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Scientists are learning more about how crab-eating macaques (shown grooming) and other primates abandoned the single lifestyle to form the large, stable societies the creatures live in today.Roy Fontaine

Primates may have evolved from living the lonely life to forming complex societies in two major steps, a new study of more than 200 species suggests. Understanding when and why the ancestors of Homo sapiens and its closest cousins adopted different social structures could help reveal more about the evolution of human society.

About 52 million years ago, primates — an order of animals that includes, among others, humans and great apes — might have stopped foraging alone and banded together in large, loosely formed, same-sex groups to search for food, anthropologist Susanne Shultz of the University of Oxford and colleagues report in the Nov. 10 Nature. Then around 16 million years ago, primates began forming more stable social groups, such as male-female pairs and harems dominated by one male, the researchers suggest.

Teaming up this way may have been prompted by a switch from a nocturnal lifestyle to moving about in the sunshine. “Being active during the day would have allowed primates to travel across larger spaces and exploit their environment more effectively, but it would have also exposed them to a huge predation risk,” says Shultz. To make it through the day, primates would have needed a new defense strategy to deal with both a greater number of predators and also new kinds of hunters.

“What’s going to nail you at night is different than what’s going to nail you during the day,” says primatologist Anthony Di Fiore of the University of Texas at Austin, who was not involved in the study. It’s tough to hide from eagle eyes in the daytime, but by joining up and serving as lookouts for each other primates would have given themselves a better chance of spotting and evading a swooping bird or other predator.

Re-creating the social behavior of animals that died millions of years ago can be tricky business. However, since behaviors are inherited, examining the ways living species interact socially can provide clues to the ways their ancestors behaved.

Shultz and colleagues examined the social behavior of 217 species of living primates, such as baboons, gibbons, and tamarins. Combining this data with information about primates’ evolutionary tree, the researchers made predictions about the likely transitions the creatures underwent between foraging alone and forming the complex societies evident in modern-day primates.

“We asked the data to tell us what the most likely transition was,” says Shultz. The researchers found that the most likely route primates took to learn today’s cooperative behavior involved two discrete evolutionary jumps in social behavior. Ancestral primates didn’t alter their social strategies freely depending on changing habitats, nor did they incrementally increase the size and complexity of their clans over time, the researchers argue.

To get a broader picture of the evolution of cooperative social systems, the next step is to test these transition theories in other mammals, say Shultz and colleagues. The development of sociality in primates may provide clues to social behavior in humans. “If we didn’t have these social groups evolving in primates, we wouldn’t have the scaffolding in place for humans to build upon,” says Shultz.

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