Chimpanzees and humans share much in common, including cooperating to kill perceived rivals, and now a new study finds that this kind of lethal aggression -- at least among chimps -- is "normal" and sadly all too common.

"Normal," in this case, means that the behavior results from natural and evolved tendencies and does not, as some other researchers have suggested, emerge in response to human pressures, such as habitat loss.

The study, published in the journal Nature, sheds light on the evolutionary roots of lethal conflict among certain primates, including humans. An accompanying "News & Views" article in the same journal, for example, points out that in 2013 alone, there were 33 armed state-level conflicts around the world. Many of them have persisted for decades.

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Chimps are just as violent.

"Most killings involve gang attacks," Michael Wilson, who led the new study, told Discovery News. "When attacking adults, many attackers pile onto the victim. They pin the victim to the ground and hit, kick and bite the victim."

"Attackers may cause massive trauma to internal organs, break bones, inflict numerous puncture wound from canine teeth, and bite or tear off fingernails genitalia, and even the throats of victims," added Wilson, who is an associate professor of anthropology, ecology, evolution and behavior at the University of Minnesota.

Male chimpanzees, he continued, also sometimes kill infants after attacking their mothers and snatching their babies away. They will then kill the infant by biting it to death or hitting it against the ground or trees.

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Such traumatic moments became evident after the researchers analyzed 426 combined years of research at 18 chimpanzee study sites and 92 combined years of research at 4 bonobo sites. Coalitional killings were documented at 15 of the 18 chimp study sites, but there was only one suspected killing among bonobos.

The muriqui, a South American monkey, is also known for peaceful behavior, based on earlier studies.

Chimps and humans, however, have similar rates of killing among rival groups.

"Males kill more often than females in both species, and humans and chimpanzees share an unusual pattern of cooperating to kill," Reynolds said.

Ferdinand, the alpha male of the Kasekela community, standing bipedally in the midst of a charge display. Ian Gilby

This method of killing appears to evolve in "fission-fusion" social systems, where males from one group split off into more temporary subgroups. Both chimps and humans have such "gangs" that may attack rival individuals or gangs, particularly when they outnumber or outmuscle the victim(s).

Reynolds believes that the behavior is driven by "both genetic and non-genetic components."

Does that then mean some chimps and humans are inherently predisposed to kill others of their own kind?

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"The data tell us that there are some ecological and demographic circumstances in which the benefits of lethal aggression exceed the costs for chimpanzees, nothing more," Joan Silk, author of the "News & Views" piece, wrote.

"Humans are not destined to be warlike because chimpanzees sometimes kill their neighbors," added Silk, a professor in Arizona State University's School of Human Evolution & Social Change.

Reynolds agrees. He believes that trade is a powerful tool that humans have to curb warfare.

"Political scientists have found that countries that trade a lot with each other are less likely to go to war with one another," he explained. "Another important tool is democracy. Countries with mature democracies almost never go to war with other democracies."