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Among Galápagos albatrosses to Arctic mammals, primates to great apes, various animals appear to kiss. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Oral Clues

It is not the first time scientists have proposed Neanderthals and Homo sapiens were closely connected. In previous studies, scientists have found modern people and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea chimed with research that has found humans of certain genetic backgrounds contain ancient genetic material in their genome, revealing interbreeding was occurring.

Intimate Interpretation

"It certainly puts a different spin on ancient interactions," Brindle said.

Writing in the publication Evolution and Human Behavior, Brindle and her team detail how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Defining Intimate Contact

"There have been some previous attempts to define a kiss, but it's largely human-centric, which means that essentially other animals don't kiss. Now we know that they likely engage, it might just not look from what our intimate contact resembles," said Brindle.

However, she noted some actions that looked like kissing were something rather different – such as the processing and food sharing, or "kiss-fighting", observed in fish called certain marine animals.

As a result the research group came up with a description of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but no transfer of nutrition.

Research Approach

Brindle explained they concentrated on accounts of intimate behavior in primates from Africa and Asia, including primates, apes and great apes, and used digital recordings to verify the observations.

The researchers then combined this data with information on the genetic connections between living and extinct types of such animals.

Evolutionary Origins

The team say the findings suggest kissing developed approximately 21.5m and 16.9m years ago in the predecessors of the great primates.

Placement of Neanderthals on this family tree means it is probable they, too, indulged in a kiss, the researchers say. But the behavior might not have been confined to their specific group.

"Reality that modern people kiss, the fact that we currently have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have kissed," the researcher added.

Biological Significance

While the evolutionary explanation is debated, the expert explained kissing could be used in reproductive situations to possibly increase reproductive success or help choose between partners, while it might help strengthen connections when practiced in a platonic way.

A separate researcher in the behavior of primates said that as kissing behavior was observed in a wide range of primates it made sense its roots extend far into our evolutionary past, and an analysis of different forms of kissing among a wider variety of animals might extend its beginnings back further still.

"Things that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we look closely at different species," he said.

Social Elements

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

"However, as humans we thrive or fail on the quality of our emotional bonds, and ways of encouraging trust and closeness will have been important for eons," the professor stated. "This could represent an image that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it should be no surprise that ancient hominins – and including Neanderthals and our human ancestors collectively – kissed."