Kissing may have started 21 million years ago with our primate ancestors

Kissing is everywhere in human culture—in movies, poetry, wedding ceremonies. Yet scientists still can't fully explain why we do it. A new study published in Evolution and Human Behavior suggests the answer might reach back further than anyone expected: to a common ancestor we shared with other apes roughly 21 million years ago.

The puzzle is real. "Kissing seems a bit of an evolutionary paradox," says Matilda Brindle, an evolutionary biologist at the University of Oxford who led the research. "It probably doesn't aid survival and could even be risky in terms of helping pathogen transmission." And yet, it persists across cultures and across species.

Brindle and her team noticed something striking: kissing isn't uniquely human. Gorillas, chimpanzees, rhesus macaques, and other apes also kiss on the lips. If so many of our closest relatives share the behavior, the researchers reasoned, maybe it wasn't invented separately by each species. Maybe it was inherited.

To test this, they first had to define what counts as a kiss. That turned out to be trickier than expected—mouth-to-mouth contact can mean many things in the primate world. They settled on "non-aggressive, mouth-to-mouth contact that did not involve food transfer," then combed through scientific literature and even YouTube videos to document kissing behavior in modern primates.

Using this data, they mapped out the evolutionary family tree of apes and ran computer simulations to trace when kissing most likely emerged. The results pointed to a common ancestor living between 21.5 and 16.9 million years ago—long before Homo sapiens evolved around 200,000 to 300,000 years ago.

"It would be very unlikely that kissing independently evolved in all of these species of ape that we're very closely related to," Brindle explains. "It makes much more sense that this is kind of an ancient trait within our primate family tree."

The study also found an 84 percent chance that Neanderthals—our extinct cousins—kissed each other. Combined with evidence that humans and Neanderthals shared similar oral microbiomes and that most non-African humans carry Neanderthal DNA, Brindle suggests they were probably kissing one another. It's a reminder that even our most intimate gestures have deep roots.

Why we kiss remains an open question

But here's where the mystery deepens. Even if kissing is ancient, nobody agrees on why we do it. Brindle offers two main theories: either it enhances reproductive success by sexually arousing partners or helping them assess each other's genetic quality, or it evolved from grooming behavior and strengthens social bonds. She points out that chimpanzees "literally kiss and make up after a fight."

Yet not everyone is convinced kissing is primarily biological. Anthropologist William Jankowiak from the University of Nevada, Las Vegas, raises a critical point: if kissing were essential to reproduction or mate choice, why have so many human cultures abandoned it entirely? His own 2015 research found that romantic kissing appears in only 46 percent of 168 sampled human cultures worldwide.

This gap between evolutionary history and cultural practice is precisely what makes kissing so interesting. Catherine Talbot, a study co-author and animal cognition researcher, frames it plainly: "The social norms and context vary widely across societies, raising the question of whether kissing is an evolved behavior or cultural invention."

The research doesn't answer that question yet. But it does something perhaps more valuable—it shows that to understand why we kiss, we need to look both backward, to our primate heritage, and outward, to the cultures that shape what kissing means. The behavior may be ancient, but its purpose remains wonderfully unclear.