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Among seabirds to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals did it too – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

This isn't the initial instance experts have proposed ancient relatives and Homo sapiens were closely connected. In previous studies, researchers have discovered modern people and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, adding that the idea chimed with research that has revealed humans of certain genetic backgrounds contain Neanderthal DNA in their genome, revealing genetic mixing was occurring.

Romantic Spin

"It certainly puts a different perspective on human-Neanderthal relations," the lead researcher said.

Writing in the journal a scientific periodical, Brindle and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not limited to how humans smooch.

Defining Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's very much been human-centric, which implies that essentially other animals don't kiss. Now we know that they likely engage, it might just not look from what our intimate contact looks like," said Brindle.

However, she said some behaviors that resembled intimate contact were something rather different – such as the chewing and transfer of food, or "kiss-fighting", observed in fish known as certain marine animals.

Consequently the team developed a description of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but no transfer of nutrition.

Study Approach

The lead researcher explained they concentrated on reports of kissing in primates from Africa and Asia, including primates, apes 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 ancient species of such animals.

Evolutionary Timeline

Researchers say the findings suggest kissing developed approximately 21.5 million and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a kiss, the researchers conclude. But the activity might not have been limited to their specific group.

"Reality that modern people engage intimately, the reality that we now have demonstrated that ancient relatives probably kissed, indicates that the two [species] are probably did engage," Brindle added.

Evolutionary Significance

While the evolutionary explanation is debated, the expert said kissing could be used in reproductive situations to possibly enhance mating outcomes or help choose between partners, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of primates commented that as kissing behavior was seen in a wide range of primates it was logical its roots extend far into our evolutionary past, and an examination of various types of intimate behavior among a broader range of species might extend its beginnings back even earlier still.

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

Social Aspects

An archaeology expert said that intimate contact had a social component as it was not universal to all human groups.

"Nonetheless, as people we succeed or struggle on the strength of our relationships, and methods of encouraging confidence and intimacy will have been significant for eons," she said. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it should be no surprise that Neanderthals – and even Neanderthals and our own species together – engaged intimately."