archaeal genes ~2,300: eubacterial genes 2,100 Mya
||Emelyanov VV. Mitochondrial connection to the origin of the eukaryotic cell. Eur J Biochem. 2003 Apr270(8):1599-618. p.1604 left column 2nd paragraphPubMed ID12694174
|| Feng, D.-F., Cho, G. & Doolittle, R.F. (1997) Determining divergence times with a protein clock: update and reevaluation. Proc. Natl Acad. Sci. USA 94, 13028–13033.PubMed ID9371794
||P.1604 left column 2nd paragraph: "Timing of the appearance of eubacterial genes in eukaryotic genomes is another way to attempt to distinguish between different hypotheses about the origin of the pro-eukaryotic genome. Available data of this kind are rather controversial. On the one hand, Feng et al. [primary source] showed that archaeal genes appeared in Eukarya about 2.3 billion years ago (Bya) while eubacterial genes appeared 2.1 Bya. It was suggested that both estimates relate to the same event, fusion between an archaebacterium and a eubacterium, and the shift in the appearance time of bacterial genes to the present day was merely due to involvement in the analysis of mitochondrial and α-proteobacterial sequences. The above small difference would thus just reflect a more recent endosymbiotic event [ref 96]."