Mayr’s distinction between proximate and ultimate explanation is justly famous, marking out a division of explanatory labor in biology. But while it is a useful heuristic in many cases, there are others in which proximate factors play an important role in shaping evolutionary trajectories, and in such cases, each project is sensitive to, and relevant to, the other. This general methodological claim is developed in the context of a discussion of human cooperation, and in particular, in a discussion on the (...) puzzling stability of the social contract over the Pleistocene–Holocene transition. For I argue that while we have a plausible account of the stability of Pleistocene cooperation, the stabilizing factors of the Pleistocene disappear in the Holocene, but cooperation does not. Holocene humans solved many collective action problems; cooperation did not collapse despite the apparent growth of free-riding elites. So the article combines a methodological claim about the interaction of proximate and evolutionary biology with a substantive one about the ecology of human cooperation. (shrink)