A putative evolutionary scenario for the origin of eukaryotic B-family DNA polymerases from prokaryotic ancestral forms. The scheme is rendered within the framework of the symbiotic scenario of the origin of eukaryotes whereby the symbiosis of an archaeon with an α-proteobacterium gave rise to the mitochondrion and triggered eukaryogenesis. The domains are designated by unique shapes as in Figure 4. PolBM, the "major" form of archaeal B-family DNA polymerase (PolBI ); PolBm, "minor" form of archaeal B-family DNA polymerase (PolBII ; PolDs, small subunit of archael PolD (active exonuclease). Inactivation of PolDs in the protoeukaryote (the Last Eukaryotic Common Ancestor, LECA) is denoted by crosses. The origin of Pol ε is depicted as insertion of a bacterial B-family polymerase between the catalytically active module derived from the archaeal PolB-M and the Zn-finger derived from the archaeal PolD.