The only people in this world who haven’t been tickled are probably infants who have not yet achieved this rite of passage. Everyone has giggled, gasped for breath, and gone into convulsions during a tickling attack, whether they still love tickling or grew to hate it. It’s one of those things that are hard to understand, similar to yawning and kissing — all of humanity has done it since the beginning of time, but how did it get started? And why do we do it?

Evolution, once again, holds the answer. Sources note it as a social behavior that bonds people in close relationships, such as family members and friends, and babies learn to laugh in response early on. “It’s one of the first forms of communication between babies and their caregivers,” neuroscientist Robert R. Provine told Popular Science. Parents also learn — to stop tickling when the baby no longer finds it humorous. “The face-to-face activity also opens the door for other interactions.”

But tickling is much more than that; research also links it to self-defense. It is no coincidence that the most ticklish parts of the body are also the most vulnerable, like the neck, the ribs and the stomach. Popular Science says kids learn to protect those vulnerable locations during the relatively safe activity of tickling each other. Adults, on the other hand, usually cease to be ticklish; their learning is through. Further strengthening that link, University of Tuebingen scientists have shown that tickling “activates the part of our brain that anticipates pain — which is why you may accidentally lash out at someone who is trying to tickle you,” the Daily Mail reports.

Outside of teaching kids to react to danger and protect weak areas during combat, laughter is also a sign of submission. According to the Daily Mail, “researchers believe that our responses to tickling date back to man's earliest evolution and developing self-awareness.”

The Tuebingen scientists saw that the kind of laughter that comes with tickling is different from other funny laughter. While both types activated the brain’s Rolandic Operculum, which controls facial movements and emotional and vocal reactions, tickling additionally stimulated the hypothalamus, which is linked to regulating body temperature, hunger, tiredness, sexual behavior, and instinctive reactions like the fight-or-flight response to danger. That would explain why people might start laughing simply when threatened with a potential tickle yet cannot tickle themselves: Their brains know there is no need to respond to a self-tickle.

In that case, our senses are working together to determine the threat level, as they do in many other scenarios; for example, how our bodies respond differently to the sound of a honking horn based on the visual information of our surroundings — if we are in the middle of the street we may become startled, but if we are in our living rooms, we are simply annoyed. With tickling, our response to the feeling is largely determined by what else we perceive. A friendly face will make tickling elicit laughter, but the same sensation created by, for instance, a spider crawling across our arm will not end the same way.

It is unclear exactly when the first tickle fight took place, but tickling and tickle-induced laughter exist in our monkey relatives, such as chimpanzees and orangutans.

The New York Times notes that tickling a baby human is similar to tickling a little chimp. “The ape has the same sensitive spots: under the armpits, on the side, in the belly. He opens his mouth wide, lips relaxed, panting audibly in the same “huh-huh-huh” rhythm of inhalation and exhalation as human laughter. The similarity makes it hard not to giggle yourself.” And the chimp also plays the tickling game in the same way a human child would: “He pushes your tickling fingers away and tries to escape, but as soon as you stop he comes back for more, putting his belly right in front of you. At this point, you need only to point to a tickling spot, not even touching it, and he will throw another fit of laughter.”