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On the basis of recently discovered RNAs with catalytic capabilities resembling those of enzymes, it is postulated that an 'RNA world' may have played a determining role in prebiotic chemistry and led evolution from prebiological to biological systems. The advent of the RNA world thus postulated, however, entails the preexistence of ribomononucleotides, and presumes that their reactions resulted in templatelike oligonucleotides. Attention is presently given to the reaction of nucleoside monophosphates with the phosphoimidazolide-activated nucleosides that (1) have successfully been used in place of the natural nucleoside triphosphates and (2) for whose prebiotic existence there is now some evidence.