By Elizabeth Pennisi, ScienceNOW

Look around and it's impossible to miss the importance of social interactions to human society. They form the basis of our families, our governments, and even our global economy. But how did we become social in the first place? Researchers have long believed that it was a gradual process, evolving from couples to clans to larger communities. A new analysis, however, indicates that primate societies expanded in a burst, most likely because there was safety in numbers.

It's a controversial idea, admits anthropologist and study author Susanne Shultz of the University of Oxford in the United Kingdom. "We're likely going to cause a bit of trouble."

Over the past several decades, researchers have gained tremendous insights into the evolution of social groups in bees and birds by comparing them with relatives with different social systems. In these animals, it seems that complex societies evolved in steps. Single individuals paired off or began living with a few offspring. These small groups gradually grew larger and more complicated, ultimately yielding complex organizations. Some anthropologists have assumed a similar history for primates.

Shultz and colleagues decided to test this idea. Their first task was figuring out which factors influenced the makeup of current primate societies. A common hypothesis is that the local environment shapes group structure. For example, food scarcity might drive individuals together so that they can help each other with hunting and foraging. But after combing the scientific literature on 217 primate species, the researchers noticed that closely related species tended to organize their societies in the same way, no matter where they lived. Baboons and macaques, for example, inhabit many places and habitats, yet for the most part they always live in a mixed company of related females and unrelated males.

Because group structure was not at the whims of the environment, Shultz and colleagues reasoned, it must be passed down though evolutionary time. And indeed, when they looked across the primate family tree, they found that the current social behaviors of a species tended to be similar to those of its ancestors.

With this in mind, the researchers inferred how the ancestors of these primates lived, trying to come up with the scenario that would require the fewest evolutionary changes to get to the current distribution of social organizations in the family tree. They ran a statistical model to determine what would happen, say, if the last common ancestor to the monkeys and apes lived in pairs or lived in groups.

To the researchers' surprise, the most sensible solution suggested that the solitary ancestor started banding together not in pairs, as scientists had thought, but as loose groups of both sexes, as the team reports online today in Nature. Given the modern distribution of social organizations, the most likely time for this shift was around 52 million years ago, when the ancestors of monkeys and apes split off from the ancestors of lemurs and other prosimian primates.

Shultz suspects that, at this time, the nocturnal ancestors of today's primates became more active during the day. It's easier to sneak around at night when you're alone, she notes, but when you start hunting during the day, when predators can more easily spot you, there's safety in numbers.

But not all of today's primates live in large, mixed-sex groups. A few, such as the New World titi monkeys, live in pairs. And some primates, such as gorillas, form harems with one male and multiple females. The analysis shows that these social structures showed up only about 16 million years ago.

"When I read the paper, I was really quite struck with what a different picture [it] gives us," says Joan Silk, an anthropologist at the University of California, Los Angeles. "[Some] theoretical models will have to be revised."

Bernard Chapais, a primatologist at the University of Montreal in Canada is impressed with how many primates the analysis included. "It's close to the total number of species in the primate order," he says. He agrees with Shultz's scenario, but he and Silk would have liked to see Shultz consider more details, such as the mode of reproduction, when classifying social systems—something she plans to do. Even without that refinement, however, "these analyses represent a welcome addition to the current study of social evolution," says Peter Kappeler, an anthropologist at the University of Göttingen in Germany.

This story provided by ScienceNOW, the daily online news service of the journal Science.

Image: Gelada baboons (Theropithecus gelada) live in large, mix-sexed groups. Dave Watts/Flickr

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