This paper considers how biological evolution shaped the elements of a simple but complete model of economic decision making. These elements are preferences, beliefs and rationality. Whereas Nature might impose preferences over consumption on the individual, Nature might optimally allow beliefs to be influenced by local knowledge and final choice to be flexible. This reinforces the usual approach. On the one hand, evolution also suggests that some extensions of this model are implausible; on the other, it suggests unexpected directions of generalization. In any case, evolution provides a basis for an overarching economic theory and maintains restrictions on this theory.