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Among seabirds to polar bears, primates to great apes, certain species appear to kiss. Now, scientists suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

It is not the first time experts have proposed Neanderthals and early modern humans were closely connected. Among earlier research, scientists have discovered humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the concept aligned with research that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Intimate Spin

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

Publishing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to come up with a description that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some efforts to define a intimate act, but it's largely focused on humans, which means that essentially non-human species do not engage in this. Now we understand that they probably do, it might just not look from what human kissing resembles," explained Brindle.

However, she noted some actions that looked like intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in fish called French grunts.

As a result the team developed a description of intimate contact centered around social behaviors involving directed oral interaction with a member of the same species, with some movement of the mouth but absence of food.

Study Methods

The lead researcher explained they focused on reports of kissing in non-human species from Africa and Asian regions, including primates, chimpanzees and orangutans, and employed digital recordings to verify the reports.

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

Historical Origins

The team say the results suggest intimate contact developed somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree means it is probable they, too, indulged in a kiss, the researchers say. But the activity may not have been limited to their own species.

"The fact that humans engage intimately, the fact that we now have shown that ancient relatives probably engaged, suggests that the two [species] are also likely to have engage," the researcher noted.

Biological Importance

While the evolutionary explanation is discussed, Brindle said intimate contact could be used in sexual contexts to possibly increase reproductive success or help choose between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of great apes commented that as intimate contact was observed in a broad spectrum of primates it made sense its origins lie deep in our evolutionary past, and an examination of various types of kissing among a wider variety of animals might extend its beginnings back even earlier still.

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

Cultural Aspects

An archaeology expert said that kissing had a cultural element as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and methods of encouraging confidence and closeness will have been important for millions of years," she said. "It might be an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it should be no surprise that Neanderthals – and including Neanderthals and our human ancestors collectively – kissed."