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Among seabirds to Arctic mammals, primates to orangutans, various animals appear to kiss. Now, researchers suggest that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Evidence

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

"Probably they were kissing," she said, explaining that the concept aligned with studies that has revealed humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Romantic Interpretation

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

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues detail how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not restricted by how humans smooch.

Defining Intimate Contact

"There have been some previous attempts to define a kiss, but it's largely human-centric, which implies that basically other animals do not engage in this. Currently we know that they likely engage, it might just not look from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some actions that looked like kissing were something rather different – such as the processing and food sharing, or "mouth contact", observed in aquatic species called French grunts.

As a result the research group developed a description of intimate contact centered around friendly interactions involving intentional oral interaction with a member of the identical group, with some motion of the mouth but absence of food.

Research Methods

Brindle said they focused on reports of kissing in primates from Africa and Asian regions, including primates, chimpanzees and orangutans, and used online videos to verify the observations.

The researchers then integrated this information with details on the evolutionary relationships between living and extinct species of such primates.

Evolutionary Timeline

Researchers say the results indicate kissing developed approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

The position of ancient hominins on this evolutionary lineage suggests it is likely they, too, engaged in a intimate act, the researchers say. But the activity may not have been confined to their specific group.

"Reality that humans engage intimately, the reality that we currently have shown that Neanderthals probably kissed, indicates that the both groups are probably did kissed," the researcher added.

Biological Importance

While the scientific reasoning is discussed, Brindle said kissing could be employed in sexual contexts to potentially enhance mating outcomes or help choose between mates, while it might help reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of great apes said that as kissing behavior was seen in a wide range of apes it was logical its roots extend far into our ancient history, and an analysis of various types of intimate behavior among a wider variety of species might extend its beginnings back further still.

"Behaviors that we think of as characteristics of human life, like kissing, are not unique to us if we look closely at different species," the expert noted.

Cultural Elements

Another professor said that kissing had a cultural element as it was not universal to all societies.

"Nonetheless, as humans we thrive or fail on the quality of our emotional bonds, and methods of encouraging trust and intimacy will have been significant for eons," she said. "This could represent an concept that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it should be no surprise that ancient hominins – and even Neanderthals and our human ancestors collectively – engaged intimately."