First, he and his team compiled everything they could find on causes of death for various mammals, accumulating some 3,000 studies over two years. Their work revealed that lethal violence aimed at others from the same species is rare but widespread. It exists in almost 40 percent of the 1,024 mammal species that the team surveyed, and varies from group to group. Contrary to Watership Down, rabbits rarely kill each either. Neither do bats or whales. As you might expect, carnivores like lions, tigers, and bears, do so more frequently. But “it was striking that lethal violence wasn’t concentrated in those groups,” says Gómez.

The primates-the order that includes us, apes, monkeys, and lemurs-seem to be especially violent. While just 0.3 percent of mammal deaths are caused by members of the same species, that rate rose to 2.3 percent in the common ancestor of primates, and dropped slightly to 1.8 percent in the ancestor of great apes. That’s the lethal legacy that humanity inherited.

That isn’t to imply determinism. Even within the apes, chimps are notably more aggressive than bonobos, which suggests that group-wide capacities for violence can be tempered by other factors. And history shows that humans have also varied greatly in our violent tendencies. We are influenced by our history, but not saddled to it.

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.