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From seabirds to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, scientists suggest that Neanderthals did it too – and might even have locked lips with modern humans.

Shared Oral Evidence

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. In earlier research, scientists have found humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with studies that has found people of non-African ancestry have bits of ancient genetic material in their genome, demonstrating genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a different spin on ancient interactions," the lead researcher said.

Publishing in the journal 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 humans smooch.

Defining Kissing

"There have been some efforts to describe a intimate act, but it's very much been focused on humans, which implies that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what our intimate contact looks like," said the evolutionary biologist.

However, she noted some actions that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "kiss-fighting", seen in fish called French grunts.

As a result the team developed a definition of kissing based on 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 Methods

Brindle explained they concentrated on accounts of intimate behavior in primates from the African continent and Asia, including primates, apes and great apes, and employed digital recordings to verify the reports.

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

Evolutionary Timeline

Researchers propose the findings suggest intimate contact evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

Placement of Neanderthals on this family tree suggests it is probable they, too, engaged in a kiss, the researchers conclude. But the behavior may not have been confined to their own species.

"Reality that modern people engage intimately, the reality that we currently have demonstrated that ancient relatives probably engaged, indicates that the both groups are probably did kissed," the researcher added.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle said kissing could be used in sexual contexts to possibly enhance reproductive success or assist in selecting between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

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 analysis of various types of kissing among a wider variety of species might push its beginnings back further still.

"Behaviors that we think of as characteristics of our species, like kissing, are not unique to us if we examine carefully at different species," he said.

Cultural Aspects

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

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and ways of encouraging confidence and closeness will have been important for millions of years," she said. "It might be an concept that appears a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but really it ought to be expected that Neanderthals – and including Neanderthals and our human ancestors collectively – engaged intimately."