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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Common Oral Clues

This isn't the initial instance experts have suggested Neanderthals and early modern humans were closely connected. Among previous studies, researchers have discovered humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the concept aligned with studies that has found 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 ancient interactions," Brindle commented.

Writing in the journal a scientific periodical, the researcher and colleagues report how, to investigate the evolutionary origins of intimate contact, they first had to come up with a description that was not limited to how humans kiss.

Describing Intimate Contact

"Previously there were some efforts to describe a kiss, but it's largely human-centric, which means that essentially other animals don't kiss. Currently we understand that they likely engage, it may appear different from what our intimate contact looks like," said Brindle.

Nonetheless, she noted some actions that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species known as French grunts.

Consequently the team developed a description of kissing centered around social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some motion of the mouth but absence of nutrition.

Research Methods

The lead researcher explained they focused on accounts of kissing in primates from the African continent and Asia, including primates, apes and great apes, and employed digital recordings to confirm the observations.

The researchers then combined this information with information on the genetic connections between living and ancient types of such primates.

Evolutionary Timeline

Researchers propose the results suggest kissing developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, indulged in a intimate act, the researchers say. But the behavior might not have been confined to their specific group.

"The fact that modern people kiss, the reality that we now have shown that ancient relatives very likely engaged, indicates that the both groups are also likely to have engage," Brindle added.

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle explained kissing could be employed in sexual contexts to possibly increase mating outcomes or help choose between mates, while it might help strengthen connections when practiced in a platonic way.

Another expert in the activities of great apes said that as intimate contact was observed in a broad spectrum of primates it was logical its origins extend far into our evolutionary past, and an examination of various types of kissing among a wider variety of animals might push its origins back further still.

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

Social Elements

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

"However, as people we succeed or struggle on the strength of our relationships, and methods of promoting confidence and closeness will have been important 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 really it should be expected that Neanderthals – and including Neanderthals and our human ancestors together – engaged intimately."