Not only did farming increase our work hours, reduce our travel, impoverish our diet, and increase social hierarchy, it also greatly increased war. From a Science book review: There are four extant great ape species. Each has a different social system, with chimpanzees being the only species to regularly evidence multimale violent coalitions. …. In chimpanzees, each female … mates with every male in her group. Males join forces to defend their territory … and each male will have access to all those females… Marriage is a universal human behavior, and it is defined by cultural and legal rules proscribing sex outside of marriage, particularly by women. … All available evidence suggests that intergroup violence, practiced primarily by males, does have a long evolutionary history in our species. However, the intensity and nature of that violence is highly variable. …