Humans rely heavily on their prosocial relationships. We propose that the experience and display of prosocial emotions evolved to regulate such relationships through inhibiting individual selfishness in service of others. Two emotions in particular serve to meet two central requirements for upholding prosociality: gratitude motivates maintenance of ongoing prosocial interactions, and guilt motivates repair of ruptured prosocial interactions. We further propose, and review developmental evidence, that nascent forms of these two emotions serve their respective functions from early in ontogeny. The (...) remarkably early emergence of these prosocial emotions allows even very young children to participate in and benefit from prosociality. We discuss the implications of and challenges to this conclusion and identify pressing future directions for this work. (shrink)