Ancient DNA Suggests Neanderthals Engaged In Kissing — And Probably Smooched Humans Too!

Two different human species meet tens of thousands of years ago. Modern humans, fresh from Africa, encounter their stockier, cold-adapted cousins in what is now Europe. Did this initial meeting lead to conflict or confusion? On the contrary, surprising evidence suggests humans and Neanderthals kissed upon crossing paths.

Scientists have long known that modern humans and Neanderthals interbred, leaving genetic traces that persist in people today. Now, a study published in Evolution and Human Behavior goes further, suggesting with 84% probability that Neanderthals engaged in kissing behavior. More tantalizing still, ancient DNA evidence hints that the two human species may have locked lips with each other.

The finding comes from the first evolutionary analysis of kissing across primates, conducted by researchers at the University of Oxford. Using statistical modeling based on kissing patterns in living apes and monkeys, the team reconstructed the behavior’s deep history. Their work shows that kissing is neither a recent cultural invention nor a uniquely modern human trait.

“Kissing can be observed across the animal kingdom,” the researchers write. Yet despite its widespread occurrence, “the fitness benefits of kissing are unclear.”

The Microbe That Tells the Story

The case for Neanderthal kissing rests partly on an unlikely witness: a microbe living in our mouths.

Modern humans and Neanderthals split into separate species between 450,000 and 750,000 years ago. For most of that time, they evolved in isolation, one lineage in Africa and the other in Europe and western Asia. When they finally met again, probably around 60,000 years ago, they were different enough to be classified as distinct species, yet similar enough to produce fertile offspring.

Here’s where the microbe comes in. Both modern humans and Neanderthals harbor a bacterium called Methanobrevibacter oralis in their mouths. It’s a commensal organism, meaning it lives in our bodies without causing harm. Scientists can trace the evolutionary history of these bacteria through DNA analysis, just as they trace the history of their human hosts.

Crucially, the strains of M. oralis in modern humans and Neanderthals only diverged between 112,000 and 143,000 years ago. That’s hundreds of thousands of years after the two human species split. Something kept these oral microbes mixing between the two lineages long after their human hosts went their separate ways.

One explanation involves close contact between Neanderthals and modern humans. The bacteria needed a route from one species to the other, and that route could have involved mouths coming into contact.

Three Ways to Share Spit

The researchers identify three plausible scenarios for how oral bacteria could jump between human species.

The first is food sharing. Both Neanderthals and modern humans are known to have shared food within their groups. If a Neanderthal and a modern human shared a meal, perhaps passing food back and forth, microbes could have hitched a ride.

The second is premastication, the practice of pre-chewing food before giving it to someone else, typically an infant or young child. This behavior occurs in all living great ape species where kissing has been documented. A Neanderthal caregiver feeding a modern human infant through premastication, or vice versa, could have transferred the microbes.

The third possibility is kissing itself.

An Ancient Ape Inheritance

The Oxford team’s evolutionary analysis strongly suggests that Neanderthals inherited kissing from their ape ancestors. The behavior likely evolved in the common ancestor of all great apes between 21.5 and 16.9 million years ago, then persisted across most lineages.

Living great apes provide the template. Chimpanzees kiss primarily to reconcile after conflicts, briefly pressing their lips together to repair social bonds. Bonobos, in contrast, engage in prolonged, tongue-involved kisses that researchers describe as having “a strikingly sensual character.” Bonobos also display affectionate mouth-to-mouth contact between mothers and infants, reinforcing bonds. Orangutans kiss rarely, with males sometimes engaging in mouth-to-mouth contact that may involve exchanging food or drink, or in other cases appears affectionate. Sexual kissing has also been documented in orangutans. Western lowland gorillas kiss occasionally in both platonic and sexual situations.

Humans clearly fit into this pattern, with kissing occurring in many (though not all) cultures worldwide. The statistical modeling that incorporates all these living species points to Neanderthals falling within the kissing group.

“Given that kissing was present in the common ancestor of all great apes,” the researchers write, “it would be surprising if both human lineages had somehow lost this behavior.”

What Neanderthal Kissing Might Have Looked Like

If Neanderthals did kiss, they probably did so for the same reasons other great apes do: to bond socially, to reconcile after conflicts, to assess potential mates, or as part of sexual behavior.

Some evolutionary hypotheses suggest that kissing might help individuals assess potential partners through smell and taste, picking up cues about health, genetic compatibility, and reproductive status. The study’s early data show a tentative association between kissing and multi-male mating systems across primates, where females mate with multiple males and competition between males is intense, but this pattern hasn’t been formally tested. Modern human societies often fall along this spectrum, while Neanderthal mating patterns are still being reconstructed from fragmentary evidence.

