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Our ability to cooperate closely with other group members and to suppress cheats means that selection at the group level rather than the individual level has been an exceptionally strong force during human evolution. It may have played a crucial role in shaping both our genes and our culture.
Rare and momentous event
Converging lines of evidence suggest that human genetic evolution represents a major evolutionary transition and one which accounts for our uniqueness among primates. In most primates, members of a group cooperate to a degree, but there is also intense competition within groups for social dominance. In contrast, most extant hunter-gatherer societies are vigilantly egalitarian, suppressing individuals who try to benefit themselves at the expense of others. As we have seen, the suppression of within-group selection is the hallmark of a major transition.
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Vigilant egalitarianism probably arose early in human evolution and was a precondition for the other attributes that make us so distinctive as a species. This is an example of gene-culture co-evolution in which it is impossible to say which came first.
Our closest primate relatives are also highly intelligent but in a way that appears predicated on mistrust. In contrast, human intelligence appears to be based on trust among members of a group, making shared awareness and coordination of activities advantageous for so many generations that they are now instinctive in our species, as developmental psychologist Michael Tomasello has argued. Even dogs, which have been co-evolving with humans for tens of thousands of years, surpass most primates with respect to adaptations based on trust (Science, vol 298, p 1634).
As with other major evolutionary transitions, the human transition was a rare event that had momentous consequences. As the only primate super-organism, we have spread over the globe, occupied hundreds of ecological niches and displaced countless other species 鈥� for better or for worse. The human transition is also by no means complete. Within-group selection is only suppressed, not stopped, so constant vigilance is needed to maintain prosocial behaviour. Indeed, many aspects of human morality can be interpreted as the apparatus that evolved to make group selection a strong force in our species.
Cultural group selection
Cultural evolution is a multilevel process, just like genetic evolution. Socially transmitted traits can spread because they benefit whole groups, or give individuals an advantage within groups, or because they act like parasites and do not benefit individuals or groups. Examples can be found at all levels but group-level selection appears to be an even stronger force for human cultural evolution than for human genetic evolution.
鈥淛ust like genetic traits, cultural traits can spread when they benefit groups鈥�
With genetic evolution, a mutant gene starts out at a low frequency within single groups. It takes many generations for a beneficial mutation to become common in a group and eventually in the total population. With cultural evolution, however, a new trait can become common within a group in less than a generation. Cultural variation between groups can even be maintained in the presence of migration, as long as individuals entering a group abandon their old customs and adopt the new ones. Finally, almost any trait can be established within groups by norms enforced by rewards and punishments. Group selection for norms that are stable within each group can be more powerful than group selection for traits that are selectively disadvantageous within groups.
Cultural group selection has resulted in the development of cooperative human societies several orders of magnitude larger than the pre-agricultural societies that existed only 15,000 years ago. Human history provides a detailed record of multilevel cultural evolution, including the expansion of the most cooperative and coordinated cultures and the collapse of cultures plagued by divisions. This dynamic might even explain the rise and fall of empires throughout human history, as ecologist Peter Turchin details in his 2005 book War and Peace and War.
Darwin鈥檚 problem is encountered at every scale of human society: from the smallest group to the global village, the behaviours that maximise relative advantage within a social unit tend to undermine the welfare of the unit as a whole. Establishing prosociality at a large scale requires a process of selection at that scale 鈥� whether a raw process of variation and selection or a more deliberative process of selecting practices by intentional planning.
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