This is why finding altruism displayed by chimps is a bit puzzling. After all, how could sacrificing your own life for that of an unrelated individual (the most extreme form of altruism) be a trait that would survive through the ages? In Santos' opinion, figuring out whether the chimpanzee or human is getting any reproductive benefit from its actions is the harder question.
"It really involves measuring and comparing reproductive fitness," she says. "That's going to be one of the harder questions from an evolutionary point of view, about why these animals might have these abilities and why they might not."
Anne Casselman is a science writer based in Vancouver, Canada.