Origin of DNA replication systems: the primordial gene pool scenario. The schematic is based on models of pre-cellular evolution discussed in [21,38]. The walls of the inorganic compartments are shown by dotted lines to emphasize their porosity. Double-headed arrows denote inter-compartmental horizontal gene transfer. C denotes a hypothetical precursor of PolC, probably, a non-templated polymerase (see text). Other designations are as in Figure 2.