"That chimpanzees and humans kill members of neighbouring groups of their own species is...a startling exception to the normal rule for animals. Add our close genetic relationship to these apes and we face the possibility that intergroup aggression in our two species has a common origin. This idea of a common origin is made more haunting by the clues that suggest modern chimpanzees are...surprisingly excellent models of our direct ancestors. It suggests that chimpanzee-like violence preceded and paved the way for human war, making modern humans the dazed survivors of a continuous, 5-million-year habit of lethal aggression."