The modern natural nucleosides are N1-ribosides of pyrimidine nucleobases and N9-ribosides of purine nucleobases. Although, for instance, N7-ribosyl purines were likely readily available under prebiotic conditions, nature elected to use N9-ribosides. As such, early metabolism was evolved to selectively furnish, degrade and re-use N9-ribosyl purines. As a result, modern (highly evolved) nucleoside-processing enzymes generally exhibit perfect control over the stereoselectivity of the attack at the anomeric center as well as the regioselectivity of the glycosylation. Although unusual glycosylation isomers of purine nucleobases are known in plants (where they serve as hormones), the other conceivable ribosylation isomers of the natural nucleobases do not occur as regular products of metabolism.