About 6 million years ago in Africa there was an ape species that would change the world. We don’t know much about that animal — we assume it was similar to the modern-day chimpanzee — but we do know that one population separated from the rest and would eventually evolve into our species, Homo sapiens. The rest of the species stayed together for another few million years.

Then, between 1 million and 2 million years ago, this group again split in two — separated by the physical barrier of the Congo River. The formerly single species would go on to evolve into chimpanzees on one side of the river, and bonobos on the other.

Six million years sounds like a long time, but in evolutionary terms it’s rather short. We are still closely related to both of those cousin ape species, and slightly more closely to bonobos than to chimps. Understanding them, therefore, is hugely important to learning about our own biology and behavior, and work at the Primate Research Institute at Kyoto University has been vital.

Decades of study in the wild has told us that chimps tend toward violence, and bonobos tend toward love and peace. There are exceptions, and we’ll come back to a shockingly gruesome finding that has just been reported by the Kyoto group, but in general chimps use violence to settle disputes, whereas bonobos, where females dominate society, use sex.

Humans can’t be so easily categorized. We are both violent and peaceful; we seem to have blended characteristics of both chimps and bonobos. I’ve been thinking about this recently, reading primatologist Richard Wrangham’s new book “The Goodness Paradox.” Wrangham is a professor of anthropology at Harvard University, and argues that since we split from chimps and bonobos, humans domesticated themselves. A species becomes domesticated, Wrangham says, if it changes genetically and becomes tame.

The key was when sophisticated language evolved, which enabled conspiracies and cooperation to flourish. This change in our ability to communicate seems to have occurred between 500,000 and 300,000 years ago. It meant, Wrangham argues, that weaker individuals could join forces and plan to attack and remove — kill — violent, stronger leaders. As the ferocious and violent males were gradually removed from the population over hundreds of thousands of years, our species as a whole become less violent. We became tame.

Incidentally this gradual reduction in the use of violence against members of our own species has continued in modern times, as documented brilliantly by another Harvard professor, Steven Pinker, in his book “The Better Angels of Our Nature.”

Like everything, language itself evolved from simple forms. A recent analysis of the gestural language used by chimps and bonobos shows that there are many similarities — if the two apes ever met up, they would probably be able to understand each other.

Takeshi Furuichi of Kyoto University, and a team of international colleagues, recorded 33 different gestures made by bonobos and compared them with those made by chimps. “Bonobos and chimpanzees share not only the physical form of the gestures, but also many gesture meanings,” the scientists say.

For example, an arm extended in front of another animal means “climb on me” in both bonobo and chimpanzee language; scratching of the arms means “groom me,” and stroking the mouth means “fetch that object.” Some gestures were different, however. If a bonobo stands up it means “let’s have sex” — but the signal doesn’t exist for chimps. You might expect that there are more signals designed to encourage sex in a species where sex is used socially.

Lest we get carried away with the idea that bonobos are completely loving and that their society is some sort of sexual utopia, let’s just review some more of Furuichi’s work with wild bonobos in Congo. Be warned this is quite stomach churning.

Furuichi’s team observed two cases of female bonobos eating their own offspring. Maternal cannibalism is very rare in apes, but the observations by Furuichi and another case in Congo suggest it is a normal part of bonobo behavior. It’s not known whether the mothers killed their own children first.

Kirsty Graham from the University of York, U.K., is one of Furuichi’s collaborators. “In future, we hope to learn more about how gestures develop through the apes’ lifetimes. We are also starting to examine whether humans share any of these great ape gestures and understand the gesture meanings, so watch this space.”

It’s worth noting that both bonobos and chimpanzees are endangered species, with populations declining toward extinction in the wild. We can’t let this happen. It’s wrong in its own right to let these animals go extinct and, if we do, we will lose the opportunity to gain huge insight into our own evolution.

Rowan Hooper is managing editor of New Scientist magazine. He tweets at @rowhoop and his book, “Superhuman: Life at the Extremes of Our Capacity,” is out now, published by Simon & Schuster.

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