A groundbreaking study delves into the evolution of kissing, suggesting that our ape ancestors and early human relatives, such as Neanderthals, likely engaged in lip-locking with both friends and sexual partners. This intimate behavior may trace its origins back an impressive 21 million years.

While the earliest documented human kisses are found in records from 4,500 years ago in Mesopotamia and Ancient Egypt, Matilda Brindle, the research’s lead author and an evolutionary biologist at Oxford University’s Department of Biology, describes kissing as an “evolutionary conundrum.”

She points out that kissing poses significant risks, including the potential spread of diseases, without offering clear benefits in terms of reproduction or survival.

“We found kissing to be a fascinating subject to explore,” Brindle, who focuses on studying sexual behaviors in primates, shared with CNN.

“Its widespread presence among animals suggests that kissing might indeed be an evolved trait,” she added.

Kissing, which the team defined as non-aggressive, mouth-to-mouth contact that doesn’t involve food, isn’t something that can be detected in the fossil record, so Brindle and her colleagues used a different approach.

From existing scientific literature, the researchers collected information on which modern primate species have been observed kissing; these included chimpanzees, bonobos, orangutans and one species of gorilla.

The team then ran a phylogenetic analysis, which allows scientists to infer information about traits in extinct species based on behavioural data from living animals. It involves reconstructing a tree or map of how different primate species are related based on genetic information, Brindle explained.

“With that information, we can kind of travel back through time,” she said.

The team deployed statistical modelling to simulate different evolutionary scenarios along the branches of the tree to estimate the probability that different ape ancestors kissed.

For example, she said chimpanzees, bonobos and humans all kiss, so it’s likely that the last shared ancestor of all those species did, too. To give robust estimates, the model was run 10 million times.

The results, published Wednesday in the journal Evolution and Human Behaviour, suggested that kissing is an ancient trait in the large apes, evolving in an ancestor of that group between 21.5 million and 16.9 million years ago.

This means that extinct human relatives, such as Neanderthals, were likely to have engaged in kissing, too. It’s also possible — since scientists know that our species, Homo sapiens, interbred with Neanderthals — that humans and Neanderthals kissed one another, the study noted.

However, the model does not reveal why or how kissing evolved, Brindle said, noting that there are multiple uses, including assessing potential mates, foreplay, bonding, mitigating social tension, and chewing food before giving it to offspring.

She added there is limited data on kissing in animals outside of ape species, making it difficult to reconstruct how the trait may have developed over time. What’s more, much of the information came from animals living in captivity or sanctuaries. Additional data on kissing in different species is needed, she said.

“What we’ve done, which is a really important first step, is showing it’s an evolved trait,” Brindle said.

“It’s really ancient. But why? And that’s the amazing next step if people want to pick up the mantle.”

Kissing is not a universal behaviour in human society, the researchers noted in the new study. It is only documented in 46 per cent of cultures, according to a 2015 paper.

“We did find a strong evolutionary signal in kissing but it doesn’t mean it has to be retained,” Brindle explained.

For some populations, she added, kissing might not be a good fit.

“Primates are extremely flexible species, very intelligent, and so kissing might be useful in some contexts but not in others,” she said.

“And if it’s not useful, it is quite risky with high potential for disease transfer.”

Kissing is more than just “mouth-to-mouth” touching, and the study doesn’t really shed much light on why humans kiss the way they do, said Adriano Reis e Lameira, an evolutionary psychologist and primatologist at the University of Warwick, who was not involved in the work.

“The large majority of kisses humans give are not mouth-to-mouth,” he said via email.