Humans have been going to war since civilization began, if not before: prehistoric man was not the amiable hunter-gatherer that some assumed, going by the 13,000-year-old battle-site recently identified in Sudan. We don't seem to have stopped rattling our sabers at each other since.

Chimpanzees are no better. They are perfectly capable of premeditated murder and even their erstwhile No. 1 fan, Jane Goodall, concluded after decades of research that this ape is downright dangerous, saying, “I didn’t know chimpanzees can rip your face off."

Open gallery view Jane Goodall, British ethologist, kisses Pola, a 14-months-old chimpanzee baby, even though she came to realize that nice, chimps are not. Credit: Bela Szandelszky, AP

Of all species in the world, humans and chimpanzees are among the few to coordinate attacks on their own kind. And we already knew that chimps didn't learn that behavior from humans, thanks to 2014 research by the Lincoln Park Zoo.

Now a collaboration of South Korean and American scientists shows, in a new paper published in PLOS Genetics Thursday, that war is in our genes – humans and chimps. Not in other apes. Apparently it's no coincidence that the chimp is our closest relative: we share about 99 percent of our DNA.

The genetic change in us and our warlike cousins is that our fight-or-flight response is ramped up, deduced Kang Seon Lee of South Korea's KAIST university and colleagues.

Open gallery view Female bonobos play together at the Planckendael zoo Credit: Georges Gobet, AFP

Macaques, for example, and some bonobos – a subspecies of chimp deeply devoted to peaceful sex - lack these genetic variants.

Fight-or-flight is also known as a state of hyperarousal. Our bodies physiologically react with hormonal flooding to the perception of imminent danger, preparing us to rip apart an aggressor, or flee it. Survival can depend on the efficiency of this mechanism. The change in humans and chimps is in a gene, called ADRA2C, which regulates the fight-or-flight response.

Technically, ADRA2C dulls the response but chimps and humans produce a molecule called NRSF that inhibits ADRA2C. So, all in all, the fight-or-flight response is increased. The researchers also postulate that the ADRA2C gene underwent selection during the domestication of chickens, which are, in the wild, pretty unpleasant.

Of course, we cannot say what came first: the primate that wanted to slaughter its brethren or the dampened negative regulator.

"If this type of aggressive behavior was common during their evolution, then the fight-or-flight response likely played a critical role in adapting to the threat of deadly conflicts," the researchers write. That makes sense: if a pack of apes are trying to brain one another, the ones with advanced fight-or-flight reactions would likely fare better, whether they were being more aggressive or running away faster.

Bonobo, mon amour

Open gallery view Did domestic chickens undergo selection pressure against fight-or-flight, to minimize aggression? Credit: Gil Cohen Magen

Epigenetics is all the rage in genetic research these days, and it turns out that the change in the chimp, the human and the odd bonobo is both genetic and epigenetic.

Such epigenetic changes are not mutations, which are changes to the DNA. These are changes in genetic expression caused by molecules that bind to DNA. The epigenetic "markers" regulate the expression of the genes – and, crucially, they can be inherited. If an epigenetic change turned off one of your genes, it may be off in your kids too.

The upshot is that humans, chimps and some bonobos acquired genetic and accompanying epigenetic changes that decreased the expression of their ADRA2C gene, intensifying the fight-or-flight response. Macaques for instance have no such variation and have not been observed waging war, says the team.

Which brings us to bonobos, which in contrast to the horrible chimp, seem to normally be amiable, though an Israeli researcher recently proved that a female bonobo could actually make a spear in an attempt to stab him. In other words the bonobo is perfectly capable of fashioning a tool of war, and using it.

The fight-or-flight regulatory change is not however universal in the bonobos. The researchers postulate that it began spreading in the population recently, possibly in response to threats. For instance the lady bonobo who tried to stab Haifa University's Itai Roffman may have thought his approach was threatening.

So in short, it seems that differences in the way this one gene is expressed powerfully affect behavior. Maybe when U.S. President Donald Trump and North Korea's Kim Jong Un snarl at each other, they're just paying heed to the call of their wild natures.