Premastication, where food is pre-chewed and then passed on, occurs in every great ape species where kissing has been documented. The authors suggest it may also have been part of Neanderthal caregiving, making it a promising candidate for how kissing could have evolved in our lineage. This behavior requires mouth-to-mouth contact and lip protrusion, the same physical movements used in kissing.

Social kissing for bonding and reconciliation would have served Neanderthals well. They lived in small, tight-knit groups where maintaining social cohesion was critical for survival. Brief kisses after disputes, as seen in chimpanzees, could have helped smooth over tensions and keep groups functioning.

When Two Human Species Met

The possibility that Neanderthals and modern humans kissed each other opens new questions about how they interacted.

Genetic evidence has already established that the two groups interbred. People of non-African descent carry 1-4% Neanderthal DNA, the result of interbreeding that occurred after modern humans left Africa but before they spread across the rest of the world. The encounters were intimate enough to produce children.

But the microbe evidence suggests something potentially more complex than simple interbreeding. The transfer of oral bacteria could indicate sustained contact between the two groups, possibly including social behaviors like kissing that aren’t strictly necessary for reproduction.

One scenario involves mixed groups where Neanderthals and modern humans lived together, at least temporarily. In such settings, social kissing could have occurred as part of normal relationship maintenance, similar to how chimpanzees kiss to reconcile.

Another scenario involves romantic or sexual relationships that included kissing as part of courtship or intimacy, much as happens in many human cultures today. If sexual kissing helped individuals assess mate quality in both species independently, it would likely have served the same function when they encountered each other.

A third possibility combines elements of both: family groups that included members of both species, with parental care behaviors like premastication or affectionate kissing crossing species boundaries.

What We Still Don’t Know

The researchers emphasize significant uncertainties in their analysis. The 84% probability for Neanderthal kissing is based on statistical modeling, not direct fossil evidence. Soft tissue behaviors like kissing don’t preserve in the archaeological record.

The microbe evidence, while suggestive, doesn’t prove that Neanderthals and modern humans kissed each other. Food sharing or premastication between the two species could explain the bacterial transfer without requiring kissing. Distinguishing between these possibilities from ancient DNA alone is difficult.

There’s also the question of variation. Kissing isn’t universal among living humans. While it occurs in many cultures, others lack the practice entirely or view mouth-to-mouth contact as inappropriate or unhygienic. If modern humans show such variation, Neanderthals might have too. Some Neanderthal populations could have kissed while others didn’t.

The type of kissing matters as well. The study’s definition encompasses both sexual and platonic kissing, but these may have different evolutionary origins and functions. Neanderthals could have engaged in one form but not the other.

Rethinking Neanderthals and Ourselves

The image of kissing Neanderthals upends lingering stereotypes. Popular culture has long portrayed them as brutish, inarticulate creatures fundamentally different from modern humans. The reality emerging from decades of archaeological and genetic research paints a different picture.

Other research over the past few decades has shown that Neanderthals made sophisticated stone tools, controlled fire, buried their dead, created art, and cared for injured group members. They had language capacity, as evidenced by their anatomy and genetics. They adorned themselves with pigments and jewelry. They were fully human in all the ways that matter, just a different variety of human than us.

Kissing fits comfortably into this picture. If Neanderthals engaged in complex social behaviors, used symbols, and formed emotional bonds, kissing would have been a natural part of their behavioral repertoire. It’s a simple behavior, requiring no tools or special skills, just the basic mammalian ability to control lip movements.

The possibility that Neanderthals and modern humans kissed each other adds another dimension to their story. It suggests interactions that went beyond simple conflict or reproduction, potentially including the kinds of social and romantic behaviors that define human relationships today.

When a modern human and a Neanderthal pressed their lips together 50,000 years ago, whether in greeting, in passion, or in reconciliation, they were expressing something ancient. Kissing had already been around for more than 15 million years by that point, passed down through countless generations of apes and hominins. Both species had inherited the capacity. Both species, it seems, put it to use.

We are, in many ways, the product of those encounters. The Neanderthal DNA scattered through our genomes, the shared microbes in our mouths, and possibly even our kissing behavior all link us to these vanished cousins. Every time modern humans kiss, we’re engaging in an act that Neanderthals likely performed too, using the same basic motor patterns, probably for many of the same reasons.

The story of human kissing reaches back millions of years into our primate past. But one of its most intriguing chapters may have been written in a fleeting moment when two human species met, recognized something familiar in each other, and leaned in close.

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