Social behavior among primates–including humans–has a substantial genetic basis, a team of scientists has concluded from a new survey of social structure across the primate family tree.
The scientists, at the University of Oxford in England, looked at the evolutionary family tree of 217 primate species whose social organization is known. Their findings, published in the journal Nature, challenge some of the leading theories of social behavior, including:
• That social structure is shaped by environment–for instance, a species whose food is widely dispersed may need to live in large groups.
• And the so-called social brain hypothesis: that intelligence and brain volume increase with group size because individuals must manage more social relationships.
By contrast, the new survey emphasizes the major role of genetics in shaping sociality. Being rooted in genetics, social structure is hard to change, and a species has to operate with whatever social structure it inherits.
If social behavior were mostly shaped by ecology, then related species living in different environments should display a variety of social structures. But the Oxford biologists–Susanne Shultz, Christopher Opie and Quentin Atkinson–found the opposite was true: Primate species tended to have the same social structure as their close relatives, regardless of how and where they live.
The researchers suggest that sociality emerged about 52 million years ago. The earliest primates sought safety by being solitary and inconspicuous, moving only at night. It seems that when they shifted to daytime activity, they sought safety in numbers.
It was from these loose, unstructured groups that more specific forms of primate social behavior began to evolve, some 16 million years ago. These included pair bonding, an arrangement adopted by gorillas and humans, and the multi-male, multi-female groups typical of baboons and chimpanzees.
The fact that related species have similar social structures, presumably because the genes for social behavior are inherited from a common ancestor, “spells trouble” for ecological explanations, Joan B. Silk, a primate expert at the University of California, Los Angeles, wrote in a commentary in Nature. Also, the finding that there has not been a steady progression from small groups to large ones challenges the social brain hypothesis, Dr. Silk said.
The Oxford survey confirms that the structure of human society, too, is likely to have a genetic basis, since humans are in the primate family, said Bernard Chapais, an expert on human social evolution at the University of Montreal.