Human Kissing May Have Evolved From Primate Grooming

No one knows when humans first began kissing. Putting your mouth against another person's, whether as an expression of friendship or love, is not a universal practice, suggesting that culture rather than instinct may be responsible.

Humans are not the only animals to do it, however. Other primates, including chimpanzees and bonobos, kiss one another too. According to Adriano Lameira, a primatologist and evolutionary psychologist at the University of Warwick in the UK, this points to a possible origin that nobody has previously considered – perhaps because it is so revolting.

A grooming origin for human kissing

Lameira suggests that this early form of snogging was once part of an ordinary grooming session among our ancestors. When a grooming primate encountered a parasite or a flake of dead skin, they would lean in, attach their lips to their partner's skin and affectionately slurp the offending material away.

Although this cleaning became less necessary as time passed, the final smack of the lips remained.

"The hygienic relevance of grooming decreased over human evolution due to fur-loss," Lameira writes in his paper, "but shorter sessions would have predictably retained a final 'kissing' stage, ultimately, remaining the only vestige of a once ritualistic behaviour for signaling and strengthening social and kinship ties in an ancestral ape."

Records show that humans were kissing thousands of years ago, so the practice has survived both the passage of time and the changing whims of fashion. That persistence raises some intriguing questions about its origins.

Other explanations for kissing

Researchers have previously proposed evolutionary origins for kissing. A popular theory links it to feeding babies food that an adult has already chewed. Kissing, however, involves pushing the lips outwards and creating some suction. Passing pre-chewed food into a baby's mouth requires an outward force instead, making that explanation somewhat less persuasive.

Another hypothesis holds that kissing grew out of sniffing someone as a form of social inspection. But that leaves the question of why the mouth would need to play a part.

The groomer's final kiss

Lameira proposes instead that the practice may have developed as a form of reassurance. Both humans and other primates are social animals that use rituals to reinforce their bonds. Among other primates, grooming is the most common way of strengthening those connections.

"Grooming consists of picking through the fur/hair of others to remove parasites, dead skin, and debris," Lameira explains. "Grooming helps to establish and maintain alliances, hierarchies, and group cohesion through social touch, with the consequent release of endorphins, which reduces stress and promotes feelings of well-being between groomer and groomed, further cementing social ties."

Humans spend 89 percent less time grooming one another than would be expected compared with other primates. That makes sense: we lack fur and have other ways to bathe and clean ourselves without another person's help. Yet as we lost our fur and spent less time grooming each other, a vestige of the ritual may have survived.

Lameira calls one such remnant the "groomer's final kiss". Searching a partner's fur for parasites and debris would have become less necessary, but each grooming session would have included, or concluded with, the groomer placing their lips on the other primate's body to remove stubborn material.

From there, a mouth-to-mouth kiss could naturally have developed as an extension of that groomer's kiss.

Unpleasant though it is, the idea sounds at least as plausible as the other explanations offered so far. We may never know for certain whether it is correct, but researchers could explore it further by studying other primates and comparing their grooming behaviour with the thickness of their coats.

"For future evolutionary insight into the evolution of human kissing, and other behaviours uniquely exhibited by our species," Lameira says, "it will be important to retain in mind and ponder the influence of the broader socioecological, cognitive, and communicative context of human ancestors."

For now, the greater challenge may be keeping thoughts of nits out of your head the next time you snog your sweetheart.

The argument was published in Evolutionary Anthropology.