All modern cells 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 actually something of a puzzle how the modern cell emerged from the prebiotic chemistry of early earth. (See Wikipedia here ).
RNA was likely to be among the very first of the modern cell components to emerge, because RNAs can do the jobs of genome storage (now done mainly by DNA) and enzymatic action (now done mainly by protein) whereas the other components cannot. Proteins might have come second, and DNA last. So how did DNA take over the job of genome storage?
This week's Nature has an amazing suggestion: DNA was "invented" by viruses as a way of evading the defenses of ancient cells. Ancient viruses, as with the ones today, could only make copies of themselves by succesfully infecting a host. So they become engines of innovation, thinking of every possible dodge to get 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.
There are some clues from comparative genomics that the DNA world developed in pieces. The means of interpreting DNA genes (transcription, translation) are very similar among all the major domains of life, suggesting that these tools were present in the common ancestor of all present-day cells. In contrast, the means of handling and copying DNA vary quite a bit. 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. This suggests a scenario in which a clever parasite brings along DNA plus the means of copying it-- a different parasite for bacteria, archaea, eukaryotes-- and hijacks the cell's existing interpretation equipment. The merger of virus plus RNA/protein cell then created the modern cell.
With this topic it is important to realize that the ideas are very speculative. 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, actually exchange whole chemical modules. Thus it remains very very hard to state what came first.
The original article (Biochimie) is here. An argument about DNA emerging in stages (TIGS) is here .