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Among seabirds to Arctic mammals, chimpanzees to great apes, certain species appear to kiss. Now, researchers propose that ancient hominins did it too – and possibly locked lips with modern humans.

Shared Oral Clues

This isn't the initial instance experts have proposed Neanderthals and early modern humans were closely connected. In previous studies, scientists have found humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were kissing," the researcher noted, adding that the concept aligned with research that has revealed humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Intimate Spin

"This offers a different perspective on ancient interactions," the lead researcher commented.

Writing in the publication Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the evolutionary origins of kissing, 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 describe a kiss, but it's largely human-centric, which implies that essentially other animals do not engage in this. Currently we understand that they probably do, it may appear different from what our intimate contact looks like," said Brindle.

However, she noted some behaviors that looked like kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", seen in fish called French grunts.

As a result the research group came up with a definition of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but absence of nutrition.

Study Approach

The lead researcher said they concentrated on reports of kissing in non-human species from Africa and Asian regions, including bonobos, apes and orangutans, and employed online videos to verify the reports.

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

Historical Origins

The team say the findings suggest intimate contact developed approximately 21.5 million and 16.9 million years ago in the ancestors of the great primates.

The position of Neanderthals on this evolutionary lineage suggests 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 modern people kiss, the fact that we now have demonstrated that ancient relatives very likely engaged, suggests that the two [species] are also likely to have engage," Brindle added.

Evolutionary Significance

While the scientific reasoning is debated, the expert said intimate contact could be employed in reproductive situations to possibly enhance mating outcomes or assist in selecting between mates, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of primates commented that as kissing behavior was seen in a broad spectrum of apes it made sense its origins extend far into our ancient history, and an examination of different forms of kissing among a wider variety of species might push its origins back further still.

"Behaviors that we consider as characteristics of human life, like kissing, are not exclusive to us if we look closely at other animals," he said.

Cultural 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 thrive or fail on the strength of our emotional bonds, and methods of promoting trust and closeness will have been important for eons," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it should be no surprise that Neanderthals – and even them and our own species collectively – kissed."