Do humans kill each other because it’s in our blood, or is it all based on our environment? Philosophers and scientists have batted around theories for centuries. But an extensive climb through the evolutionary tree of mammals brought scientists to a fresh vantage point. From there, the answer seems to be: a mix of both, but mostly, it's in our blood.

After carefully compiling more than 4 million murder records across 1,024 mammalian species, evolutionary biologists at the University of Granada found that humans are more vicious than most mammals but generally on par with our primate lineage. And this does jibe with the rest of the evolutionary tree, in which species tended to bunch as either murderous, slightly savage, or peaceful. Being territorial and social were big determinants of those bloodthirsty bunches, the authors note. Overall, the finding, published this week in Nature, offers solid support for the argument that homicidal urges stem from evolutionary roots.

However, when the researchers tracked the murder rates of human populations from 50,000 BC forward—capturing hunter-gatherers to bands, tribes, chiefdoms, and states—they noted that murder rates jumped around a lot. And the timespans for those fluctuations in ferociousness are too swift for a genetic explanation. Societies, it seems, can modify our killer instincts.

However, not everyone completely buys the analysis on humans. “The stuff we have on prehistory is very thin,” Polly Wiessner, an anthropologist at the University of Utah in Salt Lake City, told Science. She noted that the study authors weighed records such as modern homicide percentages equally with more error-prone missionary accounts and ancient battlefield excavations.

Another sticking point is that the authors clumped all homicides together. But, not all homicides are equal, argues biological anthropologist Richard Wrangham of Harvard. “In the primates, infanticide is undoubtedly the commonest type,” Wrangham told The Atlantic. But humans “kill adults at an exceptionally high rate…. That’s worth stressing in order to avoid readers leaping to the conclusion that there is nothing surprising about human violence. Humans really are exceptional.”

The lead author of the study, José Maria Gómez, agreed with the critique, explaining that there simply wasn’t enough data to tease apart the types of murders.

Still, the mammalian-wide comparison of homicidal rates is impressively compiled and interesting, Wiessner said. According to the study, the overall murder rate of mammals is only about 0.3 percent (that’s one murder in the 300 deaths). For about 60 percent of mammals, there were no records of lethal violence at all. Generally, “species we expect to be violent, such as the predatory carnivores, are violent, and species that we do not expect to be violent because they are mainly vegetarian, tend not to be,” evolutionary biologist Mark Pagel wrote in a commentary piece. But there were some surprises; the most barbaric of all mammals turns out to be meerkats, with a murder rate of about 19 percent.

For us humans, the authors predicted a murder rate of two percent at the origin of our species. That’s in line with our primate ancestors. For instance, the researchers calculated that the ancestor of the great apes had a murder rate of about 1.8 percent. From the analysis of humans from 50,000 BC forward, the authors reported that hunter gatherers held that 2 percent murder rate. However, as humans organized into warring bands, tribes, and states, it shot up to as high as 30 percent (with a lot of uncertainty in that estimate).

Our savagery has since subsided. “Rates of homicide in modern societies that have police forces, legal systems, prisons and strong cultural attitudes that reject violence are, at less than 1 in 10,000 deaths (or 0.01 percent), about 200 times lower than the authors’ predictions for our state of nature,” Pagel noted. Thus, the argument that our environment rules our violent ways hasn’t been killed off yet.

Nature, 2016. DOI: 10.1038/nature19758 (About DOIs).