Kissing is a great evolutionary mystery. This gesture, which consists of pressing one’s lips together with those of another individual, does not appear to offer any advantage in the reproduction or survival of the species, and can even cause problems, such as the transmission of diseases. However, the vast majority of humans (and many other animal species) enjoy it immensely, so much so that kissing has inspired poems, songs, paintings, and films for centuries.
Now, a study conducted by the University of Oxford has dated the beginning of this gesture, which can express love, desire, affection or reconciliation: researchers believe it is around 21 million years old. The study found that this behavior evolved in the common ancestors of humans and other great apes, and that Neanderthals probably also kissed, with a probability of nearly 84%. The results, published Wednesday in the journal Evolution and human behaviorrevealing that, far from being a recent human cultural invention, kissing is an ancient trait deeply rooted in our biology.
“This is the first time anyone has used 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,” said Matilda Brindle, lead author of the study and an evolutionary biologist in Oxford’s Department of Biology, in a statement.
The Oxford team made the first attempt to reconstruct the evolutionary history of kissing using a cross-species approach, based on the primate family tree. The first challenge was to define exactly what constitutes a kiss, something much more complex than it seems.
“A lot of mouth-to-mouth behavior looks like kissing, but it’s not,” Brindle pointed out. Because the researchers were exploring kissing in different species, they needed a definition that applied to a wide range of animals. In the end, what they got was rather prosaic: “Non-aggressive mouth-to-mouth contact that did not involve the transfer of food.”
From Bonobos to Neanderthals
Using this definition, the researchers collected data from the scientific literature on which modern primate species have been observed kissing. The results show that kissing is present in most great apes: chimpanzees, bonobos and orangutans. Outside of primates, kissing has been documented in animals as diverse as polar bears, wolves, prairie dogs, and even some birds. However, there are significant differences between the species: the bonobo kiss is “extraordinarily sensual”, says the research, while the chimpanzee kiss is “brief and tense”.
To reconstruct the evolutionary history of kissing, the researchers used a statistical approach (called a Bayesian model) to simulate different evolutionary scenarios along the branches of the tree, and then estimate the probability that different ancestors also kissed. The model was run 10 million times to provide reliable statistical estimates, they explain.
The findings indicate that kissing evolved in the ancestors of great apes between 21.5 and 16.9 million years ago, after their divergence from smaller apes such as gibbons. The behavior has persisted throughout evolution and is still present in most modern great apes.
“By integrating evolutionary biology with behavioral data, we are able to make informed inferences about traits that do not fossilize, such as kissing. This allows us to study social behavior in both modern and extinct species,” Professor Stuart West, co-author of the study and professor of evolutionary biology at Oxford, explained in a statement.
One of the study’s most surprising findings is that Neanderthals, our extinct human relatives, also very likely kissed. Phylogenetic analysis estimates an 84.3% probability that this species practiced kissing.
This finding adds to previous studies showing that modern humans and Neanderthals shared oral microbes, particularly the bacterium Oral Metanobrevibacterthrough the transfer of saliva. Although modern humans and Neanderthals diverged between 450,000 and 750,000 years ago, this oral microbe separated into distinct lineages only between 112,000 and 143,000 years ago. This suggests that the species exchanged microbes for a long period after their separation. “We suspected that Neanderthals probably kissed,” Brindle explained in a video conference, “but it was great to be able to confirm that. An 84% probability is a pretty strong result,” he added.
Paleoanthropologist Antonio Rosas, who was not involved in the study, believes that this relationship is “a bit weak”, since the transfer of the bacteria could occur through a kiss, “but it could also be a bacterium inherited from common ancestors”.
The study also explains that despite being millions of years old, the act of kissing is not universal among humans. “It is documented in only 46% of human cultures,” Catherine Talbot, co-author and assistant professor in the College of Psychology at the Florida Institute of Technology, said in a statement. “Social norms and context vary widely from one society to another, raising the question of whether kissing is an evolved behavior or a cultural invention.”
If kissing carries risks of disease transmission, there must be compensatory adaptive benefits. Researchers have explored several hypotheses. In a sexual context, kissing could allow assessment of the quality of a potential mate through olfactory cues. It may also serve to increase pre-copulatory arousal, thus increasing the chances of fertilization. Furthermore, data from this study suggests that prechewing – that is, parents giving prechewed food to their offspring – is present in all kissing species, suggesting that kissing may have evolved from this parental care behavior.
Limitations
The authors of the study themselves recognize its limitations. Data is sparse, especially outside of the great apes, and most observations come from captive animals rather than wild populations. “It is important to view our analysis as a proof of concept for future work,” cautions Brindle. The experts hope that their study can establish a framework for future research and help systematize the recording of kissing behavior in non-human animals.
“This is an interesting study that opens up a new way of thinking about a behavior that appears to be universal among great apes. This means that the probability of kissing among Neanderthals is high,” says Clive Finlayson, director of the Gibraltar Museum, who was not involved in the study. It also highlights several life history variables that correlate “reasonably, although not perfectly,” he says, with the presence of kissing among monkeys: multi-male mating systems and prechewing. “Of course we don’t know whether these variables would apply to Neanderthals,” he acknowledges.
Brintle concludes his study with a quote widely attributed to the actress Ingrid Bergman: “A kiss is a beautiful trick designed by nature to interrupt speech when words become superfluous.” A trick that, according to this new study, is 21 million years old.
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