This book addresses humanity’s dual capacity for both war and peace. Through cross-species comparisons and wide-ranging evidence from thirteen disciplines, it reveals that neither ‘deep roots’ nor ‘shallow roots’ perspectives fully capture human behavioural plasticity. A central finding is that, contrary to the binary opposition between the ‘deep roots’ and the ‘shallow roots’ of war theses, these seemingly contradictory features of war and peace both have deep roots in the hominin lineage, and they co-evolved along an incremental trajectory spanning millions of years. This trajectory is best understood as a multi-component process with a mosaic pattern of development, involving various interacting elements that emerged at different moments and gradually assembled over long time spans. Both war and peace have deep roots, and they co-evolved. Long-term biocultural evolutionary processes – with mutually reinforcing feedback loops – have shaped the patterns of lethal conflict and peaceful cooperation between communities over millions of years. These processes laid the foundation upon which sedentary civilizations, states, and empires rose and fell, and for the ways in which they conducted diplomacy, engaged in trade, and waged war. They paved the way for the continued persistence of both war and peace in our species.