Inter-group conflict may be important among chimpanzees, but Homo sapiens turned it into an art:
Although the details remain highly controversial, a series of new studies in archaeology and anthropology have debunked Rousseau’s myth of the peaceful savage. Death rates (as the percentage of adult males killed in intergroup conflict) among indigenous and prehistoric societies make the wars of the 20th century seem like skirmishes. Although humans were not always at war, human societies were always organized around its ever-present threat.
A sensitivity to human evolution and the behavior of chimpanzees and bonobos forces students of modern students of conflict to face two hard truths. First, we evolved in conditions of resource competition where fear of others, aggression and violence offered adaptive solutions to protect and provide for ourselves and our kin. We therefore need to amend Clausewitz. Humans do indeed wage war for political purposes, but long before war for raison d’etat there was war for resources. International politics is therefore not the root of war but merely an example of it—the continuation of seeking access to valuable resources by other means. Accordingly, when we consider “Why war?,” we have an answer: war is one of Mother Nature’s solutions to compete successfully for resources.
Second, the human traits of egoism, dominance, and in-group/out-group bias are at least partly adaptations to the ecological conditions prevalent in human evolution. It is not assumed that we simply inherited these wholesale from a common ancestor, or the common ancestor we share with the chimpanzee. Clearly, we have undergone many physiological and behavioral changes since then and ecology has been as or more important. But although humans and chimpanzees appear to have travelled much of the road to war together, we have gone far further. The particular socioecological setting in which humans evolved meant that aggression and war were significant behavioral adaptations. These same settings led to remarkable levels of cooperation as well, but note that this cooperation is selectively directed towards in-group members, the better to avoid exploitation by rival groups and organize for war. These adaptations, lamentably, remain with us today and influence our behavior, politics and society.