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Among seabirds to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Currently, researchers propose that Neanderthals did it too – and might even have exchanged kisses with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In previous studies, researchers have discovered modern people and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the concept aligned with research that has revealed people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating interbreeding was at play.

Intimate Spin

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

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues detail how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how people kiss.

Describing Kissing

"There have been some previous attempts to describe a intimate act, but it's largely human-centric, which means that basically non-human species don't kiss. Now we know that they likely engage, it may appear different from what our intimate contact looks like," explained Brindle.

However, she noted some behaviors that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species known as French grunts.

As a result the team developed a description of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the mouth but no transfer of nutrition.

Study Methods

The lead researcher explained they concentrated on accounts of intimate behavior in primates from Africa and Asia, including primates, chimpanzees and great apes, and used online videos to confirm the observations.

The researchers then integrated this data with details on the evolutionary relationships between extant and extinct species of such animals.

Historical Origins

The team propose the results indicate intimate contact evolved approximately 21.5 million and 16.9 million years ago in the ancestors of the great primates.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a intimate act, the scientists say. But the activity might not have been confined to their own species.

"The fact that modern people kiss, the fact that we now have demonstrated that Neanderthals very likely kissed, suggests that the two [species] are also likely to have kissed," the researcher added.

Biological Significance

Although the evolutionary explanation is debated, the expert explained intimate contact could be employed in reproductive situations to possibly enhance mating outcomes or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of primates said that as kissing behavior was observed in a wide range of primates it made sense its origins lie deep in our ancient history, and an examination of various types of kissing among a wider variety of animals might extend its origins back further still.

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

Social Elements

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting confidence and intimacy will have been significant for millions of years," she said. "It might be 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 human ancestors collectively – kissed."