George Wilkinson mulls over some interesting speculation:
DNA polymerases (the copying enzymes) in the various domains of life are in each case more closely related to viral proteins than to comparable proteins from the other domains of life. These data, and recent appreciation of the life-like capabilities of giant viruses, have led some researchers to an interesting suggestion: DNA as a genomic storage compound originated in viruses as a way of evading the defenses of ancient cells.
More @ 3QD.According to this view, ancient viruses, as with the ones today, could only make copies of themselves by succesfully infecting a host. So they become engines of innovation, using every possible dodge to get their genetic payload inside the host cell. In an early, RNA-protein world, there would not be enzymes to degrade DNA, so a virus encoded by DNA would have a big survival advantage. This suggests a scenario in which a clever parasite brings along DNA plus the means of copying DNA-- a different parasite at least for bacteria and archaea/eukaryotes-- and hijacks the cell's existing interpretation equipment. The symbiosis of virus plus RNA/protein cell eventually resulted in the modern arrangement of DNA, RNA and protein.