The appearance of eukaryotic cells was a major step in the evolution of terrestrial life. Recent phylogenetic analyses indicate that the Eukaryotes appeared from the Archaebacteria rather than being a distinct domain from Archaebacteria and Eubacteria. The Asgard archaeal group, which shares genes that are otherwise unique to Eukaryotes, has been suggested to be the closest relative to Eukaryotes. However, eukaryotic genes have also been shown to have originated from diverse groups in the Archaebacteria and Eubacteria. Asgard archaeon-like Archaea (Archaebacteria) may have been the host for endosymbiosis of the mitochondrial ancestor (Alphaproteobacteria) and might have been the ancestor of Eukaryotes; nevertheless, horizontal gene transfer from various lineages of Archaebacteria and Eubacteria also appear to have played an important role in the evolution of Eukaryotes.
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