Kissing may be millions of years older than humans

Kissing might feel like a cultural invention, but new research suggests it reaches far deeper into our evolutionary past.

Researchers from the University of Oxford analyzed primate behavior, ancient oral microbes, and genetic links between early human groups. They concluded that our ancestors – and even Neanderthals – likely shared intimate mouth-to-mouth contact.

Why kissing puzzles biologists

Kissing seems risky because saliva can carry viruses and other germs, yet people still do it in many places. From an evolutionary point of view, behaviors that spread disease but give no benefit should fade away, so this habit needs an explanation.

Led by biologist Dr. Matilda Brindle, the Oxford team wanted to test whether kissing is a purely cultural invention or a much older biological trait that humans share with other apes.

Across animals, mouth contact can mean feeding, grooming, or aggression, so the researchers had to decide what they would count as a kiss.

They settled on non-aggressive mouth-to-mouth contact without food transfer, a rule simple enough to apply to many species.

With that definition, the experts searched field reports and videos for examples of kissing in primates that evolved in Africa, Europe, and Asia.

Chimpanzees, bonobos, orangutans, and several monkey species all showed gentle mouth contact in social situations such as bonding, reconciliation, or courtship.

Modeling how kissing evolved

To turn scattered observations into a testable picture, the team used phylogenetic analysis, a statistical method that tracks traits along evolutionary family trees.

They coded each species as either having kiss-like contact or not, then placed those data onto a primate family tree built from genetics.

The researchers applied Bayesian modeling, a way of updating probabilities as new evidence is added. That approach simulated millions of possible histories for the trait.

The model compared scenarios where kissing evolved once, many times, or not at all, and scored which patterns matched the distribution across species.

Kissing rooted deep in apes

The best supported scenario placed the origin of kiss-like behavior in a common ancestor of large apes between 21.5 and 16.9 million years ago.

“This is the first time anyone has taken a broad evolutionary lens to examine kissing,” said Dr. Brindle. “Our findings add to a growing body of work highlighting the remarkable diversity of sexual behaviors exhibited by our primate cousins.”

Her remark emphasizes that intimate behavior in primates is varied and that seemingly simple actions can have deep evolutionary roots.

Microbes reveal ancient connections

Clues about Neanderthal kissing come from studies of the oral microbiome, the community of microbes that live in the mouth and on teeth.

When researchers sequence DNA from ancient plaque, they can compare bacterial communities from Neanderthals, early Homo sapiens, and living primates.

One comprehensive paper assembled dental biofilms from Neanderthals and humans spanning roughly the last 100,000 years and compared their microbial communities.

That work showed extensive overlap between Neanderthal and early modern human mouth bacteria, including lineages that seem to have persisted in our lineage for long periods.

A separate study on dental calculus, the rock-hard plaque that builds up on teeth, linked these microbial patterns to Neanderthal diet and health.

How Neanderthals passed bacteria

Together, such analyses suggest that intimate contact, shared food, and shared environments allowed mouth bacteria to circulate between Neanderthals and Homo sapiens over thousands of years.

Genetic studies of ancient skeletons also reveal repeated episodes of interbreeding, reproduction between distinct human groups that had been separated for long periods.

If genes and mouth microbes moved between populations, then face-to-face contact that sometimes included kiss-like behavior became a realistic part of interactions between Neanderthals and early Homo sapiens.

Culture reshapes a very old habit

Across human societies today, detailed records show that romantic-sexual kissing, lip-to-lip contact between partners in a sexual or courtship context, is far from universal.

A cross-cultural research project covering 168 societies found that adults engaged in such kissing mostly in socially complex cultures, with widespread absence in several low-density regions.

Ethnographers working with foraging and small-scale farming communities often describe other forms of affection, including embraces, shared meals, or gentle touching of faces or hands.

In these places, lip contact may be absent, treated as a foreign custom, or even regarded as unpleasant.

“While kissing may seem like an ordinary or universal behavior, it is only documented in 46 percent of human cultures,” said Catherine Talbot, an assistant professor of psychology at the Florida Institute of Technology.

Why human kissing may have started

Even with this new modeling, scientists still cannot say exactly what first pushed our ancestors toward kiss-like contact.

Ideas include easing tension after conflict, testing a partner’s health through smell and taste, and slowly training the immune system by sharing small amounts of microbes.

Because behaviors do not fossilize, researchers must rely on indirect traces such as ancient teeth, long-term genetic patterns, and careful observations of modern primates.

Clues hidden in genes and fossils

To interpret these conflicting lines of evidence, the researchers drew on evolutionary theory and long-term models of behavioral change.

“By integrating evolutionary biology with behavioral data, we’re able to make informed inferences about traits that don’t fossilize – like kissing,” said Stuart West, a professor of evolutionary biology. “This lets us study social behavior in both modern and extinct species,”

Future work using the same framework may add more primate species, better fossil samples, and richer cultural records, sharpening estimates of when and where kiss-like contact appears.

For now, the picture that emerges is of a very old, flexible behavior that links modern humans, our ape cousins, and Neanderthals through shared biology and shared choices about intimacy.

The study is published in the journal Evolution and Human Behavior.

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