Scientists reveal kissing began millions of years before humans

Kissing is seen in many animal species, yet it poses an evolutionary puzzle: the act can spread disease and does not appear to directly boost survival or reproduction. Although kissing holds strong emotional and cultural meaning for many human groups, its evolutionary background has rarely been examined in detail.

Reconstructing Kissing on the Primate Family Tree

In this study, researchers conducted the first cross-species attempt to trace the origins of kissing using the evolutionary relationships among primates. Their results suggest that kissing has deep roots in large apes, emerging in their ancestor between 21.5 -- 16.9 million years ago. This behavior appears to have persisted through evolution and is still observed in most species within this group.

The team also concluded that Neanderthals, our extinct human relatives, likely kissed as well. This conclusion is supported by earlier research showing that humans and Neanderthals exchanged oral microbes (via saliva transfer) and interbred, implying that kissing was part of their interactions.

Dr. Matilda Brindle, lead author and evolutionary biologist at Oxford's Department of Biology, said: "This is the first time anyone has taken a broad evolutionary lens to examine kissing. Our findings add to a growing body of work highlighting the remarkable diversity of sexual behaviors exhibited by our primate cousins."

Defining and Identifying Kissing in Animals

To perform their analysis, the team first needed to define what counts as a kiss, since many mouth-to-mouth actions resemble the behavior without being the same thing. Because they were comparing species across a wide evolutionary range, the definition had to work universally. They defined kissing as non-aggressive, mouth-to-mouth contact that does not involve food transfer.

After settling on this definition, the researchers reviewed scientific literature to identify modern primates known to engage in kissing. Their focus was on monkeys and apes that evolved in Africa, Europe, and Asia, including chimpanzees, bonobos, and orangutans.

Modelling the Evolution of Kissing

The next stage involved a phylogenetic analysis in which kissing was coded as a trait and placed onto the primate family tree. Using a statistical method (called Bayesian modelling), the team simulated millions of possible evolutionary scenarios to estimate how likely it was that various ancestors also kissed. The model ran 10 million times to provide strong statistical confidence in the results.

Professor Stuart West, co-author and Professor of Evolutionary Biology at Oxford, said: "By integrating evolutionary biology with behavioral data, we're able to make informed inferences about traits that don't fossilize -- like kissing. This lets us study social behavior in both modern and extinct species."

Cultural Variation and Future Research

The researchers note that available data are still limited, especially beyond the large apes, but the project offers a foundation for future studies. It also gives primatologists a standardized way to report kissing behaviors in nonhuman animals.

"While kissing may seem like an ordinary or universal behavior, it is only documented in 46% of human cultures," said Catherine Talbot, co-author and Assistant Professor in the College of Psychology at Florida Institute of Technology. "The social norms and context vary widely across societies, raising the question of whether kissing is an evolved behavior or cultural invention. This is the first step in addressing that question."