In order to understand how life arose, knowledge is required of the chemical pathways that permit formation of the key building blocks of life under plausible prebiotic conditions.  The RNA world hypothesis holds that in the primordial soup there existed free-floating pyrimidine and purine ribonucleotides, the fundamental molecules that combine in series to form RNA.  Complex molecules such as RNA must have emerged from relatively small molecules whose reactivity was governed by physico-chemical processes.  RNA is composed of pyrimidine and purine  nucleotides, both of which are necessary for reliable information transfer, and thus natural selection and Darwinian evolution.  Becker et al. showed how pyrimidine nucleosides can be synthesized from small molecules and ribose, driven solely by wet-dry cycles.