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The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems.This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.
The last universal common ancestor (LUCA) is the hypothesized common ancestral cell from which the three domains of life, the Bacteria, the Archaea, and the Eukarya originated. The cell had a lipid bilayer ; it possessed the genetic code and ribosomes which translated from DNA or RNA to proteins .
The three-domain system presumes that eukaryotes are more closely related to archaea than to Bacteria and are sister group to Archaea, thus, it treats them as separate domain. [29] As more new archaea were discovered in the early 2000s, this distinction became doubtful as eukaryotes became deeply nested within Archaea.
It was also found that the eukaryotes are more closely related to the Archaea than they are to the Eubacteria. Although the primacy of the Eubacteria-Archaea divide has been questioned, it has been upheld by subsequent research. [22] There is no consensus on how many kingdoms exist in the classification scheme proposed by Woese.
According to the domain system, the tree of life consists of either three domains, Archaea, Bacteria, and Eukarya, [1] or two domains, Archaea and Bacteria, with Eukarya included in Archaea. [3] [4] In the three-domain model, the first two are prokaryotes, single-celled microorganisms without a membrane-bound nucleus.
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.
They suggested and formally defined the terms Bacteria, Archaea and Eukarya for the three domains of life. [22] It was the first tree founded on molecular phylogenetics and microbial evolution as its basis. [23] [24] The model of a tree is still considered valid for eukaryotic life forms.
When Carl Woese first published his three-domain system in 1990, [3] [4] it was believed that the domains Bacteria, Archaea, and Eukaryota were equally old and equally related on the tree of life. However certain evidence began to suggest that Eukaryota and Archaea were more closely related to each other than either was to Bacteria.