The violent tearing apart of a group of chimpanzees suggests the evolutionary roots of war

A once harmonious group of wild chimpanzees split in two, leading to intractable conflicts and escalating violence. Researchers say the rift suggests that human warfare is a deeply ingrained part of our nature, rather than something that emerged recently as our culture became more complex.

Aaron Sandel at the University of Texas at Austin and colleagues analyzed 24 years of social networks, 10 years of GPS-based measurements, and 30 years of demographic data on chimpanzees (Mr. Troglodytes) in Kibale National Park, Uganda.

“We want to be especially careful with the words we use,” says Sandel. “These are chimpanzees. War and civil war are concepts that have special meaning for humans. What we saw is not civil war. But it has important parallels. In particular, the changes in group identities that underlie deadly conflict.”

Chimpanzees are well known for inflicting horrific violence on each other, but this is usually reserved for outsiders or infants born to rival males.

The Ngogo chimpanzees, with a population of 150 to 200, belonged to the largest known group of primates, which together with bonobos (Paniscus), are the closest relatives of humans.

Between 1995 and 2015, the group was considered cohesive, living as a cooperative unit and exhibiting fission and fusion dynamics, the researchers say. This means that, like all chimpanzee populations, they form temporary associations throughout the day as individuals, moving around a shared common territory before reuniting in the evening.

Females mostly disperse at puberty, while males stay with their group for life. Before 2015, adult males in Ngogo grouped together with females, hunting together and cooperating in territorial patrols.

Then, on June 24, 2015, the members of the group met in the middle of their territory. One cluster of Ngogo chimpanzees, known as the Central Group, drove out another, known as the Western Group.

From this day the cohesion began to break down; by 2018, the two groups had split permanently. Between 2018 and 2025, the Western group carried out 24 attacks, killing at least seven adult men and 17 infants in the second group.

Sandel says it’s not clear which group initiated the conflict, although it was the central chimpanzees who first chased the western chimpanzees. “Both the western and central groups were actively engaged in territorial behavior when new groups emerged and the split was completed,” says Sandel. “But the Western group have become the aggressors and are responsible for all the deadly attacks.

Scientists suggest that several factors may have led to the breakup. The first may have been a conflict over food resources, then the death of five important men and a woman in 2014, which probably weakened social ties. A change in the alpha male followed. The final blow to the prospects for peace was an outbreak of respiratory diseases.

The disease resulted in the death of 25 members of the Ngogo chimpanzee in January 2017, including the last two males, spanning both the western and central groups. As a result of this tragedy, the last hopes of reconciliation seem to have been lost.

Sandel and his colleagues argue that the way conflict developed may have implications for understanding the evolutionary roots of human conflict. The polarization and war that occurs among people today is typically attributed to ethnic, religious, or political divisions. But a complete focus on these cultural factors overlooks social processes that are also present in our closest animal relatives, the researchers say.

“In some cases, it may be in small, everyday acts of reconciliation and reconciliation between individuals that we find opportunities for peace,” the team writes in their research.

Maud Mouginot at Boston University in Massachusetts says there are roughly two camps when it comes to speculation about how warfare evolved and escalated among humans. The first argues that war is a relatively recent innovation rooted in human culture that emerged with the rise of agriculture and nation-states. The other camp claims that the roots of war go much further back in our evolution. “I think the Ngogo data makes a significant contribution to the entrenched case,” says Mouginot.

“This study shows that the social dynamics of group fission and subsequent warfare can occur without any of the cultural markers we often attribute to human warfare—differences in faith, language, religion, dress,” he says. Luke Glowackialso at Boston University.

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