Evolution and genetics seem to have baked a certain amount of murder into humans as a species, but civilization has tamed some of the savage beast in us, according to a new study.
Scientists calculated the rate at which more than 1,000 mammal species kill their own kind, and noticed how closely related species have similar rates of lethal violence. They essentially found that where a species is on evolutionary tree of life tells a lot about how violent the species is to its own kind. And we’re in a rough neighborhood.
Humans are “in a position within a particularly violent mammalian clade, in which violence seems to have been ancestrally present,” the study in the journal Nature says. That means that based on other rather murderous species closely related to us, humans have “inherited their propensity for violence.”
As a group, mammals average a lethal violence rate against their own of about three killings of their own species in 1,000 deaths. The “root” violence rate of early humans and many of our closer primate cousins is about 20 in 1,000, said study lead author Jose Maria Gomez at the University of Granada in Spain. But in the medieval period, between 700 and 1500 A.D., that deadly rate shot up to about 120 per 1000.
But we’ve gotten less murderous.
On average, modern humans now kill each on a rate of 13 in 1000, Gomez said, basing his calculations on World Health Organization data. But he says the exact numbers are rough and depend on many technical variables, so what is more accurate is to say “violence has decreased significantly in the contemporary age.”
“It seems that we are in the present time less violent than we were in the past,” Gomez said in an email interview.
While humans are killing each other less than we once did, we are not nearly as peaceful as the killer whale – which despite the name, has a rate of interpersonal violence of pretty much zero (though Gomez notes that only a small sample of killer whales was examined). Many whale species, bats and anteaters are particularly peaceful to their own kind, the study finds.
But humans are far less violent than the cougar, certain baboons, lemurs and chinchillas that have murder rates of well over 100 per 1,000.
The study looks at violence through the lens of phylogenetics – the study of evolutionary relationships, or how closely related species seem to share common traits.
“We found that closely related mammal species tend to have similar levels of violence,” Gomez said. The more closely related species are, the more similar are their violence levels.
Gomez and colleagues used 1,044 studies that looked at 1,024 different mammal species, with the causes of death determined for more than 4 million individual mammals. Then for each species, the researchers counted the number of deaths due to a member of the species killing another. They didn’t use studies where they couldn’t find causes of death. And because of the study’s historical nature, Gomez could only examine killings, not wounding.
Moving up in evolutionary complexity often seems a walk on the wild side, especially in the branches of the tree of life where humans come from.
If mammals have a three in 1,000 violence rate, a superorder of species called Euarchontogilires that includes our ancestors, hares, and rodents has a violence rate of about 11 in 1000. Further on, the large grouping called Euarchonta that includes us, other primates, tree shrews and flying lemurs has a rate of about 23 per 1,000. It drops to about 18 for great apes.
It was hard to calculate lethal violence rates for early humans; Gomez examined several thousand prehistoric people’s deaths across the globe in archaeological sites.
Culture, government and other factors tamp down the inherited violence, Gomez and colleagues conclude.
“Our study suggests that the level of lethal violence is reversible and can increase or decrease as a consequence of some ecological, social or cultural factors,” Gomez said.
These conclusions are similar to what Harvard psychologist Steven Pinker found in his book “The Better Angels of Our Nature.” He charted long-term reductions in war, murder, rape and bigotry.
Pinker praised the Gomez study as creative and thorough.
“Based on three biological facts – we are apes, we are social and we are territorial – one would predict that humans should engage in lethal violence in our natural conditions,” Pinker wrote in an email. “Modern societies have developed, especially the rule of law, that have reduced rates of lethal violence below what would expect for a mammal with our ancestry and ecology.”
Harvard biological anthropologist Richard Wrangham, who studies the roots of violence, praised the breadth Gomez study, but said there some issues remain. He’s especially concerned about too closely linking primates killing their own, which is more the murdering of infants, to humans whose killing their own mostly involves adults.
Few species beyond humans and some social territorial carnivores like wolves and lions are part of the “adult-killing club” of their own species, Wrangham said. “Humans really are exceptional.”