A kiss may feel like the most human expression of affection, but a groundbreaking new study says the first lip-to-lip moment happened millions of years before humans even existed.
Published in the journal Evolution and Human Behavior, the research suggests that kissing evolved between 21.5 and 16.9 million years ago, in the common ancestor of large apes, long before Homo sapiens emerged around 300,000 years ago.
Lead researcher Matilda Brindle, an evolutionary biologist at the University of Oxford, calls kissing a “deep-rooted evolutionary behavior” shared across primates and inherited by early humans.
Kissing Is Much Older Than Humanity, Scientists Say
To uncover the origins of kissing, scientists built one of the most extensive phylogenetic models ever run for social behavior, conducting 10 million simulations to trace how mouth-to-mouth contact evolved across primate species.
Their definition of kissing was precise, non-aggressive, mouth-to-mouth contact with no food transfer.
This helped distinguish true kissing from similar behaviors, like orangutans feeding infants mouth-to-mouth or fish engaging in “kiss fighting.”
Across modern species such as bonobos, chimpanzees, gorillas, orangutans, macaques, and baboons, researchers found multiple instances of genuine kissing, strong evidence that the behavior is ancient and biologically embedded.
Did Humans and Neanderthals Kiss? Study Says It’s Likely
The analysis goes further: kissing probably existed in early human relatives, including Neanderthals.
And since Neanderthals and modern humans mated, and exchanged oral microbes, scientists believe that Homo sapiens and Neanderthals likely kissed each other.
Brindle calls this a “more romantic spin” on human–Neanderthal interactions, contrasting with the often brutal stereotypes around prehistoric life.
Why Did Kissing Evolve? Scientists Still Aren’t Sure
While the study uses cutting-edge modeling to trace kissing back to our ape ancestors, the purpose of kissing remains a scientific mystery.
Researchers suggest several possibilities:
- assessing potential mates through chemical cues
- strengthening social bonds
- reducing tension within groups
- aiding reproductive success
- evolving from food-sharing behaviors
But kissing isn’t universal even today, it’s documented in only 46% of human cultures. That, researchers say, means the behavior is both biological and culturally shaped.
Kissing Isn’t Just Human: Polar Bears, Primates and More Do It Too
Adding to the surprise, the team found examples of kissing-like behavior in multiple species far beyond primates, including polar bears.
This challenges the belief that kissing is a uniquely human expression of intimacy.
“It puts humans firmly in the middle of the animal kingdom,” Brindle said, “rather than above it.”
A First Step in Understanding One of Our Most Intimate Behaviors
Experts not involved in the study agree it breaks new ground by viewing kissing through an evolutionary lens, rather than purely cultural or psychological.
The research also opens the door to future questions, "Who do individuals choose to kiss? Why do some societies kiss, while others don’t? And how did a gesture with risks, including disease transmission, survive millions of years?"
For now, scientists can say one thing with confidence - kissing didn’t begin with humans, we simply inherited it.
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