Laughter is, evolutionarily speaking, pretty old news. It’s a behavior we humans share with many of our primate relatives, who make similar vocalizations during play. In fact, based on the species that display this behavior, the precursor to all modern primate forms of laughter is thought to have emerged around 20 million years ago.
But humans differ from our cousins in one very important way: we’ve learned to voluntarily control an imitation of laughter. Much like crying, yawning, or screaming, laughter is at its core an involuntary emotional reaction for most primates. But humans have developed such control over our breathing and vocal apparatus that we can imitate these vocalizations.
The thing is, our imitations aren't exactly like the real thing. Real laughter has different acoustic features from imitation laughter and has more in common with the laughter of other primates—it’s louder and higher in pitch, among other things. Gregory Bryant, a UCLA researcher who studies the evolution of vocal communication, came up with a way to test whether these acoustic differences can actually be perceived by people.