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Among Galápagos albatrosses to polar bears, primates to orangutans, certain species appear to kiss. Now, researchers propose that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Oral Clues

This isn't the initial instance experts have proposed Neanderthals and early modern humans were intimately acquainted. In previous studies, researchers have found humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, adding that the idea aligned with research that has revealed humans of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Interpretation

"It certainly puts a different perspective on ancient interactions," Brindle commented.

Writing 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 description that was not limited to how people kiss.

Defining Kissing

"Previously there were some previous attempts to define a kiss, but it's largely focused on humans, which means that basically other animals don't kiss. Currently we know that they likely engage, it may appear different from what human kissing resembles," explained the evolutionary biologist.

However, she noted some behaviors that resembled kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species known as certain marine animals.

As a result the team came up with a description of intimate contact based on friendly interactions involving directed oral interaction with a member of the same species, with some movement of the mouth but no transfer of nutrition.

Study Approach

The lead researcher explained they concentrated on accounts of kissing in non-human species from Africa and Asia, including primates, chimpanzees and orangutans, and used online videos to confirm the observations.

The researchers then combined this information with details on the evolutionary relationships between extant and ancient species of such primates.

Historical Timeline

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

Placement of ancient hominins on this evolutionary lineage means it is likely they, too, engaged in a intimate act, the researchers say. But the behavior may not have been confined to their own species.

"Reality that modern people engage intimately, the reality that we currently have shown that Neanderthals probably engaged, suggests that the both groups are probably did engage," Brindle noted.

Biological Importance

While the evolutionary explanation is debated, the expert explained kissing could be used in sexual contexts to potentially increase reproductive success or assist in selecting between mates, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of great apes said that as kissing behavior was seen in a wide range of apes it was logical its roots lie deep in our evolutionary past, and an examination of different forms of intimate behavior among a broader range of animals might push its beginnings back further still.

"Things that we think of as signatures of human life, like kissing, are not exclusive to us if we look closely at other animals," he said.

Cultural Elements

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

"However, as humans we thrive or fail on the strength of our relationships, and ways of promoting trust and intimacy will have been significant for millions of years," she said. "It might be an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be no surprise that Neanderthals – and even Neanderthals and our own species collectively – kissed."