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Among seabirds to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Now, researchers propose that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have discovered humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Likely they were kissing," she said, explaining that the concept aligned with research that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Interpretation

"This offers a more romantic perspective on ancient interactions," the lead researcher commented.

Writing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not restricted by how humans smooch.

Defining Kissing

"There have been some previous attempts to describe a kiss, but it's largely focused on humans, which implies that essentially non-human species do not engage in this. Currently we know that they probably do, it might just not look from what human kissing looks like," explained Brindle.

Nonetheless, she noted some behaviors that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in fish called certain marine animals.

As a result the research group came up with a description of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but no transfer of nutrition.

Study Approach

Brindle explained they focused on reports of intimate behavior in primates from the African continent and Asia, including primates, apes and orangutans, and employed online videos to confirm the observations.

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

Historical Timeline

The team say 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 ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a intimate act, the scientists say. But the behavior may not have been confined to their own species.

"Reality that humans engage intimately, the reality that we now have shown that ancient relatives probably kissed, suggests that the both groups are also likely to have kissed," Brindle added.

Evolutionary Significance

Although the evolutionary explanation is debated, Brindle said kissing could be used in sexual contexts to possibly increase mating outcomes or help choose between partners, while it might help strengthen connections when practiced in a platonic way.

A separate researcher in the activities of great apes said that as intimate contact was seen in a broad spectrum of primates it was logical its roots extend far into our ancient history, and an examination of various types of kissing among a broader range of animals might push its beginnings back further still.

"Behaviors that we consider as signatures of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Elements

An archaeology expert said that kissing had a cultural element as it was not universal to all societies.

"However, as humans we succeed or struggle on the quality of our emotional bonds, and ways of encouraging confidence and closeness will have been important for millions of years," she said. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be no surprise that Neanderthals – and including them and our own species collectively – kissed."