Hardcore violence is a feature passed on in primate lineage for millions of years, inherited by humans over the course of our evolution.

Violence can emerge even in peaceful bonobo communities. Image credits: San Diego Zoo.

Seventeenth-century philosopher Thomas Hobbes wondered whether humans are inherently violent, as did Jean-Jacques Rousseau and countless social scientists after that. Violent behavior directed towards members of the same species is not uncommon for primates, but it is virtually unheard of in other intelligent species such as whales. So where does that leave us?

Jean-Jacques Rousseau believed that almost everything we learn is cultural, it’s modeled by our surrounding environment. But researchers have found it extremely difficult to separate the evolutionary from the sociological traits.

If they want to assess a certain trait of a species, biologists commonly study the history of the species’ biological evolution, reconstructing probably ancestral features. José María Gómez from the Universidad de Granada in Spain assessed the level of lethal violence in 1,024 mammal species from 137 taxonomic families and in about 600 human populations, ranging from about 50,000 years ago to the present.

They compared how many instances of violence were lethal in mammalian species to the same instances in prehistoric humans, using phylogenetic models to make predictions as well as readily available data. Their comparison revealed a similar figure: 2%. In other words, 1 in 50 fatalities was caused by violence.

“Humans emerged from an evolutionary lineage with a long history of higher-than-average levels of lethal violence towards members of the same species,” the study writes. Even so, followers of Rousseau might step in to say that our species’ figure of 2% tells us nothing about our innate tendencies; it might merely reflect a calculated or environmentally induced response to the environments in which early humans lived.”

But this doesn’t mean that we’re meant to be violence, because as Rousseau believed, culture also has a huge impact on us. Even though 1 in 50 prehistoric fatalities might have been violent, today we really don’t see that kind of violence.

“But societies can also modify our innate tendencies. 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 .01%), about 200 times lower than the authors’ predictions for our state of nature. Hobbes has landed a serious blow on Rousseau, but not quite knocked him out.”

Online paper: http://nature.com/articles/doi:10.1038/nature19758