Smithsonian.com

Anyone who has visited a zoo can attest to the human-like qualities of our close relatives. Whether you’re watching chimpanzees, bonobos, orangutans or gorillas, it’s the facial expressions and social interactions that most make them appear similar to humans. Now researchers have evidence of another behavior shared between humans and nonhuman primates: laughter.

A study released last Thursday in Current Biology suggests that the origins of human laughter can be traced back 10 to 16 million years ago, to the last common ancestor of humans and all the modern great apes.

Marina Davila Ross and her fellow researchers recorded and analyzed the acoustics of “tickle-induced vocalizations” (see video below) in infant and juvenile orangutans, gorillas, chimpanzees, bonobos and humans. The similarities support the idea that laughter is an emotional expression shared among all five species.

According to Davila Ross’ study, the laughter of our last common ancestor probably consisted of long, slow calls in a short series. Human laughter evolved distinctive features, like regular vocal cord vibrations that are more even, as a result of selection from variation present in that last common ancestor.

The study’s findings also add evidence to the theory of continuity between nonhuman primates’ displays and human expressions—something that Charles Darwin posited in his 1872 book, The Expression of the Emotions in Man and Animals. Darwin’s work was popular not only for the text, but also the photographs and sketches that showed the striking similarities among humans, nonhuman primates and other animals as they expressed emotions like helplessness and anger.

'Expressions of Suffering - Weeping' from The Expression of the Emotions in Man and Animals (National Media Museum)

Darwin focused on the involuntary signs of emotions in humans and animals in this 1872 work:

We can understand how it is, that as soon as some melancholy state passes through the brain, there occurs a just perceptible drawing down of the corners of the mouth, or a slight raising up of the inner ends of the eyebrows, or both movements combined, and immediately afterwards a slight suffusion of tears … The above actions may be considered as vestiges of the screaming fits, which are so frequent and prolonged during infancy.

While both Darwin and Davila Ross note similarities in emotional expressions, something is still missing. Davila Ross ends her paper about laughter by writing:

The question left unaddressed is of course why those particular acoustic properties emerged, and what functions they may have served as laughter became a pervasive and characteristic component of human social communication.

We know that we’ve been laughing for millions of years, but we’re still not sure why.