The main elemental constituents of living organisms are subjected to biogeochemical cycles that result in both temporal and spatial variation in their abundance. Since organisms have to find their molecular constituents in the environment, this variability may affect the synthesis of biopolymers, in a manner that depends on their atomic (elemental) composition. For example, a biopolymer that is rich in a particular element may become more difficult to synthesize when that element is scarce. Indeed, several studies have now demonstrated that nutritional constraints can shape the evolution of biopolymers specifically in relation to their atomic composition; that is, at a more intimate scale than amino acid or base composition. We discuss these studies, and propose that the increasing availability of data on genomes, proteomes, and RNAs be exploited to study adaptive imprints in the atomic composition of biopolymers, and the factors driving these adaptations.