The thrill of sudden conflict is unmistakable. Your heart races, your breathing rate increases; you may even start to tremble as your vision narrows to a tunnel. At that moment, you’re prepared to do one of two things: fight or flee. Humans, inventors of UFC and the World War, are frequently more than happy to do the former. The only other primate that shares this readiness to throw down is our notoriously war-loving cousin, the chimpanzee, and as scientists report in a study published in PLoS Genetics on Thursday, it may come down to a genetic quirk that primes us both to tackle conflict head-on.

In the study, a team of Korean and American researchers present a key difference between the genomes of battle-ready humans and chimpanzees and our comparatively peaceful primate relatives, the rhesus macaques and bonobos. Chimpanzees and humans, they reveal, have relatively lower expression of a gene called ADRA2C, which is associated with inhibiting an individual’s fight or flight response.

Oji zoo, Kobe, Japan
Chimpanzees and humans are the only primates with decreased expression of a gene known to tamp down the sympathetic system.

All the telltale changes involved with the “fight or flight” response are a part of the sympathetic nervous system, which, very broadly speaking, prepares the body for sudden action by shifting its resources toward the functions that are most important to attacking or running away, like supplying the muscles with blood and oxygen (increased pulse and breathing rate) and seeing what’s directly in front of you (tunnel vision). In contrast, the parasympathetic or “rest and digest” system deals with all the essential-but-not-urgent activities of the body and generally tries to conserve energy.

Chimps and humans, it seems, are really good at engaging the sympathetic system because their DNA carries a series of changes that decrease the production of ADRA2C, thereby decreasing its ability to shut down the fight or flight response. Put another way, chimps and humans are physiologically well prepared to run into battle, at least compared to bonobos and macaques.

Macaca mulatta
Rhesus macaques have relatively higher levels of ADRA2C, which leads to a more regulated flight-or-flight response.

The scientists already knew about ADRA2C’s function going into the study, so they made it a point to look for changes in its expression in their analysis of the primate genomes. It makes sense, after all, that species that love to fight would have lower expression of a gene that’s linked to lower aggression. Sure enough, after combing through genomic data from humans, chimps, bonobos, and macaques, together with additional genetic data collected first-hand, the researchers found that humans and chimps have a number of quirks in their DNA that lead to lower ADRA2C expression. These took the form of multiple “regulatory sequences” on their DNA, which various other proteins bind to and thereby tamp down expression.

Unlike chimps and humans, macaques and bonobos aren’t known to kill each other. Accordingly, macaques don’t have the same ADRA2C-related changes; some bonobos, however, do. This latter observation suggested to the researchers that selection for these genetic changes happened among primates relatively recently, perhaps in response to an increased threat of war from aggressive groups within the same species. If this ‘intergroup aggression’ theory is true, then it means that warfare within a species can shape that species’ evolution — which in turn has many profound and controversial repercussions.

Humans and chimpanzees are the only primates known to engage in lethal warfare, and the question of why we are so extraordinarily violent is a long-standing philosophical question as well as an evolutionary one. For obvious reasons, many scientists are reluctant to peg our violent nature to our genetics alone, as doing so might too easily excuse the more horrific parts of our species’ barbaric history. While the current study suggests there is an evolutionary basis to our readiness to fight, it doesn’t mean we evolved to seek engagement in fights. Future research into the evolution of ADRA2C will elucidate the evolutionary roots of warfare — if they exist at all.