Abstract: What makes humans moral beings? This question can be understood either as a proximate ‘how’ question or as an ultimate ‘why’ question. The ‘how’ question is about the mental and social mechanisms that produce moral judgments and interactions, and has been investigated by psychologists and social scientists. The ‘why’ question is about the fitness consequences that explain why humans have morality, and has been discussed by evolutionary biologists in the context of the evolution of cooperation. Our goal here is to contribute to a fruitful articulation of such proximate and ultimate explanations of human morality. We do so by developing an approach focusing on recent developments in the study of mutualistic forms of cooperation and on their relevance to fairness-based morality.