Like chimps and bonobos, humans live in social groups full individuals who help each other. Also like our monkey relatives, we make war with outsiders. But our social groups can be millions strong. What makes humans so good at forming mass societies?
Anthropologist Kim Hill and her colleagues decided to find out by looking at tribal people whose cultures resemble what researchers think early human cultures might have been like. Hill and colleagues compared our human tribal structures with the group structures found among chimps and bonobos. And the main difference appears to be that humans conceive of our social groups as nested – smaller groups within larger and larger ones. So how did we learn to think in those terms, and later build societies as large as nations with billions of citizens – or an internet with just as many?
The researchers believe it has to do with the idea of pair-bonding, or perhaps more specifically marriage. Once we created stable families – whether monogamous or poly – humans could establish extended kinship structures that included things like uncles, siblings, and all that second-cousin and third-once-removed stuff that nobody understands but still honors as part of the family structure. Having these extended families laid the groundwork for the idea of nested social groups.
But there are other important differences between our tribes and the ape groups. Hill and colleagues found that among humans, it was common for both males and females to leave one group for another. Among apes, either males moved around or females – never both. Also when humans moved to another tribe, they always stayed in touch with people in their original tribe, thus solidifying that sense of extended social connection. When apes switch groups, they lose contact with the group where they were born.
In general, our tribes favour generous mixing between groups, leading to communities whose members are often unrelated. But we also maintain very complicated connections. Hill and her colleagues think it’s this combination – the connected dispersal of a network – that led to the rise of human culture. They write in a paper published today in Science:
When people reside together, they have frequent opportunities to observe innovations, evaluate their success, and imitate traits judged most successful or most common. Our analyses suggest that the increased network size that follows a unique shift in ancestral human residential structure may have led to greater exposure to novel ideas worth copying, and may explain why humans, but not other animals, evolved costly social learning mechanisms (such as high-fidelity over-imitation or conformity-biased transmission) that may have resulted in cumulative cultural evolution. This unique expansion of network size in our hominin ancestors can be detected archaeologically by the emergence of long-distance flows of tools and raw materials that appear at least as early as the middle Pleistocene.
Humans were able to trade and share information over vast distances, learning a great deal in the process, very early in our evolution. And it’s all because we stayed in touch even as we scattered to our new families and communities.
Read the full scientific paper via Science