The historical and archaeological records reveal that humans became especially good at killing 'outsiders' from other groups, tribes or nations. Some animals do engage in such conflicts, but humans excel. We are also uniquely receptive to socialization and learning, and can achieve the heights of altruistic behaviour. Economist Samuel Bowles argues that these two extremes may be related: generosity and solidarity towards one's own may have emerged only in combination with hostility towards outsiders. Both may be part of what it is to be human. All essays in the 'Being human' series are available free via http://tinyurl.com/55ncjj.From the article:
Among ancestral humans, parochial altruists may have provoked conflicts between groups over scarce natural and reproductive resources, and at the same time contributed to a group's success in these conflicts. Altruism would have facilitated the coordination of raiding and ambushing on a scale known in few other animals, while parochialism fuelled the antipathy towards outsiders. Additionally, with the development of projectile weapons, humans became adept at killing from a distance, which would have reduced the costs of aggression.
Support for this idea comes from artificial histories of early human evolution that my co-authors and I simulated by computer. In these simulations, we allowed groups of agents, tolerant or parochial, altruistic or selfish, to interact over thousands of generations under conditions likely to have been experienced by our Late Pleistocene and early Holocene ancestors. We designed the simulations so that violent conflict between two groups is likely if at least one group contains a preponderance of parochialists. We also made each group's fighters the parochial altruists (non-altruists are happy to let someone else do the fighting; tolerant members prefer to stay on friendly terms with outsiders). Thus, the groups with the most parochial altruists tend to win conflicts. Our objective was to see how the frequency of warfare, and the fraction of the different types of agent, would evolve.
In millions of simulated evolutionary histories, the populations emerging after thousands of generations of selection tend to be either tolerant and selfish, with little warfare, or parochial and altruistic with frequent and lethal encounters with other groups. Occasional transitions occur between the selfish peaceful states and the warring altruistic states. But neither altruism nor parochialism ever proliferate singly; they share a common fate, with war the elixir of their success.