In the face of threats from other groups, humans, chimpanzees and select other species tend to move closer to their own kind. Now, an international team led by Kyoto University has shown that even our more peaceful cousins, the bonobos — who have never killed outsiders — display a moderate version of this effect, suggesting the behavior may have evolved millions of years ago, before our lineages went their separate ways.

A connection betweenoutgroupthreats andin-groupCohesion has been considered an adaptation for group-based competition since Darwin’s time. In the years since, studies of everything from chimpanzees to cichlids to mongooses have found evidence to support this view, but a crucial question has remained unanswered: What about species without strong intergroup competition?

To find an answer, the KyotoU team set up an experiment that was as close as possible to an earlier study with chimpanzees: playback of vocalizations from other groups. In total, eight groups of bonobos at five locations in four countries were studied.

“We had no idea how this would end,” said lead researcher James Brooks.

“Without lethal competition between groups, a relationship between within-group cohesion and outside-group competition would not be as adaptive, but if the effect dates back to before the evolutionary split between humans, chimpanzees, and bonobos, then there might still be remnants of the effect in modern bonobos.”

The team’s findings, published in the journalPLOS ONEindicate that the bonobos observed were alert and attentive to the calls of other groups, but showed only a small increase in affiliation with their own group compared to chimpanzees. The bonobos were observed sitting more upright and resting less, with a subtle increase in the amount of social grooming, an important behavior for strengthening social bonds.

The authors propose that our common ancestor, which lived 5–6 million years ago, may have experienced group conflict, but that as its intensity decreased over the bonobos’ evolutionary history, the strength of the effect also decreased.

“While our research reveals deep roots of group conflict within our species, the key lesson is that it can be overcome,” Brooks adds, “not just in individual cases, but also at the species level.”

All other ape species, including gorillas, orangutans, chimpanzees, gibbons, and humans, have been observed killing each other in the wild. Bonobos may have found a way to break this pattern, not only because they no longer engage in lethal aggression today, but more importantly because they somehow stopped doing it at some point in the last few million years.

“Humans are capable of both: we can do terrible things to people we see as outsiders, but we can also cooperate and collaborate across borders,” said lead author Shinya Yamamoto.

“Bonobos teach us that the way our ancestors treated other groups does not seal our fate. Our own species has elements of both chimpanzee and bonobo group relationships, so it is crucial that we understand how both can and have evolved.”