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Among seabirds to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that Neanderthals did it too – and might even have exchanged kisses with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have suggested Neanderthals and Homo sapiens were closely connected. Among previous studies, scientists have found humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were kissing," she said, explaining that the idea aligned with studies that has found people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Romantic Spin

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle said.

Publishing in the publication Evolution and Human Behavior, the researcher and her team report how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not restricted by how people smooch.

Defining Intimate Contact

"There have been some previous attempts to describe a intimate act, but it's very much been human-centric, which means that essentially other animals do not engage in this. Currently we know that they likely engage, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she noted some behaviors that looked like intimate contact were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species called French grunts.

As a result the team came up with a description of kissing centered around friendly interactions involving directed oral interaction with a member of the same species, with some motion of the mouth but no transfer of food.

Study Methods

Brindle explained they concentrated on reports of kissing in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and used online videos to confirm the observations.

Scientists then combined this information with details on the evolutionary relationships between extant and ancient types of such primates.

Evolutionary Origins

Researchers propose the results indicate kissing developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this family tree means it is probable they, too, indulged in a intimate act, the scientists say. But the behavior might not have been confined to their specific group.

"Reality that modern people kiss, the fact that we now have shown that Neanderthals very likely kissed, suggests that the both groups are probably did engage," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is debated, the expert explained kissing could be employed in sexual contexts to potentially enhance mating outcomes or assist in selecting between partners, while it might help reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of primates commented that as intimate contact was observed in a broad spectrum of apes it was logical its origins extend far into our ancient history, and an analysis of different forms of kissing among a wider variety of animals might extend its origins back even earlier still.

"Behaviors that we think of as signatures of our species, like kissing, are not exclusive to us if we look closely at different species," he said.

Cultural Elements

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

"Nonetheless, as people we succeed or struggle on the quality of our relationships, and methods of encouraging trust and intimacy will have been important for millions of years," she said. "This could represent an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be expected that ancient hominins – and even Neanderthals and our own species collectively – engaged intimately."