NADH and FADH2 redox reactions are built deep into our biochemistry. For example, pyridine nucleotides are involved in >500 enzymatic reactions. When we look at the structure of deamido-NAD+, it is strikingly symmetrical:
The same is true for FAD, a two-headed monster of a nucleotide
in which we can recognize the two heads, belonging to xanthosine (a purine nucleotide) and alpha-ribazole 5'-phosphate, the latter of which is produced from flavin in the course of B12 synthesis:
The phosphoribose can be swapped between alpha-ribazole 5'-phosphate and nicotinic acid.
Given such enticing hints, it is tempting to speculate that one or more of these nucleotides might be so fundamental because it was actually part of some ancient RNA primary sequence, giving RNA containing this code the power to do serious catalytic chemistry with acid and amide groups, powerful options for redox and methyltransferase activity and more.
I know this is a hard question to answer, but is there any evidence that weighs in, or could weigh in, on whether this happened? For example, if these functional groups are too unstable in regard to intramolecular reactions on a strand of RNA, or if there is no way to envision a potential base pair that would be reliably distinguished from the known base pairings. Or if RNA editing sometimes creates them in some organism. Or if aptamers made from these subunits show improved catalytic possibilities. (That doesn't seem so easy to look up, since many papers look for aptamers binding these as prosthetic groups, rather than incorporating them) It would be useful simply to have a notion of what evidence for such an idea would look like.