Despite kissing carrying cultural and emotional significance in many human societies, up to now researchers have paid little attention to its evolutionary history.
In a new study from Oxford researchers carried out the first attempt to reconstruct the evolutionary history of kissing 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.
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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.
“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,” says Dr Matilda Brindle, an evolutionary biologist at Oxford’s Department of Biology.
To run the analyses, the team first defined what constitutes a kiss. This was challenging, because many mouth-to-mouth behaviors 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. By integrating evolutionary biology with behavioral data, “we’re able to make informed inferences about traits that don’t fossilise – like kissing. This lets us study social behavior in both modern and extinct species,” says Brindle.
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 modeling) 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.
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 behaviors in nonhuman animals using a consistent definition.
“While kissing may seem like an ordinary or universal behavior, it is only documented in 46% of human cultures,’ said Catherine Talbot, co-author of the study who works at the 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.”
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