Humans and chimpanzees have a much more active fight-or-flight response than any other primates, a new study reports. The findings point to the role aggression and warfare played in our evolutionary history and that of our closest relatives.

Fist.

Image via Pxhere.

Biology has no qualms letting organisms get dirty, even downright aggressive, to secure their interests. Faced with so many potential crazies, our brains (along with those of most complex organisms) have evolved a protocol whose single purpose is to determine whether we should throw the towel or a fist when faced with a threat. New research suggests that this protocol is unusually active in humans and our closest relatives, the chimpanzees, suggesting an evolutionary history fraught with aggression and large-scale conflict.

Knuckleheads

Aptly dubbed the ‘fight-or-flight’ response, this hard-wired security protocol is primarily controlled by the automatic nervous system (ANS) — a division of the nervous system that oversees the activity of smooth muscles (those in your organs) and glands throughout the body. For the most part, the ANS works outside of our conscious perception or control and regulates heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal.

According to a new paper by a team of US and Korean researchers, this system is much more active in humans and chimps compared to other primates. They note that humans and chimps are the only primates known to frequently engage in warfare (i.e. large-scale conflict), suggesting that this adaptation of the fight-or-flight response evolved in response to frequent aggression or threat of conflict.

To find evidence of this adaptation, the team looked at how different primate species regulate a gene called ADRA2C. The gene’s main function is to temper the activity of the sympathetic nervous system, which plays a central role in forming the fight-or-flight response. To that end, they analyzed the genomes, transcriptomes (the entire set of genes expressed in a cell), and epigenomes (the compounds external to DNA which can bind to it to affect gene expression) from humans, chimps, and other primates.

They report that humans and chimps evolved both genetic and epigenetic changes that decrease ADRA2C expression — in other words, that increase both the fightiness and the flightiness of their brains. Macaques don’t have these changes, and they’re not universal in bonobos, suggesting that the gene variants spread through the population recently (after we parted ways on the tree of life), most likely in response to the threat of conflict. The team further used CRISPR/Cas9 to show that reverting to the genetic states of macaques and bonobos will restore ADRA2C expression to higher levels.

Variations in ADRA2C gene expression can have significant effects on behavior. For example, changes in the gene that occurred during chicken domestication likely resulted in the somewhat aloof and un-aggressive birds we know today. The team notes that finding adaptations associated with a reduced expression of ADRA2C in chimps and humans but not in their relatives suggests that it was inter-group aggression — rather than an outside threat, such as that of predators — which shaped these genes. They also believe such adaptations could form the evolutionary roots of human warfare — which, in turn, could have shaped much of society as we know it.

The paper “Selection on the regulation of sympathetic nervous activity in humans and chimpanzees” has been published in the journal PLOS Genetics.

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