Time and again humans have domesticated wild animals, producing tame individuals with softer appearances and more docile temperaments, such as dogs and guinea pigs. But a new study suggests that one of our primate cousins—the African ape known as the bonobo—did something similar without human involvement. It domesticated itself.

Anthropologist Brian Hare of Duke University's Institute for Brain Sciences noticed that the bonobo looks like a domestic version of its closest living relative, the chimpanzee. The bonobo is less aggressive than the chimp, with a smaller skull and shorter canine teeth. And it spends more time playing and having sex. These traits are very similar to those that separate domestic animals from their wild ancestors. They are all part of a constellation of characteristics known as the domestication syndrome.

The similarities between bonobos and domesticated species dawned on Hare during a large departmental dinner, where he listened to Harvard University anthropologist Richard Wrangham hold forth on bonobos. "He was talking about how bonobos are an evolutionary puzzle," Hare recalls. "'They have all these weird traits relative to chimps and we have no idea how to explain them,'" Wrangham had noted. "I said, 'Oh that's like the silver foxes!' Richard turned around and said, 'What silver foxes?'"

The foxes that Hare mentioned were the legacy of Russian geneticist Dmitri Belyaev. In the 1950s Belyaev started raising wild silver foxes in captivity and breeding those that were least aggressive toward their human handlers. Within just 20 generations, he had created the fox equivalent of our domestic pooches. Instead of snarling when humans approached, they wagged their tails. At the same time, their ears became floppier, tails curlier and skulls smaller.

Belyaev's experiments showed that if you select for nicer animals, the other parts of the domestication syndrome follow suit. Hare thinks that a similar process happened in bonobos, albeit without human intervention.

Rape, murder and warring neighbors are all regular aspects of chimp life. Bonobo societies, however, are far more peaceful. Hare thinks that the chimplike ancestors of bonobos found themselves in an environment where aggressive individuals fared poorly. By selecting for the most cooperative ones, evolution forged a "self-domesticated" ape, just as Belyaev produced domestic foxes by picking the most docile ones.

Hare has now laid out his ideas in a new paper, written with Wrangham and Harvard colleague Victoria Wobber, and published online January 20 in Animal Behaviour.

Bonobos and chimpanzees diverged from a common ancestor between one million and two million years ago, after the formation of the Congo River separated one population of apes into two. Considering that neither species can swim, the two populations "might as well have been on different planets," Hare says.

Both groups faced very different environments. Hare thinks that the northern population, which would eventually become chimps, had more competition from gorillas for their food. They were forced to compete fiercely with one another for whatever was left. Females got a particularly short shrift, and were easily overpowered by males for both sex and resources.

"In bonobo-land in the south, the story was different," Hare explains. "The river would have protected the ancestors of bonobos from gorillas." With more food to go around, females could gather in larger groups, form tight social bonds, and better resist the advances of males. In this land of plenty, the least aggressive males, who opted for alliances rather than brute force, were most likely to mate. South of the river, the nicer apes thrived.

As a result, Hare thinks that they started maturing more slowly. Many domestic animals evolved to become less aggressive by slowing the pace of development, so adults retained juvenile traits. For example, as Belyaev’s foxes became tamer, their minds and bodies became more like those of puppies than wild adults.

The same thing probably happened as domestic dogs and bonobos evolved from their respective ancestors. Their physiques changed—faces became shorter, skulls shrank, sex differences narrowed, teeth shortened and sections of their fur lost coloration. Their bodies responded to stress in a more muted way. They behaved differently, playing, grooming and mating more often. The tamer generations also became more sensitive to social cues. Simply by maturing more slowly, they all evolved the same set of domesticated traits.

"People have been thinking about domestication as a human-centered thing: purposeful, directed, something we do to animals," says Greger Larson of Durham University in England, who was not involved in this study. "But what Brian says is that this process, which we imbue with all this human-centric meaning, is something that takes place in nature. That's super cool."

Not everyone is convinced by Hare's idea. "I'm not buying it," says bonobo expert Frans de Waal of Emory University. He notes that it is not clear if bonobos evolved from a chimplike ancestor or vice versa. If the latter is true, then the question is why chimps became so aggressive, rather than why bonobos evolved to be nicer. Hare admits the problem. "It's a real challenge, especially since we don't have a single fossil for either species," he says.

De Waal also wonders if other female-dominated species, such as spotted hyenas or ring-tailed lemurs, would also show signs of self-domestication. Hare agrees. "To really test the hypothesis," he remarks, "you need to have a dozen species that you believe are self-domesticated to understand what it is about the ecology that might explain these changes."

Ultimately Hare hopes that more research on bonobo genetics, behavior and ecology will demonstrate whether his idea is right or wrong.