Fig. 1.

The three viruses, three domains theory. Dotted lines correspond to RNA cell lineages, and bold lines correspond to DNA cell lineages. FvA, FvB, and FvE are founder viruses for Archaea, Bacteria, and Eukarya, respectively. The cellular tree is rooted in the “bacterial branch” (1), but other rootings are possible (discussed in ref. 6). The arrows between FvA and FvE are connected to symbolize specific evolutionary relationships between the two viruses that provide their DNA replication machineries to the archaeal and the eukaryal domains.