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From seabirds to polar bears, chimpanzees to great apes, certain species appear to kiss. Now, scientists propose that Neanderthals did it too – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

It is not the first time experts have proposed ancient relatives and early modern humans were intimately acquainted. Among previous studies, researchers have discovered humans and their thick-browed cousins possessed the identical oral bacteria for millions of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, adding that the concept aligned with research that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating interbreeding was at play.

Intimate Spin

"This offers a more romantic spin on ancient interactions," Brindle said.

Publishing in the publication Evolution and Human Behavior, the researcher and her team detail how, to explore the historical roots of kissing, they first had to develop a description that was not limited to how people smooch.

Describing Kissing

"Previously there were some efforts to describe a kiss, but it's largely human-centric, which means that essentially other animals do not engage in this. Currently we understand that they probably do, it might just not look from what human kissing looks like," explained the evolutionary biologist.

However, she said some actions that resembled kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in fish called French grunts.

As a result the research group developed a description of intimate contact based on friendly interactions involving intentional oral interaction with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Research Methods

Brindle explained they concentrated on reports of kissing in non-human species from the African continent and Asian regions, including primates, apes and great apes, and used online videos to verify the observations.

The researchers then integrated this data with details on the evolutionary relationships between living and ancient species of such primates.

Evolutionary Timeline

Researchers propose the findings suggest kissing evolved approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree suggests it is likely they, too, indulged in a intimate act, the researchers say. But the behavior may not have been limited to their own species.

"Reality that modern people engage intimately, the reality that we now have demonstrated that ancient relatives probably kissed, suggests that the two [species] are probably did kissed," the researcher added.

Biological Significance

Although the scientific reasoning is discussed, Brindle explained intimate contact 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 activities of great apes commented that as kissing behavior was seen in a wide range of primates it was logical its origins lie deep in our ancient history, and an examination of different forms of intimate behavior among a broader range of species might extend its origins back further still.

"Behaviors that we consider as signatures of human life, like kissing, are not exclusive to us if we look closely at different species," he said.

Cultural Aspects

Another professor said that kissing had a social component as it was not common to all human groups.

"However, as people we succeed or struggle on the quality of our relationships, and ways of promoting trust and closeness will have been significant for millions of years," she said. "It might be an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but actually it should be no surprise that ancient hominins – and including them and our human ancestors collectively – kissed."