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• Posted November 20, 2025

Scientists Say Kissing Began Long Before Humans Existed

Kissing may feel like a very human habit, but new research suggests it has much deeper roots. A team of scientists says the behavior likely began more than 20 million years ago, long before modern humans existed.

Researchers from Oxford University in England reviewed decades of studies on primates to understand how kissing may have evolved.

By comparing the behavior of living species such as chimpanzees, bonobos, orangutans and one gorilla species, the team used statistical modeling to estimate when kissing first appeared.

Their results, published Nov. 19 in the journal Evolution and Human Behavior, suggest that an ancient ancestor of today’s apes likely engaged in mouth-to-mouth contact between 16.9 million and 21.5 million years ago.

Kissing presents an "evolutionary conundrum," study lead author Matilda Brindle, an evolutionary biologist at Oxford, said to CNN.

She said the behavior is surprising because it carries some obvious risks, including spreading germs, and may not directly help with survival.

Yet across primate species, kissing and similar behaviors may serve many purposes, Brindle said. These include:

• finding potential partners

• foreplay

• building social bonds

• soothing tension

• helping parents feed infants by chewing food first

Because the behavior isn't a fossil, researchers must study living species to better understand the past.

The oldest written records of human kissing date back about 4,500 years to ancient Mesopotamia and Egypt. But kissing today isn’t universal. A 2015 study found that only 46% of humans practiced kissing.

"We did find a strong evolutionary signal in kissing but it doesn’t mean it has to be retained," Brindle explained. "Primates are extremely flexible species, very intelligent, and so kissing might be useful in some contexts but not in others. And if it’s not useful, it is quite risky with high potential for disease transfer."

The team ran more than 10 million simulations to estimate how likely it was that early ape ancestors kissed.

Their findings strongly support the idea that extinct relatives, like Neanderthals, who lived alongside early humans, probably kissed as well.

But Brindle says the model can’t reveal the original purpose of kissing or how it's evolved over time. Much of the behavioral data also comes from animals in captivity, which means scientists need more information from primates in the wild.

Still, experts say the study lays the groundwork for future research.

"This is a wonderful example of the interplay of nature and nuture, including for a behavior many of us humans consider so deeply intimate," Justin Garcia, an evolutionary biologist and director of The Kinsey Institute at Indiana University in Bloomington, who was not involved in the study, told CNN in an email.

"Kissing is both biological and cultural, it is a behavior that invokes the bodily senses and clearly has some evolutionary origins, but we also know it varies across individuals and populations," he said.

More information

Read the full analysis here.

SOURCE: CNN, Nov. 19, 2025