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Among Galápagos albatrosses to polar bears, primates to great apes, certain species appear to kiss. Now, researchers propose that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Common Oral Evidence

It is not the first time experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In previous studies, scientists have discovered modern people and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the idea chimed with studies that has found humans of non-African ancestry contain Neanderthal DNA in their genome, revealing genetic mixing was occurring.

Romantic Spin

"It certainly puts a different perspective on human-Neanderthal relations," Brindle commented.

Writing in the publication Evolution and Human Behavior, Brindle and colleagues detail how, to explore the historical roots of kissing, they first had to develop a description that was not limited to how humans smooch.

Defining Intimate Contact

"Previously there were some previous attempts to describe a intimate act, but it's largely focused on humans, which means that basically non-human species don't kiss. Currently we know that they probably do, it might just not look from what human kissing resembles," said Brindle.

Nonetheless, she noted some behaviors that looked like kissing were something rather different – such as the processing and food sharing, or "mouth contact", observed in aquatic species known as certain marine animals.

As a result the research group developed a description of kissing centered around friendly interactions involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the mouth but no transfer of nutrition.

Research Methods

The lead researcher explained they concentrated on reports of intimate behavior in primates from the African continent and Asian regions, including bonobos, apes and orangutans, and employed digital recordings to confirm the reports.

Scientists then integrated this data with details on the genetic connections between living and extinct types of such primates.

Historical Origins

Researchers say the results suggest kissing developed approximately 21.5 million and 16.9 million years ago in the ancestors of the large apes.

The position of Neanderthals on this family tree means it is likely they, too, indulged in a kiss, the scientists conclude. But the activity might not have been limited to their specific group.

"Reality that modern people kiss, the fact that we now have demonstrated that ancient relatives probably engaged, indicates that the two [species] are also likely to have kissed," the researcher noted.

Biological Importance

Although the scientific reasoning is debated, the expert said kissing could be employed in sexual contexts to potentially increase mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was seen in a wide range of primates it was logical its origins extend far into our evolutionary past, and an analysis of various types of kissing among a broader range of animals might push its origins back further still.

"Behaviors that we consider as signatures of our species, like kissing, are not unique 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.

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and methods of encouraging confidence and intimacy will have been important for millions of years," she said. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be expected that ancient hominins – and even them and our human ancestors collectively – kissed."