Scientists trace kissing back 21 million years — And yes, Neanderthals did it too!

Kissing occurs in a variety of animals, but presents an evolutionary puzzle: it appears to carry high risks, such as disease transmission, while offering no obvious reproductive or survival advantage. Despite kissing carrying cultural and emotional significance in many human societies, up to now researchers have paid little attention to its evolutionary history, reads the University of Oxford's Department of Biology website.

In the new study, the researchers carried out the first attempt to reconstruct the evolutionary history of kissing using a cross-species approach based on the primate family tree. The results indicate that kissing is an ancient trait in the large apes, evolving in the ancestor to that group 21.5 - 16.9 million years ago. Kissing was retained over the course of evolution and is still present in most of the large apes.

The team also found that our extinct human relatives, Neanderthals, were likely to have engaged in kissing too. This finding, together with previous studies showing that humans and Neanderthals shared oral microbes (via saliva transfer) and genetic material (via interbreeding), strongly suggests that humans and Neanderthals kissed one another.

Lead author Dr Matilda Brindle 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 behaviours exhibited by our primate cousins.”

To run the analyses, the team first defined what constitutes a kiss. This was challenging, because many mouth-to-mouth behaviours look like kissing. Since the researchers were exploring kissing across different species, the definition also needed to be applicable to a wide range of animals. They therefore defined kissing as non-aggressive, mouth-to-mouth contact that did not involve food transfer.

Having established this definition, the researchers collected data from the literature on which modern primate species have been observed kissing, focusing on the group of monkeys and apes that evolved in Africa, Europe and Asia. This included chimpanzees, bonobos, and orangutans, all of which have been observed kissing.

They then ran a phylogenetic analysis; treating kissing as a ‘trait’ and mapping this to the family tree of primates. They used a statistical approach (called Bayesian modelling) to simulate different evolution scenarios along the branches of the tree, to estimate the probability that different ancestors also engaged in kissing. The model was run 10 million times to give robust statistical estimates.

Co-author Professor Stuart West said: "By integrating evolutionary biology with behavioural data, we’re able to make informed inferences about traits that don’t fossilise - like kissing. This lets us study social behaviour in both modern and extinct species."

While the researchers caution that existing data are limited - particularly outside the large apes - the study offers a framework for future work, and provides a way for primatologists to record kissing behaviours in nonhuman animals using a consistent definition.

“While kissing may seem like an ordinary or universal behaviour, it is only documented in 46% of human cultures," said Dr 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 behaviour or cultural invention. This is the first step in addressing that question.”