A prebiotic synthesis of the purine DNA nucleosides (deoxyadenosine and deoxyinosine) in which the pyrimidine RNA nucleosides (cytidine and uridine) persist has implications for the coexistence of DNA and RNA at the dawn of life. The nature of the first genetic polymer is the subject of major debate(1). Although the 'RNA world' theory suggests that RNA was the first replicable information carrier of the prebiotic era-that is, prior to the dawn of life(2,3)-other evidence implies that life may have started with a heterogeneous nucleic acid genetic system that included both RNA and DNA(4). Such a theory streamlines the eventual 'genetic takeover' of homogeneous DNA from RNA as the principal information-storage molecule, but requires a selective abiotic synthesis of both RNA and DNA building blocks in the same local primordial geochemical scenario. Here we demonstrate a high-yielding, completely stereo-, regio- and furanosyl-selective prebiotic synthesis of the purine deoxyribonucleosides: deoxyadenosine and deoxyinosine. Our synthesis uses key intermediates in the prebiotic synthesis of the canonical pyrimidine ribonucleosides (cytidine and uridine), and we show that, once generated, the pyrimidines persist throughout the synthesis of the purine deoxyribonucleosides, leading to a mixture of deoxyadenosine, deoxyinosine, cytidine and uridine. These results support the notion that purine deoxyribonucleosides and pyrimidine ribonucleosides may have coexisted before the emergence of life(5).