For example, we know that interband warfare is very common among the chimps, our closest evolutionary relatives. Warfare is also nearly ubiquitous among the small-scale societies of hunter-gatherers and farmers. The anthropologist Lawrence H. Keeley presents evidence that somewhere between 20 percent and 60 percent of males in these societies die in wars. By an argument of “interpolation,” therefore, if both chimps and modern pre-state people practiced extensive warfare, so must have our human ancestors.
