The most wretched sounds in creation are reserved for the phrase “like fingernails on a blackboard.” Despite the fact that blackboards are antiquated (does anybody even use them anymore?), the simile holds up across generations. So why does this noise inspire such universal pain and queasiness?

Well, evolutionary scientists have pointed out that the screech of fingernails on a blackboard is similar to the howls of certain monkeys, who reserve the shriek for emergency situations, such as when a predator is approaching. The theory, then, is that our primate ancestors let loose with a spine-chilling cacophony to give the subtle message “Holy shit! A leopard is closing in, let’s get the fuck out of here!” Millions of years later, a portion of our brain tells us that this particular noise is bad and it’s time to freak out.

It’s a fascinating theory, and one that verifies our common ancestry. It’s something to think about as we divide into our respective tribes and bellow at each other over minor differences in skin color or facial characteristics or vocal inflection.

If we’re all just monkeys in the same troop, why are we brimming with hostility for one another? If we could band together millions of years ago for the good of our species on the Serengeti Plain, why do so many of us melt down if the next-door neighbor turns out to be darker or lighter than us?

At one point, we could get along, but as our supposedly big brains developed, we turned on each other. The group has fragmented, with homicidal results that you’ll never see in a capuchin. It’s enough to make Darwin weep.

By the way, our monkey ancestry is also theorized to be the reason why total strangers feel compelled to touch a pregnant woman’s belly. It’s apparently a drive in primates to verify that the baby inside is ok and that the next generation will be healthy. So if you’re pregnant and get annoyed whenever somebody reaches for a belly rub, just let out a howler monkey screech and watch the offender scurry into the treetops.