capsids and helical tails;[49][51] two other distinct groups (called 'skuldviruses') are distantly related to tailless archaeal and bacterial viruses with icosahedral capsids of the realmVaridnaviria;[49][50] and the third group of viruses (calledwyrdviruses) is related to archaea-specific viruses with lemon-shaped virus particles (familyHalspiviridae).[49][50] The viruses have been identified in deep-sea sediments[49][51] and a terrestrial hot spring of the Yellowstone National Park.[50] All these viruses display very low sequence similarity to other known viruses but are generally related to the previously described prokaryotic viruses,[52] with no meaningful affinity to viruses of eukaryotes.[53][49]
In addition to viruses, several groups of crypticmobile genetic elements have been discovered throughCRISPR spacer matching to be associated with Asgard archaea of the Promethearchaeia, "Thorarchaeia" and "Heimdallarchaeia" lineages.[49][54][2] These mobile elements do not encode recognizable viral hallmark proteins and could represent either novel types of viruses or plasmids.
^abEme, Laura; Spang, Anja; Lombard, Jonathan; Stairs, Courtney W.; Ettema, Thijs J. G. (November 2017). "Archaea and the origin of eukaryotes".Nature Reviews. Microbiology.15 (12):711–723.doi:10.1038/nrmicro.2017.133.PMID29123225.S2CID8666687.
^Nobs, Stephanie-Jane; MacLeod, Fraser I.; Wong, Hon Lun; Burns, Brendan P. (May 2022). "Eukarya the chimera: eukaryotes, a secondary innovation of the two domains of life?".Trends in Microbiology.30 (5):421–431.doi:10.1016/j.tim.2021.11.003.PMID34863611.S2CID244823103.
![Metabolic pathways of Asgard archaea, varying by phyla,[28] now de-ranked[2]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAsgard_archaea_Phyla_(cropped).png)
![Metabolic pathways of Asgard archaea, varying by environment[28]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAsgard_archaea_in_various_environments_(cropped).png)
