The multi-copy single-stranded DNA (msDNA) is yielded by the action of reverse transcriptase of retro-element in a wide range of pathogenic bacteria. Upon this phenomenon, it has been shown that msDNA is only produced by Eubacteria because many Eubacteria species contained reverse transcriptase in their special retro-element. We have screened around 111 Archaea at KEGG (Kyoto Encyclopedia of Genes and Genomes) database available at genome net server and observed three Methanosarcina species (M.acetivorans, M.barkeri and M.mazei), which also contained reverse transcriptase in their genome sequences. This observation of reverse transcriptase in Archaea raises questions regarding the origin of this enzyme. The evolutionary relationship between these two domains of life (Eubacteria and Archaea) hinges upon the phenomenon of retrons. Interestingly, the evolutionary trees based on the reverse transcriptases (RTs) and 16S ribosomal RNAs point out that all the Eubacteria RTs were descended from Archaea RTs during their evolutionary times. In addition, we also have shown some significant structural features among the newly identified msDNA-Yf79 in Yersinia frederiksenii with other of its related msDNAs (msDNA-St85, msDNA-Vc95, msDNA-Vp96, msDNA-Ec78 and msDNA-Ec83) from pathogenic bacteria. Together the degree of sequence conservation among these msDNAs, the evolutionary trees and the distribution of these ret (reverse transcriptase) genes suggest a possible evolutionary scenario. The single common ancestor of the organisms of Eubacteria and Archaea subgroups probably achieved this ret gene during their evolution through the vertical descent rather than the horizontal transformations followed by integration into this organism genome by a mechanism related to phage recognition and/or transposition.