We had our first kiss about 21 million years ago

BarcelonaWhat's the point of kissing? What biological function can a make-out session have? Or is it an evolutionary function? These are questions that are unlikely to cross your mind while watching Burt Lancaster and Deborah Kerr on the beach in that iconic scene from From there to eternity. But this is the question that some Oxford scientists asked themselves, leading them to investigate the kissing history of some animal species, especially primates. Because humans aren't the only ones who kiss: ants, birds, and polar bears do it too, to name a few. We're talking about kissing on the mouth, an action to which we attribute a romantic component that science seems to have trouble understanding, since—according to the new study—this action "doesn't appear to aid survival or reproduction in any obvious way, while the potential costs of transferring a disease are high." The very definition these Oxford scientists use strips away all the romanticism: kissing is "non-aggressive interactions involving directed, intraspecific oral-oral contact with some movement of the lips/mouthparts and no transfer of food."

The bad news is that they haven't found a clear answer to their initial question, but they have "developed a framework for future research" and, along the way, discovered that the first kiss between primates must have occurred approximately 21 million years ago. According to the study A comparative approach to the evolution of the kiss, published in the magazine Evolution and Human Behavior, Kissing evolved in an ancestor of the great apes between 21.5 and 16.9 million years ago. This finding was obtained after creating a primate family tree and performing phylogenetic analysis to deduce the behaviors of their ancestors using computer evolutionary simulations.

In fact, even Neanderthals kissed on the mouth, and furthermore, the study concludes that some of these Neanderthals once kissed humans of our species, Homo sapiens, because microbial species have been found that coincide in the mouths of both. "There are studies that have shown that Neanderthals and Homo sapiens we shared an oral microbe (Ethanobrevibacter oralis) 200,000 years after we went our separate ways. So we know we exchanged saliva, although that could have been from eating together. But we also know, from the evolutionary simulations we've done, that there's an 84% chance Neanderthals kissed each other. If you add to this the fact that it's been shown that Neanderthals and Homo sapiens “They had sexual relations, and we know that both species kiss, so I think it’s very likely that they kissed each other,” explains one of the study’s authors, Matilda Brindle, from the Department of Biology at the University of Oxford, by phone. Her study attempted to differentiate between sexual kisses and platonic kisses. “The kisses a mother gives her offspring, or even two chimpanzee friends who fight and then kiss and reconcile, would be platonic kisses,” that is, all those that are not part of sexual behavior, Brindle explains. “They push their mouths together and it looks like they’re kissing, but it’s actually a territorial display,” she adds.

The evolutionary function of kissing on the mouth

Thus, gorillas, chimpanzees, bonobos, orangutans, baboons, and humans, Neanderthals or Homo sapiensThey all kiss. However, there are differences. Gorillas do it, but rarely, and in fact, there is one species of gorilla (the eastern gorilla) that doesn't kiss. But in bonobos, kisses, both sexual and platonic, are very common. In chimpanzees, platonic kissing seems to be common, but sexual kissing is rare, and in some orangutan species, both forms of kissing have also been observed, but they are uncommon. "We don't have data on gibbons, which are small apes, and on baboons and macaques, we only have data for some, but there are gaps in the information, which doesn't mean they don't kiss, but rather that we don't have enough data to say so," explains Brindle, who dedicated himself to finding all the scientific literature on each of them to corroborate this.

One limitation noted in their own research is that some of the primate species studied could only be analyzed in captivity. "For example, to our knowledge, bonobos have not yet been observed kissing in the wild, even though this behavior occurs regularly in captivity," and "this is surprising, since the occurrence of other behaviors, such as masturbation or the construction and use of sex toys, does not seem to vary depending on whether the individuals are in the wild."

Brindle says she is convinced that kissing on the mouth does have an evolutionary function for the animal species that do it, although for now all we have are hypotheses. "We think that platonic kissing has to do with strengthening bonds and maintaining social relationships, which is very useful, and as for sexual kissing, it could be a way of evaluating a partner to find out if an individual is of sufficient quality to mate with. Perhaps if they have bad breath, they are sick and no longer good for kissing. There are studies that say we use pheromones to know if we are genetically compatible with other individuals, so that offspring have more diverse genes, especially with regard to immune function." She concludes with an invitation to other colleagues: "We have shown that the evolutionary history of kissing can be traced; we have seen how it evolved, but we still don't know exactly why: we need other primatologists and zoologists to continue the search."