Bonobos look like tame versions of chimpanzees. They're much less aggressive than chimps, their features are softer and smaller than their cousins, and they famously have sex for pleasure. Basically bonobos are domesticated... except humans had nothing to do with it.
The differences between chimps and bonobos are a good match for similar pairings of wild and domesticated animals, such as wolves and dogs. But there's no reason to think that ancient humans once went around trying to domesticate chimpanzees, so whatever happened here must have been without any outside intervention. And according to Duke researcher Brian Hare, bonobos really are an example of a self-domesticated ape, a species where aggressive males were weeded out by natural selection so that only the meeker, more good-natured bonobos remained.
As Hare explains to Scientific American, the divergence between chimps and bonobos likely began around one to two million years ago, when the newly formed Congo river separated two populations of their chimp-like common ancestor. The northern apes found themselves sharing territory with gorillas, who had a huge advantage in acquiring food. The smaller apes had to fight viciously for the remaining food, with males likely coming to dominate females both in terms of sex and resources. The result? Chimps.
But, according to Hare, the Congo River isolated the southern apes from gorillas, which meant food remained plentiful. As Scientific American explains, this was great news for females:
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.
Assuming it's true, this means that domestication isn't all about humans. Instead, anywhere a species encounters an environment where aggression isn't valued, they will become tamer and, ultimately, more childlike. The problem, however, is that we can't be sure that Hare's story of bonobo evolution is correct, because we don't have any fossils of their common ancestors with chimps. In the absence of clear evidence to support the theory bonobos may have to remain a singularly weird species, albeit a very pleasant one.