The evolution of human cooperation is described as one of biology’s great mysteries. How can natural selection favor traits that are disadvantageous to an individual? Numerous theories and models have been developed to answer this question and they all share the same fundamental solution: positive phenotypic assortment. Cooperation can only evolve if the benefits of cooperation preferentially flow between those who cooperate. Now, a central challenge is determining which theories best explain assortment using ecologically relevant data for the setting of human evolution. Here, I provide insight into the evolutionary origins of cooperation using data from one of the few remaining hunter-gatherer populations in existence – the Hadza of Northern Tanzania. The findings highlight the adaptive nature of human cooperative behavior—particularly its responsiveness to local social environments—as a feature important in generating the assortment necessary for cooperation to evolve.