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From seabirds to Arctic mammals, primates to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins did it too – and might even have locked lips with modern humans.

Common Oral Evidence

It is not the first time scientists have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, scientists have found modern people and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Probably they were kissing," she said, adding that the concept aligned with research that has revealed humans of non-African ancestry contain ancient genetic material in their genome, demonstrating genetic mixing was at play.

Intimate Spin

"It certainly puts a different perspective on human-Neanderthal relations," the lead researcher said.

Publishing in the journal a scientific periodical, the researcher and colleagues report how, to investigate the evolutionary origins of kissing, they first had to come up with a description that was not limited to how humans kiss.

Defining Intimate Contact

"Previously there were some efforts to describe a kiss, but it's largely focused on humans, which implies that basically other animals don't kiss. Now we know that they probably do, it may appear different from what human kissing looks like," said the evolutionary biologist.

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

As a result the team developed a description of intimate contact centered around friendly interactions involving directed oral interaction with a individual of the identical group, with some movement of the oral area but no transfer of nutrition.

Study Methods

Brindle explained they focused on reports of intimate behavior in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and used online videos to verify the observations.

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

Historical Timeline

The team propose the findings indicate intimate contact developed somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals on this family tree suggests it is likely they, too, engaged in a kiss, the scientists conclude. But the behavior may not have been limited to their specific group.

"Reality that modern people engage intimately, the fact that we currently have demonstrated that ancient relatives probably engaged, suggests that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle said kissing could be used in sexual contexts to possibly enhance mating outcomes or assist in selecting between partners, while it could assist strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was seen in a broad spectrum of apes it was logical its roots lie deep in our ancient history, and an examination of different forms of kissing among a wider variety of species might extend its origins back further still.

"Things that we consider as signatures of our species, like kissing, are not unique to us if we examine carefully at other animals," he said.

Cultural Aspects

An archaeology expert explained that kissing had a social component as it was not common to all human groups.

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and methods of promoting confidence and intimacy will have been significant for eons," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be no surprise that Neanderthals – and including Neanderthals and our own species together – engaged intimately."