by George Wilkinson
Cells across the tree of life are built with very similar components: A wall to keep everything in, DNA which stores the genome, and RNA and protein which take care of the mechanics and metabolism. These components are heavily interdependent, and it is an ongoing puzzle how the modern cell emerged from the prebiotic chemistry of early earth. (See Wikipedia here ). There are two major approaches to try to bridge this gap. The first is to try to synthesize biotic compounds from a soup of plausible precurors, as in Miller and Urey’s “lightning experiment.” [Miller shown performing the experiment in photo on the right.] The converse approach is to scrutinize living organisms for hints of their ancestors, specifically the Last Universal Cellular Ancestor (LUCA).
RNA was likely to be among the very first of the modern information polymer classes to emerge, because RNAs can do the jobs of genome storage (now done mainly by DNA) and enzymatic action (now done mainly by protein). Proteins might have come second, and DNA last. So how did DNA take over the job of genome storage?
There are some clues from comparative genomics that the DNA world developed subsequent to LUCA. The handling of RNA (transcription, translation) is similar among all the major domains of life, suggesting that these tools were present in the common ancestor of all present-day cells and diverged subsequently in archaea, bacteria, and eukaryotes. In contrast, the means of handling and copying DNA vary quite a bit, with, for example, the major bacterial DNA processing enzymes lacking archaeal/eukaryotic homologs. 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.
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.
With this topic it is important to realize that the ideas are very speculative and contentious. The attraction of this idea- that the parasite-prey relationship is a very old evolutionary engine- actually also makes trouble because modern day parasites and prey, especially in the microbial world, exchange and exapt whole genetic modules. There is thus even a middle view, in which two-way “technology transfers” between viruses and canonical cellular life will have completely obscured DNA's origins.