This chapter argues that intergroup lethal conflict and peaceful cooperation both have deep evolutionary roots in the hominin lineage and co-evolved over millions of years through a mosaic-like pattern. The inclination for intense male–male aggression was first inherited from Miocene apes, where it mainly functioned to threaten and deter rivals rather than to kill. The capacity for intergroup coalitionary killings was then likely present from the time hominins engaged in group hunting, an aptitude possibly already found in the last common ancestor of chimpanzees, bonobos, and humans. This capacity was enhanced in the genus Homo through cultural innovations such as weapon technology and cultural benefits such as status and prestige. In parallel, the inclination for peaceful intergroup cooperation also emerged gradually, as first suggested by a relative reduction in aggression levels in hominins compared to other apes, indicated by changes in canine morphology. This shift became intertwined with cumulative cultural evolution, enabling practices such as negotiation, peacemaking, intergroup marriage, and exchange networks. In Homo sapiens, cumulative culture became supplemented by a selection against reactive aggression (also called ‘self-domestication’), enhancing prosocial intergroup relationships. The combination of cumulative cultural evolution and selection against reactive aggression further increased the scale of peaceful cooperation, as reflected in the emergence of long-distance exchange networks in the Palaeolithic archaeological record.

The Introduction sets out the central question that this book aims to tackle: when, why, and how did we evolve the peculiar and paradoxical inclination for both war and peace that characterises our species? The book argues that the inclination for both intergroup coalitionary killing and peaceful cooperation have deep roots and that they co-evolved through a gradual, incremental trajectory over millions of years. This co-evolution unfolded in a mosaic-like pattern, with distinct but interacting components emerging at different moments and becoming integrated over evolutionary time.