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Among seabirds to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.

Shared Microbial Clues

It is not the first time experts have proposed Neanderthals and Homo sapiens were intimately acquainted. In previous studies, scientists have discovered modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were kissing," she said, adding that the concept aligned with research that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing interbreeding was occurring.

Intimate Interpretation

"It certainly puts a different spin on human-Neanderthal relations," the lead researcher said.

Publishing in the publication a scientific periodical, the researcher and colleagues detail how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how people kiss.

Defining Intimate Contact

"Previously there were some efforts to describe a kiss, but it's very much been human-centric, which implies that basically non-human species don't kiss. Now we understand that they probably do, it may appear different from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she said some actions that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "kiss-fighting", seen in fish known as certain marine animals.

As a result the research group developed a description of kissing centered around friendly interactions involving directed oral interaction with a member of the same species, with some movement of the oral area but no transfer of nutrition.

Study Methods

Brindle said they focused on reports of kissing in primates from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and employed online videos to verify the reports.

Scientists then combined this information with details on the evolutionary relationships between living and extinct species of such animals.

Historical Origins

The team propose the findings indicate kissing evolved somewhere between 21.5m and 16.9 million years ago in the ancestors of the great primates.

The position of Neanderthals on this evolutionary lineage means it is probable they, too, indulged in a intimate act, the scientists say. But the activity might not have been limited to their specific group.

"Reality that modern people kiss, the fact that we now have shown that Neanderthals very likely kissed, indicates that the two [species] are probably did kissed," Brindle added.

Evolutionary Significance

While the evolutionary explanation is discussed, Brindle said kissing could be used in sexual contexts to potentially enhance mating outcomes or help choose between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

A separate researcher in the behavior of great apes said that as kissing behavior was seen in a broad spectrum of apes it was logical its roots extend far into our evolutionary past, and an analysis of various types of intimate behavior among a wider variety of species might push its beginnings back further still.

"Behaviors that we think of as characteristics of our species, like intimate contact, are not unique to us if we examine carefully at different species," the expert noted.

Social Elements

Another professor said that intimate contact had a social component as it was not universal to all human groups.

"Nonetheless, as people we succeed or struggle on the quality of our emotional bonds, and ways of promoting confidence and closeness will have been significant for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and even them and our own species collectively – kissed."