Victoria Gil
BBC science
Humans kiss, monkeys kiss, even polar bears kiss. And now researchers have reconstructed the evolutionary origins of kissing.
Their study suggests that mouth-to-mouth kissing emerged more than 21 million years ago, and was likely practiced by the common ancestor of humans and other great apes.
The same research concludes that Neanderthals probably kissed, and even that humans and Neanderthals may have kissed each other.
Scientists have studied kissing because it represents a kind of evolutionary puzzle: it has no obvious benefit for survival and reproduction, yet it is present not only in many human societies, but also in many animal species.
By finding evidence that other animals kiss, scientists have been able to create an "evolutionary family tree" to determine when this behavior most likely originated.
To ensure they were comparing the same behavior across species, the researchers had to come up with a very precise, and rather unromantic, definition of a "kiss."
In a study published in the journal Evolution and Human Behaviour, they defined kissing as non-aggressive, directed mouth-to-mouth contact "with some movement of the lips or parts of the mouth, without the exchange of food."
"Humans, chimpanzees and bonobos - they all kiss," explains lead researcher Dr Matilda Brindle, an evolutionary biologist at the University of Oxford.
From this, she concluded that "it is likely that their most recent common ancestor also kissed."
"We think kissing probably evolved around 21,5 million years ago in great apes."
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In this study, scientists found behavior that fits their scientific definition of kissing in wolves, prairie dogs, polar bears (pretty slobbery because they use their tongue a lot), and even albatrosses.
They focused on primates, and especially great apes, to build an evolutionary picture of the origin of the human kiss.
The same study also concluded that Neanderthals, our closest ancient relatives who became extinct around 40.000 years ago, also kissed.
Earlier research on Neanderthal DNA also showed that Homo sapiens and Neanderthals have the same type of bacteria found in our saliva.
"This means they must have been exchanging saliva for hundreds of thousands of years after the two species split," explains Dr Brindle.
Although this study was able to determine when kissing originated, it did not answer the question of why it appeared.
There are already numerous theories: that it originated in our ape ancestors due to mutual grooming, or as an intimate way of assessing the health and even compatibility of partners.
Dr. Brindle hopes this research will open the door to finding answers to this question.
"It's important to understand that this behavior was also present in our non-human relatives," she says.
"We should be studying this behavior, not dismissing it as something silly just because it has a romantic connotation in people."
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