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Among seabirds to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Oral Evidence

It is not the first time scientists have proposed Neanderthals and early modern humans were intimately acquainted. Among previous studies, scientists have found humans and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea chimed with studies that has revealed people of certain genetic backgrounds contain ancient genetic material in their genome, revealing interbreeding was at play.

Intimate Interpretation

"This offers a different spin on ancient interactions," Brindle said.

Writing in the journal Evolution and Human Behavior, the researcher and her team report how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not restricted by how humans smooch.

Describing Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's very much been focused on humans, which means that essentially non-human species don't kiss. Now we know that they probably do, it may appear different from what human kissing looks like," explained the evolutionary biologist.

However, she said some actions that looked like kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species called French grunts.

Consequently the team developed a description of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but absence of food.

Research Methods

The lead researcher said they focused on reports of kissing in primates from Africa and Asia, including primates, chimpanzees and great apes, and employed digital recordings to verify the observations.

Scientists then integrated this data with information on the evolutionary relationships between living and ancient types of such animals.

Evolutionary Timeline

Researchers propose the findings indicate kissing evolved approximately 21.5m and 16.9 million years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, indulged in a intimate act, the researchers conclude. But the behavior might not have been confined to their own species.

"The fact that humans kiss, the reality that we currently have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have engage," Brindle added.

Evolutionary Significance

While the scientific reasoning is debated, Brindle explained kissing could be used 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.

A separate researcher in the behavior of great apes said that as intimate contact was observed in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an examination of various types of intimate behavior among a broader range of species might extend its origins back even earlier still.

"Things that we think of as signatures of human life, like intimate contact, are not exclusive to us if we look closely at different species," the expert noted.

Social Aspects

Another professor said that intimate contact had a cultural element as it was not common to all societies.

"However, as humans we thrive or fail on the strength of our relationships, and ways of encouraging confidence and intimacy will have been important for eons," she said. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be expected that Neanderthals – and including Neanderthals and our own species together – engaged intimately."