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|Ignicoccus hospitalis (and its symbiote Nanoarchaeum equitans)|
|Domain:||Archaea or Arkarya|
Lake et al. 1988
|Domain & Regnum|
The Eocyte hypothesis is a biological classification that indicates eukaryotes emerged within the prokaryotic Crenarchaeota (formerly known as eocytes), a phylum within the archaea. This hypothesis was originally proposed by James A. Lake and colleagues in 1984 based on the discovery that the shapes of ribosomes in the Crenarchaeota and eukaryotes are more similar to each other than to either bacteria or the second major kingdom of archaea, the Euryarchaeota.
The eocyte hypothesis gained considerable attention after its introduction due to the interest in determining the origin of the eukaryotic cell. This hypothesis has primarily been in contrast with the three-domain system introduced by Carl Woese in 1977. Additional evidence supporting the eocyte hypothesis was published in the 1980s, but despite fairly unequivocal evidence, support waned in favor of the three-domain system.
With advancements in genomics, the eocyte hypothesis experienced a revival beginning in the mid-2000s. As more archaeal genomes were sequenced, numerous genes coding for eukaryotic traits have been discovered in various archaean phyla, seemingly providing support for the eocyte hypothesis. In addition to a Crenarchaeal origin of eukaryotes, some studies have suggested that eukaryotes may also have originated in the Thaumarchaeota. A superphylum - TACK - has been proposed that includes the Thaumarchaeota, Crenarchaeota, and other groups of archaea, so that this superphylum may be related to the origin of eukaryotes.
As a result of metagenomic analysis of material found nearby hydrothermal vents, another superphylum -- Asgard -- has been named and proposed to be more closely related to the original eukaryote and a sister group to TACK more recently. The eocyte tree root may be located in the RNA World, that is the root organism may have been a Ribocyte (aka Ribocell). For cellular DNA and DNA handling an "out of virus" scenario has been proposed, i. e. string genetic information in DNA may have been an invention performed by viruses later handed over to Ribocytes twice, once transforming them into bacteria and once transforming them into archaea. All these findings do not change the eocyte tree as given here in principle, but zoom into a higher resolution of it.
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