Using the last recorded formal duel in the U.S. as an introductory framework, this essay explores how adjudi-cation and conciliation, two distinct modes of dispute processing, interact to affect social evolution. Evolutionary transitions and the fitness of more complex entities, such as pro-social animal societies, require ongoing stable cooperation among constituent members. Cultural mechanisms of law and conciliation harness different aspects of evolved reciprocal behavior to address these requirements. Building on the work of mid-20th century legal philosopher Lon Fuller, the essay examines how adjudication and conciliation may work in social evolution. It concludes with examples of how law itself attempts to address the conflict between these two modes of dispute processing.