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 21 million years ago.

There is wide debate about when humans began kissing romantically. Ancient texts suggest that sexual kissing was practiced in ancient Mesopotamia and Egypt at least 4,500 years ago, but because such kissing has only been documented in about 46% of human cultures, some argue that it is a cultural phenomenon that emerged relatively recently in human history.

However, there is some evidence to suggest that Neanderthals exchanged oral bacteria with Homo sapiensand chimpanzees, bonobos and orangutans have all been observed kissing. It is therefore possible that this behavior goes back much further than historical texts reveal.

To search for answers, Matilda Brindle of the University of Oxford and her colleagues attempted to trace the evolutionary history of kissing. “Kissing seems a bit of an evolutionary paradox,” she says. “It probably doesn’t help with survival and might even be risky in terms of transmitting pathogens.”

The researchers first proposed a definition of kissing that would work for many species, relying on non-antagonistic mouth-to-mouth contact and involving lip movement, but not food transfer.

This leads to the exclusion of many kisses, including kisses elsewhere on 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 explains. “Humans are taking kissing to a new level.”

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

To estimate the likelihood that various ancestral species also kissed, Brindle and his colleagues mapped this information into a 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 ancestral apes around 21.5 million to 16.9 million years ago and that there is an 84 percent chance that our extinct human relatives, the Neanderthals, also engaged in kissing.

“Obviously it’s just Neanderthals kissing; we don’t know who they’re kissing,” says Brindle. “But with the evidence that humans and Neanderthals had a similar oral microbiome and that most humans of non-African origin have Neanderthal DNA, we would say that they probably kissed, which definitely puts a much more romantic spin on the relationships between humans and Neanderthals.”

There isn’t yet enough data to explain why kissing evolved, Brindle says, but she suggests two hypotheses.

“In terms of sexual kissing, it could improve reproductive success by allowing animals to assess mate quality. If someone has bad breath, you may choose not to breed with them,” she says.

Sexual kissing might also contribute to post-copulation success by promoting arousal, she says, which can speed up ejaculation and change 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 bonds and easing social tensions. “Chimpanzees literally kiss and make up after a fight,” says Brindle.

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

As for whether kissing is an evolved behavior or a cultural invention, “I think our results show very clearly that kissing evolved,” Brindle says.

Troels Pank Arbøll, of the University of Copenhagen in Denmark, who has traced the earliest records of kissing in cuneiform script from ancient Mesopotamia, agrees. “This provides a stronger basis for arguing that kissing has existed for a long time in humans,” he says.

But that’s unlikely to be the whole story, given that many groups of people don’t kiss. “I’m sure there’s a strong cultural element and it probably came and went with different cultural preferences,” Clay says.

Ancient caves, human origins: northern Spain

Discover some of the oldest known cave paintings in the world in this idyllic part of northern Spain. Travel back 40,000 years to discover how our ancestors lived, played and worked. From ancient Paleolithic art to impressive geological formations, each cave tells a unique story that transcends time.