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Among seabirds to Arctic mammals, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists propose that Neanderthals did it too – and might even have exchanged kisses with modern humans.

Common Oral Clues

This isn't the initial instance scientists have suggested ancient relatives and early modern humans were closely connected. In previous studies, researchers have found humans and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," she said, explaining that the concept chimed with research that has found humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, demonstrating interbreeding was at play.

Intimate Interpretation

"This offers a more romantic perspective on human-Neanderthal relations," Brindle said.

Publishing in the journal a scientific periodical, Brindle and colleagues report how, to explore the historical roots of kissing, they first had to develop a definition that was not limited to how humans kiss.

Defining Kissing

"There have been some previous attempts to define a intimate act, but it's very much been human-centric, which means that basically other animals don't kiss. Currently we understand that they probably do, it might just not look from what human kissing resembles," said Brindle.

Nonetheless, she noted some behaviors that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in fish called certain marine animals.

Consequently the team developed a description of intimate contact centered around social behaviors involving directed oral interaction with a individual of the identical group, with some motion of the mouth but no transfer of nutrition.

Research Methods

The lead researcher explained they concentrated on accounts of intimate behavior in non-human species from Africa and Asian regions, including bonobos, apes and great apes, and employed online videos to verify the observations.

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

Evolutionary Timeline

Researchers propose the findings indicate kissing evolved somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the researchers say. But the activity may not have been limited to their own species.

"Reality that modern people kiss, the fact that we now have demonstrated that Neanderthals probably kissed, suggests that the two [species] are probably did engage," the researcher noted.

Biological Significance

While the evolutionary explanation is discussed, Brindle said kissing could be employed in reproductive situations to potentially enhance reproductive success or help choose between partners, while it could assist reinforce bonding when used in a platonic way.

Another expert in the behavior of primates commented that as intimate contact was observed in a wide range of primates it made sense its roots lie deep in our ancient history, and an examination of different forms of kissing among a broader range of animals might extend its origins back even earlier still.

"Things that we consider as signatures of human life, like intimate contact, are not unique to us if we look closely at other animals," the expert noted.

Social Aspects

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

"However, as humans we thrive or fail on the quality of our emotional bonds, and ways of encouraging confidence and closeness will have been significant for eons," she said. "It might be an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be no surprise that Neanderthals – and including Neanderthals and our human ancestors collectively – kissed."