Speculation on the RNA precursor problem.

While no convincing prebiotic synthesis of RNA building blocks has been demonstrated, at least two synthetic analogues of nucleic acids have been described whose properties are remarkably similar to those of RNA. The RNA backbone is thus not the only possible solution to the problem of replicating the information stored in a sequence of purines and pyrmidines. These results indirectly support the suggestion that RNA might have been preceded in evolution by a related molecule which, perhaps, was more easily synthesized than RNA. New results on the prebiotic chemistry of phosphonic acids suggest a possibility that a backbone structure based on ribose-2,4-diphosphonic acid may have formed via some surprisingly simple chemistry.