Evrim Yazgin
Cosmos science journalist
“A kiss is a lovely trick designed by nature to stop speech when words become superfluous.”
These romantic words attributed to Swedish actress Ingrid Bergman may be closer to the truth than she realised.
Scientists have been trying to figure out where kissing came from for a long time. New research suggests that the answer is to be found in the behaviour of ancient ape ancestors of humans – and other forms of touch becoming “superfluous” a la Bergman.
Today, kissing in humans is part of many social interactions, from romance and greetings to displays of respect. It has a documented history going back 4,500 years. But how kissing originated has remained a mystery.
The new research published in Evolutionary Anthropology looks at different theories of how kissing evolved.
Kissing-like behaviour has been seen in modern primates, suggesting it’s something coming from our shared ancestry. But often snout-to-snout touching in primates coined “kissing” doesn’t actually fit the bill of the pursed lips and sucking that typifies a human kiss.
“Kissing has also been described as a greeting behaviour in chimpanzees, but alas, without sufficient detail to allow determining the degree to which it mirrors the behaviour in humans,” writes sole author Adriano Lameira, an associate professor in psychology at the University of Warwick, UK.
Only bonobos have been determined to kiss like we do. For these apes, kissing is for social cohesion and comfort. They even “kiss and make up” after fights.
Given humans share 98.7% of our DNA with bonobos, some connection isn’t surprising. But it still doesn’t explain how the practice evolved.
One hypothesis is that kissing derived from “sniffing” for social inspection or through sexual ornamentation. “But these hypotheses struggle to explain why kissing takes the form it does,” Lameira writes.
Another more promising idea is that kissing comes from nursing behaviours of apes. Mothers will often chew food before passing it to their infants using protruded lips. But Lameira highlights that this behaviour doesn’t translate to the context and function of kissing – particularly among adults. Nor does it explain why no other nursing animal has evolved kissing.
“The evolution of kissing is best understood through the biology and behaviour of great apes, who offer living proxies of human’s hominid ancestors,” Lameira says. “Among terrestrial nonhuman primates, including great apes, the dominant and most prevalent signal of social bonding is grooming.”
Lameira notes that grooming – the practice of picking through hair to remove parasites and dead skin – helps maintain alliances, hierarchies and social cohesion.
Humans spent less time grooming as we evolved less body hair.
Lameira suggests that the final step in the grooming process – a kiss – was maintained while other aspects of physical grooming died away.
“According to the “groomer’s final kiss” hypothesis, it is predicted that mutual, mouth-to-mouth kissing emerged in, and stemmed from, social contexts when ancestral apes originally mutually groomed each other at the same time, although this type of grooming is rare among extant great apes compared to one-way grooming,” he writes.
“Current comparative evidence supports that kissing isn’t a derived signal of affection in humans, it instead represents a surviving devolved, vestigial form of primate grooming that conserved its ancestral form, context, and function. What was once a time- and labour-intensive ritual to cement and strengthen close social ties became gradually compressed until a groomer’s final kiss turned into a crystalised symbol of trust and affiliation.”
