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Among seabirds to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Evidence

It is not the first time experts have proposed ancient relatives and early modern humans were intimately acquainted. Among previous studies, researchers have discovered modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, explaining that the idea aligned with research that has found humans of non-African ancestry contain ancient genetic material in their genome, revealing interbreeding was occurring.

Romantic Spin

"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 intimate contact, they first had to develop a definition that was not limited to how humans smooch.

Describing Intimate Contact

"There have been some efforts to define a intimate act, but it's largely human-centric, which implies that essentially other animals don't kiss. Now we know that they probably do, it might just not look from what our intimate contact resembles," said Brindle.

However, she noted some behaviors that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species known as French grunts.

Consequently the research group came up with a definition of kissing centered around social behaviors involving directed oral interaction with a member of the same species, with some motion of the mouth but absence of nutrition.

Study Approach

The lead researcher explained they focused on accounts of intimate behavior in non-human species from the African continent and Asia, including bonobos, apes and great apes, and employed digital recordings to confirm the reports.

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

Historical Timeline

The team propose the results suggest kissing developed approximately 21.5m and 16.9m years ago in the predecessors 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 conclude. But the activity might not have been confined to their specific group.

"The fact that modern people kiss, the fact that we currently have shown that ancient relatives probably engaged, suggests that the two [species] are also likely to have kissed," Brindle added.

Evolutionary Importance

Although the evolutionary explanation is debated, the expert said intimate contact could be used in reproductive situations to potentially enhance reproductive success or assist in selecting between mates, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates said that as kissing behavior was observed in a broad spectrum of primates it was logical its origins lie deep in our evolutionary past, and an analysis of various types of kissing among a broader range of animals might push its origins back even earlier still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not exclusive to us if we look closely at different species," he said.

Cultural Elements

Another professor explained that kissing had a cultural element as it was not common to all societies.

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and ways of promoting confidence and closeness will have been important for eons," she said. "This could represent an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be no surprise that ancient hominins – and including them and our own species collectively – engaged intimately."