Human ultra-sociality—or the ability to cooperate with huge numbers (millions and more) of genetically unrelated individuals— is a particularly intriguing and novel evolutionary puzzle because it is not explained by genetic relatedness. Most ultra-sociality in nature occurs among genetically related individuals (mothers, grandmothers, sisters, brothers, fathers, grandfathers, related aunts and uncles). In eusocial insects (an advanced level of social organization, in which a single female or caste produces the offspring and nonreproductive individuals cooperate in caring for the young)—the highest level of organization of sociality—workers tend to be related siblings. In the eusocial mammal—naked mole rats—individuals seldom migrate from their natal burrow creating underground colonies of highly related individuals.
But humans cooperate in groups comprised of hundreds, thousands, and even millions of genetically unrelated individuals. A growing body of research suggests that our extreme reliance on culture—socially transmitted knowledge, beliefs, and norms—might be the key ingredient that fueled this transition. Language too may be critical because it allows cooperative reputations to spread and become widely known, and noncooperators are also known by all.