In several taxa, the ability to feed omnivorously from more than one trophic level has evolved from more specialized feeding habits. A prerequisite for this evolutionary scenario is initial coexistence (on the same or different trophic levels) of the consumers subsequently involved in omnivory. Evolution of omnivory should also be favored by prior asymmetries in consumer interactions. I use a dynamic model to explore conditions for coexistence of two consumers depending on a single biotic resource in simple. food webs of increasing asymmetry between consumers: (1) pure resource competition, (2) resource competition with unidirectional interference, (3) omnivory, and (4) pure predation. If consumers are energy limited, omnivory is unlikely to evolve from a competitive system, because stable coexistence of two consumers on a single resource is impossible, even if there is a trade-off between resource exploitation and interference abilities. Initial coexistence of two consumers is easier to achieve, if the species are arranged in a food chain. If costs of diet generalization are low, selection should then favor the inclusion of the resource in the top consumer's diet. In systems with high resource carrying capacity, the intermediate consumer will then, however, be frequently driven to extinction-in which case omnivory is not maintained. Low nutritional quality of the resource can stabilize the omnivory system and allow three-species coexistence at high resource carrying capacities. Low nutritional quality of the resource may also create conditions for the evolution of omnivory from a competitive system. If the content of an essential nutrient in resource biomass is dynamic, stable coexistence of two competitors becomes possible if both are limited by the nutrient content of the resource. In this case, selection may favor the, evolution of omnivory, because competitor biomass is usually of higher nutritional quality than resource biomass. An explicit treatment of flexible resource stoichiometry thus broadens the possibilities for the evolution of omnivory.