Kissing may have evolved in an ape ancestor 21 million years ago

Early humans like Neanderthals probably kissed, and our ape ancestors may have done so as early as 21 million years ago.

There is considerable debate about when people started kissing romantically. Ancient texts hint at this. Sexual kissing was practiced in ancient Mesopotamia and Egypt at least 4,500 years ago.but because such kisses were only approximately documented 46 percent of human culturesSome argue that it is a cultural phenomenon that arose relatively recently in human history.

However, there are hints that Neanderthals exchanged oral bacteria with wise manand chimpanzees, bonobos and orangutans have also been spotted kissing. It is therefore quite possible that this behavior has much deeper roots than the historical texts indicate.

To look for answers Matilda Brindle from the University of Oxford and her colleagues tried to figure out the evolutionary history of kissing. “Kissing seems like something of an evolutionary paradox,” she says. “It probably doesn't promote survival and may even be risky in terms of transmitting pathogens.”

The researchers first came up with a definition of kissing that would work for many species, settling on mouth-to-mouth contact that is non-antagonistic and involves lip movement but not food transfer.

This leads to the exclusion of many kisses, including kisses on other parts of the body. “If you kiss someone on the cheek, I would say it's a kiss, but by our definition it's not a kiss,” Brindle says. “People are taking kissing to the next level.”

The team then reviewed the scientific literature and contacted primate researchers to find reports of kissing in modern apes that evolved in Africa, Europe and Asia.

To estimate the likelihood that different ancestral species also engaged in kissing, Brindle and her colleagues plotted this information on the primate family tree and ran a statistical approach called Bayesian modeling 10 million times to simulate different evolutionary scenarios.

They found that kissing likely evolved in the ancestors of apes around 21.5 to 16.9 million years ago, and there is an 84 percent chance that our extinct human relatives, the Neanderthals, were also kissers.

“Obviously, these are just Neanderthal kisses; we don’t know who they’re kissing,” Brindle says. “But coupled with evidence that humans and Neanderthals had similar oral microbiomes and that most people of non-African descent have some Neanderthal DNA, we can say that they probably kissed each other, which certainly puts a much more romantic spin on the human-Neanderthal relationship.”

Brindle says there isn't enough data yet to explain why kissing came about, but she has two hypotheses.

“As for sexual kissing, it can increase reproductive success by allowing animals to judge the quality of a mate. If someone has bad breath, you may want to avoid breeding with them,” she says.

Sexual kissing can also promote post-coital success by promoting arousal, which can speed up ejaculation and change the pH of the vagina to make it more receptive to sperm, she says.

The other main idea is that non-sexual kissing resulted from courtship and is useful for strengthening connections and easing social tensions. “Chimpanzees literally kiss and make up after a fight,” says Brindle.

“I think that, based on the evidence they have, kissing definitely has an affiliative function,” says Zanna Clay at Durham University, UK. “For example, we know that in chimpanzees it plays an important role in reestablishing social relationships. But for me, the sexual aspect is a bit of a question mark.”

On the question of whether kissing is an evolved behavior or a cultural invention, “I think our results show very clearly that kissing has evolved,” Brindle says.

Troels Punk Arbøll A researcher at the University of Copenhagen in Denmark, who traced early records of kissing in cuneiform writing from ancient Mesopotamia, agrees. “This gives more evidence that people have been kissing for a long time,” he says.

But that's hardly the whole story, given that many groups of people don't kiss. “I'm sure there's a strong cultural element to it, and it's probably come and gone with different cultural preferences,” Clay says.

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