If you think kissing is just a romantic quirk humans came up with somewhere along the way, think again.
A new study led by researchers at the University of Oxford suggests kissing is far older and far more deeply rooted in our primate family tree than anyone expected.
According to the team, the behavior likely evolved in the common ancestor of humans and other large apes somewhere around 21 million years ago.
And here is the surprising part. Neanderthals probably kissed, too.
This work, published in Evolution and Human Behavior, is the first attempt to trace the whole evolutionary history of kissing using a broad, cross-species approach. It sounds simple, but the researchers were tackling an old puzzle. Kissing carries obvious risks.
It can spread disease, involves exposure to another individual’s saliva, and does not present an immediate survival benefit. So why do so many humans do it? And why do several of our primate relatives do it, too?
The researchers began by defining what actually counts as a kiss. It was not as evident as it sounds. Many mouth-to-mouth behaviors across animals resemble kissing, but do not quite fit the idea.
For example, food sharing looks like kissing but serves a clear nutritional purpose. So the team landed on a definition that works across species. A kiss is non-aggressive, mouth-to-mouth contact with a bit of lip or mouth movement, and no food transfer.
With the definition set, they began reviewing decades of research on primate behavior to document which species engage in kissing. Chimpanzees kiss. So do bonobos and Orangutans do as well.
Once they completed the list, they entered the data into a phylogenetic model, using statistics to understand a primate family tree and infer if ancient ancestors also kissed. The analysis ran ten million simulations to create a clear picture.
The result was striking. Kissing almost certainly existed in the ancestor of today’s large apes, which lived somewhere between 21.5 and 16.9 million years ago. It means kissing has been around far longer than humans, far longer than Neanderthals, and long before any modern culture decided it was romantic.
The study also examined whether Neanderthals kissed. Direct evidence is impossible, of course. Behaviors do not fossilize. But other clues line up. Past studies show that humans and Neanderthals shared oral microbes, which implies saliva exchange.
Genetic analysis confirms that the two species interbred. When you put the pieces together, the most straightforward explanation is that kissing was part of their social and sexual behavior.
Dr Matilda Brindle, the study’s lead author, says this is the first time anyone has looked at kissing through such a wide evolutionary lens. She highlights how diverse sexual behavior can be in our primate relatives and how much we still have to learn about where our own behaviors come from.
The study authors also emphasize that while kissing feels universal, it is not. Only about 46 percent of human cultures engage in romantic or social kissing.
In some societies, it is common. In others, it simply does not exist. This raises an interesting question. Is kissing an evolved behavior that some cultures later abandoned, or is it a cultural tradition that developed only in certain places? This study does not fully answer that question, but it lays a foundation for scientists to explore it.
Kissing can increase the risk of catching Gonorrhoea
Professor Stuart West, a co-author from Oxford, says that integrating behavioral observations with evolutionary biology helps researchers infer traits that cannot be preserved in bones. It opens a window into the softer, more intimate aspects of our ancestors’ lives.
The message is clear, when you see two chimpanzees gently pressing their mouths together, you are not witnessing a random act. Instead, you are observing a behavior that has existed for many years.
This behavior has helped form bonds and social ties long before the first humans walked the earth. It likely connected Homo sapiens and Neanderthals in ways that we are just beginning to understand.
Journal Reference
- Brindle, M., Talbot, C. F., & West, S. (2025). A comparative approach to the evolution of kissing. Evolution and Human Behavior, 106788. DOI: 10.1016/j.evolhumbehav.2025.106788
