New scientific research mapping the surprising origins of kissing, revealing how ancient apes and early humans may have locked lips millions of years before modern cultures recorded the act.
Kissing is often treated as a simple gesture of affection, yet a growing body of research shows the act may be far older and more complex than most people think. A new study by evolutionary biologists at Oxford University challenges long-held assumptions by suggesting that kissing may have evolved in ancient ape ancestors between 21.5 and 16.9 million years ago. The findings raise fresh questions about why early humans and other primates kissed at all, especially given the risks and apparent lack of clear survival benefits. For a behavior so common and intimate, its origins have been surprisingly mysterious. Now, scientists say we may finally be able to place kissing on the evolutionary map.
Why does kissing exist at all?
Kissing has long been viewed as an “evolutionary conundrum” because it seems to offer no direct advantage for survival or reproduction. It can spread disease, consumes energy, and carries social risks. Yet humans continue to kiss across cultures, and many animals do something similar. This puzzle led Oxford evolutionary biologist Matilda Brindle and her team to explore whether kissing is an instinctive trait inherited from ancient ancestors.
The challenge is clear: kissing leaves no trace in the fossil record. Scientists cannot search for fossilized lips or preserved moments of affection. Instead, the researchers turned to modern primates, where behaviors can be observed, compared and mapped back through evolutionary history. Their goal was to understand not only whether primates kiss, but also which species do so and how this behavior might have emerged.
The team defined kissing as non-aggressive, mouth-to-mouth contact that does not involve food sharing. Using existing scientific observations, they confirmed that chimpanzees, bonobos, orangutans, and one gorilla species all engage in some form of kissing. These findings provided the starting point for a deeper investigation.
Through phylogenetic analysis, which helps scientists track traits across different species and their ancestors, the team built an evolutionary tree of primates. They ran millions of simulations to estimate the probability that ancient ancestors of today’s apes also kissed. Because humans, chimpanzees, and bonobos all kiss, the model suggests that their last common ancestor almost certainly did, too. That ancestor lived millions of years before modern humans appeared, long before the earliest recorded kisses in ancient Mesopotamia or Egypt.
This places the origin of kissing far earlier than cultural history suggests, giving weight to the idea that kissing is not simply a social invention but a behavior with evolutionary roots.
What prehistoric kisses reveal about ancient humans and extinct relatives
The study’s modeling places the first appearances of kissing in ancient apes between 21.5 and 16.9 million years ago. This timeline is striking because it suggests that kissing was already established long before early humans evolved. If kissing existed in ancestral apes, it is likely that it was passed down to later species, including extinct human relatives such as Neanderthals.
This idea is supported by what scientists already know about Neanderthals. They showed complex social behavior, lived in groups, cared for the sick, and created symbolic objects. If kissing helped with bonding, reducing conflict, or assessing potential mates, then it would have been as useful to Neanderthals as to any early human group. And because modern humans interbred with Neanderthals, it is even possible both species kissed each other.
However, what remains unclear is why early humans and apes kissed. The study does not determine the original purpose, but it highlights several possibilities. Kissing may have helped individuals detect chemical signals on another’s breath or skin, providing clues about health or fertility. It may also have helped partners bond, strengthen alliances, or reduce social tension. In some primates, mouth contact is used to comfort distressed individuals, hinting that kissing may have begun as a soothing behavior rather than a romantic one.
The research also reveals something often forgotten: kissing is not a universal human behavior. Only 46 percent of documented cultures engage in romantic mouth-to-mouth kissing. Many societies show affection through other gestures, and some consider kissing unnecessary or even unpleasant. This diversity suggests that kissing may be flexible, appearing only when it offers a benefit within a particular culture or environment.
The findings create a clearer picture of the deep human past, showing that affectionate behaviors we see today may have roots stretching far beyond written history.
How the science works: tracing a behavior with no fossil record
Understanding the origins of kissing required scientists to rely on indirect evidence. Most behaviors leave no clear marks, so researchers must examine living species. Brindle’s team used a method known as phylogenetic inference. This statistical approach compares traits across modern species and the structure of their evolutionary relationships to infer what ancient ancestors likely did.
The team ran ten million simulations to test how kissing might have developed along the branches of the primate family tree. Each simulation produced a slightly different evolutionary scenario, allowing the researchers to find patterns that appeared consistently. When multiple species within a group share a trait, the model assumes their ancestor probably shared it, too.
This approach provided surprisingly strong evidence that kissing emerged early among large apes. Still, the researchers emphasize the limitations. Much of the data on primates comes from captivity or sanctuaries. Wild populations may behave differently, and many species are poorly studied. The absence of kissing in some primates might reflect lack of observation rather than true absence.
The study also distinguishes between mouth-to-mouth kissing and other forms of affectionate touch. Humans kiss in many ways that are not included in this research. Forehead kisses, cheek kisses and cultural greetings fall outside the definition used. Some scientists argue that these broader forms of contact must also be part of the evolutionary picture.
Even so, the new study provides an important foundation. It demonstrates that kissing is an evolved trait, not simply a cultural invention, and that its roots likely stretch deeper than any written record. As more data become available from primates and human populations, researchers hope to refine the evolutionary timeline and uncover the function of kissing in early societies.
What this ancient instinct means for modern humans
Kissing may feel deeply personal, but the study shows that it sits at the intersection of biology and culture. It is shaped by instincts inherited over millions of years, yet also by social rules, traditions and personal preferences. This mix explains why kissing can feel intimate even in societies where it is rare or limited to specific contexts.
Researchers say the findings open the door to new questions: Why do some people prefer certain types of kisses? Why do some cultures avoid kissing entirely? What sensory information do humans really exchange during a kiss? And how might early human groups have used kissing to manage relationships, reduce conflict or choose mates?
The study’s authors argue that the next step is to investigate kissing across more primate species and human populations. Doing so could reveal patterns that link ancient instincts with modern behavior, offering a clearer look at how emotions and social bonds evolved.
Kissing may not be universal, and it may even be risky in many settings, but its survival across millions of years suggests that it once served an important role—and perhaps still does. What began as a simple gesture among ancient apes has become a complex part of human connection, shaping relationships, emotions and culture in ways that continue to evolve today.
As scientists explore the origins of intimate behavior, kissing reminds us that even the most familiar gestures may carry histories deeper than we imagine.
