"Well, like everything in biology, we assume that it's going to increase our reproductive success," Langergraber said.
That success can be direct, like finding a mate and having offspring. Or it can be indirect, like helping out a relative and thus advancing the family bloodline.
The family-bloodline scenario is the basis of a theory called kin selection, which holds that animals should prefer to cooperate only with their relatives.
In so doing, they reap the indirect but substantial benefit of seeing their family genes passed on—by becoming an uncle or an aunt in addition to, or instead of, a parent.
"Most people had assumed that in animals it's mainly … through kin selection that cooperative behavior can evolve," Langergraber said.
"But here we're suggesting that's not entirely the case with chimpanzees, who are famous for being one of the most cooperative animals in the world."
Emory University primatologist Frans de Waal said: "I think we have long known or suspected that chimpanzee males cooperate very well with nonrelatives."
Nonetheless, some economists and anthropologists have "preferred to depict chimpanzee cooperation as mainly kin-based" to make the claim that human cooperation is unique, he noted.
"Now we finally have a study that includes not only [chimp] behavior but also genetics, giving us the ultimate proof that non-kin cooperation is extremely well-developed in wild chimpanzees," de Waal wrote in an email.
"This study will put to rest once and for all that only humans know reciprocity-based cooperation. As such, it is highly significant."
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