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Among seabirds to polar bears, chimpanzees to orangutans, certain species appear to kiss. Currently, scientists suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Oral Evidence

This isn't the initial instance scientists have suggested Neanderthals and early modern humans were intimately acquainted. Among previous studies, researchers have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the idea aligned with studies that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing interbreeding was occurring.

Intimate Interpretation

"It certainly puts a different perspective on human-Neanderthal relations," Brindle commented.

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the historical roots of kissing, they first had to come up with a definition that was not restricted by how people kiss.

Describing Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's largely focused on humans, which means that essentially other animals do not engage in this. Now we know that they probably do, it might just not look from what human kissing looks like," explained the evolutionary biologist.

However, she noted some actions that looked like intimate contact were something rather different – such as the processing and transfer of food, or "kiss-fighting", observed in fish known as French grunts.

As a result the team developed a description of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some motion of the oral area but no transfer of food.

Study Methods

The lead researcher explained they focused on reports of intimate behavior in primates from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and employed digital recordings to verify the observations.

Scientists then combined this data with details on the genetic connections between living and ancient species of such animals.

Evolutionary Origins

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

Placement of ancient hominins on this family tree means it is likely they, too, indulged in a kiss, the researchers say. But the behavior may not have been confined to their own species.

"The fact that humans kiss, the fact that we currently have shown that Neanderthals probably kissed, indicates that the two [species] are also likely to have engage," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in reproductive situations to potentially increase mating outcomes or assist in selecting between partners, while it might help reinforce bonding when used in a platonic way.

Another expert in the activities of primates said that as intimate contact was seen in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an analysis of various types of intimate behavior among a broader range of species might extend its beginnings back even earlier still.

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

Social Elements

An archaeology expert said that kissing had a social component as it was not common to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and methods of promoting trust and closeness will have been important for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it should be expected that ancient hominins – and even them and our human ancestors collectively – kissed."