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Among seabirds to Arctic mammals, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals did it too – and might even have exchanged kisses with modern humans.

Common Microbial Clues

This isn't the initial instance experts have suggested ancient relatives and early modern humans were closely connected. In earlier research, researchers have found modern people and their thick-browed cousins possessed the identical oral bacteria for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, adding that the concept aligned with studies that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing genetic mixing was at play.

Romantic Interpretation

"This offers a more romantic spin on ancient interactions," Brindle said.

Publishing in the journal Evolution and Human Behavior, Brindle and colleagues detail how, to explore the historical roots of intimate contact, they first had to develop a description that was not restricted by how humans smooch.

Defining Kissing

"Previously there were some efforts to describe a intimate act, but it's largely human-centric, which means that essentially other animals don't kiss. Currently we know that they likely engage, it may appear different from what our intimate contact looks like," explained Brindle.

Nonetheless, she said some actions that looked like kissing were something rather different – such as the chewing and transfer of food, or "mouth contact", observed in fish called certain marine animals.

As a result the research group developed a description of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but no transfer of nutrition.

Research Approach

Brindle explained they concentrated on accounts of kissing in primates from the African continent and Asia, including primates, chimpanzees and orangutans, and employed digital recordings to confirm the observations.

The researchers then integrated this information with information on the evolutionary relationships between extant and ancient types of such primates.

Historical Timeline

Researchers say the findings indicate intimate contact evolved somewhere between 21.5m and 16.9 million years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree suggests it is likely they, too, engaged in a kiss, the researchers conclude. But the behavior might not have been limited to their specific group.

"The fact that humans engage intimately, the reality that we currently have demonstrated that Neanderthals very likely kissed, suggests that the two [species] are also likely to have engage," the researcher noted.

Evolutionary Importance

Although the evolutionary explanation is discussed, Brindle explained kissing could be employed in reproductive situations to potentially enhance mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when used in a platonic way.

A separate researcher in the activities of great apes commented that as intimate contact was seen in a broad spectrum of apes it was logical its origins lie deep in our ancient history, and an examination of various types of kissing among a broader range of species might extend its beginnings back further still.

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

Social Elements

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

"Nonetheless, as people we thrive or fail on the strength of our relationships, and ways of encouraging confidence and closeness will have been important for eons," she said. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that ancient hominins – and including Neanderthals and our human ancestors collectively – kissed."