Darwin wondered whether lethal warring between neighboring groups might have caused humans to evolve to be more helpful and kind to each other. At first, the idea seems paradoxical. But Darwin thought this could have happened if the more cohesive, unified, caring groups had been better able to triumph over their more disunited rivals. If so, the members of those cohesive, yet warlike, groups would have left more descendants.
For a long time, the idea languished. Why? A couple of reasons. First, it appears to depend on “group selection.” This is the idea that some groups evolve characteristics that allow them to outcompete other groups, and it’s long been out of favor with evolutionary biologists. In general, natural selection works much more effectively on individuals than it does on groups, unless the groups are composed of close kin. That’s because group selection can be effective only when the competing groups are genetically distinct. Members of a kin group tend to be genetically similar to each other, and different from members of other kin groups. In contrast, groups composed of non-kin tend to contain considerable genetic variation, and differences between such groups are generally much smaller. Moreover, contact between the groups—individuals migrating from one to another, say—will reduce any genetic differences that have started to accumulate. So unless natural selection within the groups is different—such that what it takes to survive and reproduce in one group is different from what it takes in another—migration quickly homogenizes the genetics of the whole population.
A second reason Darwin’s idea has been ignored is that it seems to have a distasteful corollary. The idea implies, perhaps, that some unpleasant human characteristics—such as xenophobia or even racism—evolved in tandem with generosity and kindness. Why? Because banding together to fight means that people must be able to tell the difference between friends (who belong in the group) and foes (who must be fought). In the mid-1970s, in a paper that speculated about how humans might have evolved, Hamilton suggested that xenophobia might be innate. He was pilloried.
But times have changed. Last year, the science journal Nature published a paper that tested the idea of “parochial altruism”—the notion that people might prefer to help strangers from their own ethnic group over strangers from a different group; the experiment found that indeed they do. In addition, the idea that natural selection might work on groups—at least in particular and narrow circumstances—has become fashionable again. And so Darwin’s idea about the evolution of human kindness as a result of war has been dusted off and scrutinized.
Sam Bowles, an economist turned evolutionary biologist who splits his time between the Santa Fe Institute, in New Mexico, and the University of Siena, in Italy, notes that during the last 90,000 years of the Pleistocene Epoch (from about 100,000 years ago until about 10,000 years ago, when agriculture emerged), the human population hardly grew. One reason for this was the extraordinary climactic volatility of the period. But another, Bowles suggests, was that our ancestors were busy killing each other in wars. Working from archaeological records and ethnographic studies, he estimates that wars between different groups could have accounted for a substantial fraction of human deaths—perhaps as much as 15 percent, on average, of those born in any given year—and as such, represented a significant source of natural selection.
Bowles shows that groups of supercooperative, altruistic humans could indeed have wiped out groups of less-united folk. However, his argument works only if the cooperative groups also had practices—such as monogamy and the sharing of food with other group members— that reduced the ability of their selfish members to outreproduce their more generous members. (Monogamy helps the spread of altruism because it reduces the differences in the number of children that different people have. If, instead, one or two males monopolized all the females in the group, any genes involved in altruism would quickly disappear.) In other words, Bowles argues that a genetic predisposition for altruism would have been far more likely to evolve in groups where disparities and discord inside the group—whether over mates or food—would have been relatively low. Cultural differences between groups would then allow genetic differences to accumulate.