section_path: Origin and evolution/Relation to eukaryotes
section_name: Relation to eukaryotes
paragraph_text: Relation to eukaryotes
  
  The evolutionary relationship between archaea and eukaryotes remains unclear. Aside from the similarities in cell structure and function that are discussed below, many genetic trees group the two.
  
  Complicating factors include claims that the relationship between eukaryotes and the archaeal phylum Thermoproteota is closer than the relationship between the " Euryarchaeota " and the phylum Thermoproteota and the presence of archaea-like genes in certain bacteria, such as Thermotoga maritima, from horizontal gene transfer. The standard hypothesis states that the ancestor of the eukaryotes diverged early from the Archaea, and that eukaryotes arose through symbiogenesis, the fusion of an archaean and a eubacterium, which formed the mitochondria; this hypothesis explains the genetic similarities between the groups. The eocyte hypothesis instead posits that Eukaryota emerged relatively late from the Archaea.
  
  A lineage of archaea discovered in 2015, Lokiarchaeum (of the proposed new phylum " Lokiarchaeota "), named for a hydrothermal vent called Loki's Castle in the Arctic Ocean, was found to be the most closely related to eukaryotes known at that time. It has been called a transitional organism between prokaryotes and eukaryotes.
  
  Several sister phyla of "Lokiarchaeota" have since been found (" Thorarchaeota ", " Odinarchaeota ", " Heimdallarchaeota "), all together comprising a newly proposed supergroup Asgard.
  
  Details of the relation of Asgard members and eukaryotes are still under consideration, although, in January 2020, scientists reported that Candidatus Prometheoarchaeum syntrophicum, a type of Asgard archaea, may be a possible link between simple prokaryotic and complex eukaryotic microorganisms about two billion years ago.