QUOTE : Long before this, our ancestors lived on a thinly populated planet, with altogether only a couple of million people. Their density may have resembled that of the Bushmen, who live on ten square miles per capita. There are even suggestions that before this, about seventy thousand years ago, our lineage was at the edge of extinction, living in scattered small bands with a global population of just a couple of thousand. These are hardly the sort of conditions that promote continuous warfare. Furthermore, our ancestors probably had little worth fighting over, again like the Bushmen, for whom the only such exceptions are water and women. But Bushmen share water with thirsty visitors, and regularly marry off their children to neighboring groups. The latter practice ties groups together and means that the men in one group are often related to those in the other. In the long run, killing one’s kin is not a successful trait.