My own tin-hat theory on that is DNA probably co-evolved with RNA pretty quickly. The main difference between RNA and DNA is that a single RNA strand can fold

My own tin-hat theory on that is DNA probably co-evolved with RNA pretty quickly. The main difference between RNA and DNA is that a single RNA strand can fold up and bond with itself to make interesting() 3d shapes while DNA can't self-fold. I think single-stranded DNA initially served as a durable, robust structure for RNA to assemble against before it went off and folded up and did whatever it did chemically. This might help explain why RNA polymerase only runs 3'->5'; in fact one of the early functions of one of the RNA polymerase components was maybe to keep the single DNA strand "tidy" and free of random base-pairings. Also, it might help explain why the 3'->5' DNA strand has to jump through so many extra hoops to get replicated - double-stranded DNA came later. It might also explain the reason reverse transcriptase exists at all. Again, I am probably provably a crank on this. () This is the whole appeal of the "RNA world" hypothesis, you have one molecule that can serve both as "code" and catalyst for other chemical reactions.