Kissing occurs in most living great apes and probably also occurred among Neanderthals, first appearing in the group's ancestor 21.5 to 16.9 million years ago, according to a new study led by Oxford University scientists.
Kissing occurs in a wide variety of animals, but poses an evolutionary conundrum: It appears to carry high risks, such as disease transmission, but does not provide obvious reproductive or survival benefits.
Although kissing has cultural and emotional significance in many human societies, until now researchers have paid little attention to its evolutionary history.
In a new study, Dr Matilda Brindle from the University of Oxford and her colleagues made the first attempt to reconstruct the evolutionary history of kissing using a cross-species approach based on the primate family tree.
The results suggest that kissing is an ancient trait of the great apes, evolving in the group's ancestor between 21.5 and 16.9 million years ago.
Kissing has been preserved through evolution and is still present in most great apes.
The researchers also discovered that our extinct human relatives, the Neanderthals, probably kissed too.
This discovery, along with previous studies showing that humans and Neanderthals shared oral microbes (through saliva transfer) and genetic material (through interbreeding), strongly suggests that humans and Neanderthals kissed each other.
“This is the first time anyone has looked at kissing from a broad evolutionary perspective,” Dr Brindle said.
“Our findings add to a growing body of work highlighting the remarkable diversity of sexual behavior exhibited by our primate cousins.”
To conduct the analysis, scientists first determined what constitutes a kiss.
This was difficult because many mouth-to-mouth actions resemble kissing.
Since the authors studied kissing in different species, this definition should also be applicable to a wide range of animals.
Therefore, they defined kissing as non-aggressive mouth-to-mouth contact that does not involve the transfer of food.
Having established this definition, the researchers collected data from the literature that observed kissing in modern primate species, focusing on a group of monkeys and apes that evolved in Africa, Europe and Asia.
These included chimpanzees, bonobos and orangutans, all of whom were seen kissing.
They then performed a phylogenetic analysis; consider kissing as a “trait” and relate it to the primate family tree.
They used a statistical approach (called Bayesian modeling) to simulate different evolutionary scenarios along the branches of the tree to estimate the likelihood that different ancestors also kissed.
The model was run 10 million times to obtain reliable statistical estimates.
“By combining evolutionary biology with behavioral data, we can make informed inferences about traits that don't fossilize – like kissing,” said Oxford University professor Stuart West.
“This allows us to study the social behavior of both living and extinct species.”
Although the researchers caution that existing data are limited, especially outside of great apes, the study offers a framework for future work and gives primatologists the ability to record kissing behavior in non-human animals using a consistent definition.
“Although kissing may seem like a common or universal behavior, it is documented in only 46% of human cultures,” said Dr. Catherine Talbot of the Florida Institute of Technology.
“Social norms and context vary widely across societies, raising the question of whether kissing is an evolved behavior or a cultural invention.”
“This is the first step in addressing this issue.”
study will be published this week in the magazine magazine Evolution and human behavior.
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Matilda Brindle etc.. 2025. A comparative approach to the evolution of kissing. Evolution and human behaviorin print; doi: 10.1016/j.evolhumbehav.2025.106788
