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.