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One of the more disturbing nature documentaries I’ve ever seen shows a dozen chimpanzees punching, kicking, biting a member of their own troop, chasing him up a tree and leaving him there to slowly die of his wounds over a span of three painful days.

Primatologists aren’t sure why it happened, but they weren’t suprised. Instances of intraspecies assault and murder are far from aberrations: Apes, monkeys, lemurs, bush babies...primates of almost all kinds, really, kill each other at rates far beyond that of any taxonomic order. The surprise here is that humans are the outlier. Believe it or not, we are way more peaceful than we have a phylogenetic right to be.

In an attempt to determine how human murder rates fit into the evolutionary background—and whether civilization has made us more, er, civilized—a team at the University of Granada undertook the first large-scale survey of intraspecies violence. Their study was published in Nature today; this chart, from The Atlantic, shows the murderingest mammals.

Humans are nowhere to be seen on that chart, but, fascinatingly, might have been there 1,000 years ago.

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Gómez’s team showed that by poring through statistical yearbooks, archaeological sites, and more, to work out causes of death in 600 human populations between 50,000 BC to the present day. They concluded that rates of lethal violence originally ranged from 3.4 to 3.9 percent during Paleolithic times, making us only slightly more violent than you’d expect for a primate of our evolutionary past. That rate rose to around 12 percent during the bloody Medieval period, before falling again over the last few centuries to levels even lower than our prehistoric past.

The precise numbers, and the methods used to extrapolate them, are controversial, but no one denies that human-on-human violence is lower than it’s ever been. For a book that’ll actually have you feeling optimistic about things, I can recommend Steven Pinker’s The Better Angels of our Nature. The pacifying influence appears to be large, stable states, where the rule of law is functionally enforceable.

Not so the case among animals, and those animals are just out there fucking each other up! Notice all the primates on that chart—as The Atlantic’s recap of the study posits, “it’s likely that primates are especially violent because we are both territorial and social—two factors that respectively provide motive and opportunity for murder.”

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The same factors are in play for just about all the non-primates on that list, including the murder champions of the mammal world, the meerkats. Nearly one in five meerkats meets its end at the paws of a fellow meerkat—or more specifically, a fellow meerkat’s mom, something that appears to be a reproductive strategy in a hierarchal society with limited resources.

Such is the paradox: It appears nature has trouble creating cooperative social systems that aren’t rife with murder. The obvious, controversial question, one that’s long troubled philosophers and evolutionary psychologists, is whether the existence of murder in humans is a byproduct of evolution or a benefit. Either way, we’ve been a lot worse.

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[Nature | The Atlantic]