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Among seabirds to Arctic mammals, chimpanzees to great apes, certain species appear to kiss. Now, researchers suggest that Neanderthals did it too – and possibly exchanged kisses with modern humans.

Common Microbial Clues

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. In earlier research, scientists have discovered humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with research that has found people of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Romantic Interpretation

"This offers a different perspective on ancient interactions," Brindle commented.

Publishing in the publication Evolution and Human Behavior, the researcher and her team report how, to explore the historical roots of intimate contact, they first had to develop a description that was not limited to how humans kiss.

Defining Kissing

"Previously there were some previous attempts to describe a intimate act, but it's largely human-centric, which means that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what human kissing looks like," explained Brindle.

However, she noted some behaviors that looked like kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in fish called certain marine animals.

As a result the team developed a description of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the oral area but absence of nutrition.

Research Approach

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

Scientists then combined this information with information on the evolutionary relationships between living and extinct types of such primates.

Historical Origins

Researchers propose the results indicate kissing evolved approximately 21.5 million and 16.9 million years ago in the predecessors of the great primates.

Placement of ancient hominins on this evolutionary lineage suggests it is likely they, too, engaged in a kiss, the scientists say. But the activity may not have been confined to their own species.

"Reality that humans engage intimately, the fact that we currently have shown that Neanderthals probably kissed, suggests that the both groups are probably did engage," Brindle noted.

Biological Significance

Although the evolutionary explanation is discussed, Brindle said kissing could be employed in sexual contexts to potentially enhance reproductive success or help choose between partners, while it might help reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of primates said that as intimate contact was observed in a broad spectrum of apes it made sense its origins extend far into our evolutionary past, and an analysis of different forms of kissing among a broader range of animals might extend its origins back further still.

"Things that we think of as signatures of our species, like intimate contact, are not exclusive to us if we examine carefully at different species," he said.

Cultural Aspects

Another professor explained that intimate contact had a cultural element as it was not common to all human groups.

"However, as humans we succeed or struggle on the quality of our relationships, and methods of encouraging trust and closeness will have been significant for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be no surprise that Neanderthals – and including Neanderthals and our human ancestors collectively – engaged intimately."