We as a species have some incredibly violent traits, and now a new study out of the University of Granada in Spain and published in the journal Nature sheds light on how we came to be this way. Researchers looked into the murder rathes for a thousand mammal species and then looked at how they compared to the rates of related animal species.

They found that murder rates closely correlate with species that are on the same branch of the evolutionary tree, so to speak. Humans, unfortunately, are a particularly violent species, and mammals in general tend to be pretty violent, with three killings per every 1,000 deaths.

Early humans and primates ranked well higher than even that, with 20 killings per 1,000 deaths. And in medieval times, humans topped out at 120 per 1,000 deaths, which has since declined to 13 per 1,000, suggesting that civilization is finally taming the beast inside of us.

Jose Maria Gomez, the lead author of the study, used data from the World Health Organization to come to their conclusions. His calculations determined that violence has declined significantly in the contemporary age, although it’s difficult to come to exact numbers.

What’s the least murderous in the animal kingdom? Surprisingly, it’s the killer whale, which has a rate of murder of almost zero. Anteaters and bats are also fairly non-violent toward their own kind. On the other end are baboons, lemurs and, surprisingly, little chinchillas, who are rates of more than 100 killings per 1,000 deaths.

Scientists used the theory of phylogenetics and evolutionary relationships to examine shared traits between species that are related. They found a strong correlation in species that were more closely related.

They also examined a thousand previous studies to examine more than a thousand mammals species, investigating typical causes of death and then determining how many were related to murder.

Euarchontogilires, a superorder of species that includes early humans as well as rodents and hairs, had a murder rate of about 11 per 1,000 deaths. A smaller group, Euarchonta, had a rate of 23 per 1,000. Great apes average 18 per 1,000.

Government and culture appear to have played a role in driving down killings. Social factors also likely play a role.

“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. Modern societies have developed, especially the rule of law, that has reduced rates of deadly violence below what would expect for a mammal with our ancestry and ecology,” stated Steven Pinker, author of the book The Better Angels of Our Nature.

“The psychological, sociological and evolutionary roots of conspecific violence in humans are still debated, despite attracting the attention of intellectuals for over two millennia,” the paper’s abstract reads. “Here we propose a conceptual approach towards understanding these roots based on the assumption that aggression in mammals, including humans, has a significant phylogenetic component.

“By compiling sources of mortality from a comprehensive sample of mammals, we assessed the percentage of deaths due to conspecifics and, using phylogenetic comparative tools, predicted this value for humans,” it continues. “The proportion of human deaths phylogenetically predicted to be caused by interpersonal violence stood at 2%. This value was similar to the one phylogenetically inferred for the evolutionary ancestor of primates and apes, indicating that a certain level of lethal violence arises owing to our position within the phylogeny of mammals.

“It was also similar to the percentage seen in prehistoric bands and tribes, indicating that we were as lethally violent then as common mammalian evolutionary history would predict. However, the level of lethal violence has changed through human history and can be associated with changes in the socio-political organization of human populations. Our study provides a detailed phylogenetic and historical context against which to compare levels of lethal violence observed throughout our history.”