A study suggests kissing has evolutionary roots, dating back 21.5 to 16.9 million years. Photo / Getty Images
The first kiss in history probably took place over 16.9 million years ago – long before humans even existed, a new study suggests.
Despite its popularity in many cultures among lovers, family members and friends, kissing has long perplexed evolutionary biologists, given it has no obvious benefit to either reproducing or finding food, while carrying the risk of disease transmission.
A 2015 study even found that fewer than half of cultures actually practised romantic-sexual kissing – leading some to speculate that kissing could be a purely cultural trait.
Now, researchers say they have uncovered evidence indicating that kissing does in fact have some evolutionary basis – and that it is an ancient trait we share with our ancestors.
“Kissing has deep evolutionary roots, seeming to stretch back to at least 21.5 to 16.9 million years ago in all our closest primate relatives,” Matilda Brindle, an Oxford University evolutionary biologist, said in a phone interview.
By contrast, modern humans only began walking the Earth between 200,000 and 300,000 years ago.
Brindle’s study, which was published today in the journal Evolution and Human Behaviour, sought to understand the function and history of kissing by examining whether kissing has an evolutionary basis.
Brindle and her co-authors built a map of all the examples of modern primates that are known to kiss one another.
Their first challenge was working out how to define kissing – settling on the definition of directed, non-aggressive, mouth-to-mouth contact that did not involve food transfer – which, they argued, was the best way to define kissing across different species.
They compiled their list by reviewing existing academic literature, as well as videos of primates on YouTube, and categorising kissing as either “present” or “not observed” in each species.
They found kissing had been observed in orangutans, bonobos, chimps and one gorilla species – as well as humans.
To work out if kissing could be an evolutionary trait, the researchers then plotted a giant evolutionary family tree, known as a phylogenetic analysis, and used the modern observational data to estimate which of our ancestors kissed.
They ran the model millions of times, using 10,000 slightly varied family trees for added statistical robustness. They were then able to work out that the common ancestor of all large apes (a group that includes humans) probably practised kissing.
The resulting analysis found that all the kissing apes shared a common kissing ancestor between 21.5 and 16.9 million years ago, after small apes split off into a different evolutionary branch. In other words, kissing had an “evolutionary signature”, rather than being “purely a cultural behaviour”, argued Brindle.
“It would be very unlikely that kissing independently evolved in all of these species of ape that we’re very closely related to,” said Brindle.
“It makes much more sense that this is kind of an ancient trait within our primate family tree.”
While there was no available data on whether Neanderthals – humans’ closest hominid cousin – practised kissing, a statistical analysis of the evolutionary family tree led the researchers to conclude that Neanderthals “most likely” kissed as well.
Given that other research suggests Neanderthals were “sharing saliva with humans” and had “interbred with humans”, Brindle argues there is “evidence that humans and Neanderthals probably kissed each other” too.
Brindle said it remained a possibility that kissing evolved in different branches of hominid more recently than their last common ancestor, but she said the statistical analysis suggested this was unlikely.
Brindle and her co-authors cautioned that their study had limitations and only represented a starting point in researching the evolution of kissing.
The existing data on non-human animal kissing practices is limited, so more research is needed to help pinpoint how different types of kissing evolved.
Classing kissing behaviour in each species as either “present” or “not observed” is also a blunt measure, since it does not indicate how common kissing is in each species, and kissing could have occurred in species where the behaviour was not observed by researchers.
For now, kissing remains a mystery for evolutionary biologists – including why we kiss at all.
Separate research has suggested that kissing may have developed from premastication, or sharing pre-chewed food, or that it may be a remnant of apes’ behaviour of grooming one another. It may also have a tangible reproductive benefit as well, others have argued – by helping to judge the suitability of a potential mate.
William Jankowiak, an anthropologist at the University of Nevada at Las Vegas, who was not involved in the study, and who co-authored the 2015 study finding that kissing was not universal among humans, said that it was useful for researchers to look at other primates.
“If some version of the behaviour is present among primates, then one can reasonably infer that the human expression of the trait may have a deeper evolutionary heritage,” he said.
But he said evolutionary biologists had not been able to answer a key question: “If kissing were essential to mate evaluation or to reproductive success, why have so many cultures ceased to practise it?”
Other experts have questioned how the study defines kissing – something that is complicated by the fact the definition varies widely even when it comes to human behaviour.
Adriano Lameira, an evolutionary psychologist at England’s Warwick University, argued that the study’s definition of kissing as “mouth-to-mouth touching” was too broad – he defines kissing in humans as “protruded lips with a slight suction movement”, particularly because “the large majority of kisses humans give are not mouth-to-mouth”.
Brindle said the definition she used was deliberately broad, to allow for an evolutionary perspective and a definition applicable to other animals.
Robin Dunbar, an anthropologist at the University of Oxford who was also not involved in the study, argued that “one would have to ask whether kissing as seen in great apes really is the same thing as kissing as seen in humans”.
Dunbar pointed out that kissing has different contexts, making it a challenge to define even in a single species.
“I guess we might all be said to kiss our offspring on the head [or even on the mouth] … but that isn’t quite the same thing that most people would mean when they think of kissing in humans,” he said.
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