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

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

There is wide debate over when humans began kissing romantically. Ancient texts hint that sexual kissing was practised in ancient Mesopotamia and Egypt at least 4500 years ago, but because such kissing has been documented in only about 46 per cent of human cultures, some argue it is a cultural phenomenon that emerged relatively recently in human history.

However, there are hints that Neanderthals exchanged oral bacteria with Homo sapiens, and chimpanzees, bonobos and orangutans have all been observed kissing. So it is possible that the behaviour goes back far further than historical texts reveal.

To look for answers, Matilda Brindle at the University of Oxford and her colleagues have attempted to work out the evolutionary history of kissing. “Kissing seems a bit of an evolutionary paradox, she says. “It probably doesn’t aid survival and could even be risky in terms of helping pathogen transmission.”

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

This leads to many smooches being excluded, including kisses elsewhere on the body. “If you kiss someone on the cheek, then I would say that is a kiss, but by our definition, it isn’t kissing,” says Brindle. “Humans take kissing to a new level.”

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

To estimate the likelihood that various ancestral species also engaged in kissing, Brindle and her colleagues mapped out this information in a family tree of primates and ran a statistical approach called Bayesian modelling 10 million times to simulate different evolution scenarios.

They found that kissing probably evolved in ancestral apes some 21.5 million to 16.9 million years ago and there is an 84 per cent chance that our extinct human relatives, Neanderthals, engaged in kissing too.

“Obviously, that’s just Neanderthals kissing; we don’t know who they’re kissing,” says Brindle. “But together with the evidence that humans and Neanderthals had a similar oral microbiome and that most humans of non-African descent have some Neanderthal DNA, we would argue they were probably kissing each other, which definitely puts a much more romantic spin on human-Neanderthal relations.”

There isn’t enough data yet to tell why kissing evolved, says Brindle, but she does suggest two hypotheses.

“In terms of sexual kissing, it could enhance reproductive success by letting animals assess mate quality. If someone has bad breath, then you can choose not to reproduce with them,” she says.

Sexual kissing could also help with post-copulation success by promoting arousal, she says, which can speed up ejaculation and change the vaginal pH to make it more hospitable to sperm.

The other main idea is that non-sexual kissing developed from grooming and is useful for strengthening bonding and mitigating social tension. “Chimpanzees will literally kiss and make up after a fight,” says Brindle.

“I think from the evidence that they have, kissing definitely has this affiliative function,” says Zanna Clay at Durham University, UK. “We know, for example, in chimps that it does seem to form this important role in repairing social relationships. But to me, the sexual aspect is a little bit of a question mark.”

As to the issue of whether kissing is an evolved behaviour or a cultural invention, “I think our results show very clearly that kissing has evolved,” says Brindle.

Troels Pank Arbøll at the University of Copenhagen in Denmark, who traced early records of kissing in cuneiform writing from ancient Mesopotamia, agrees. “This provides a more well-developed basis to argue that kissing has been with humans for a long time,” he says.

But that is unlikely to be 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,” says Clay.

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