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From seabirds to polar bears, primates to orangutans, certain species appear to kiss. Now, researchers suggest that Neanderthals did it too – and might even have exchanged kisses with modern humans.

Common Microbial Evidence

It is not the first time experts have suggested Neanderthals and Homo sapiens were closely connected. In earlier research, scientists have found modern people and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," the researcher noted, explaining that the idea aligned with research that has revealed humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing interbreeding was occurring.

Intimate Spin

"This offers a different spin on human-Neanderthal relations," Brindle commented.

Writing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not restricted by how humans smooch.

Defining Intimate Contact

"Previously there were some previous attempts to describe a intimate act, but it's largely focused on humans, which implies that essentially non-human species don't kiss. Currently we know that they probably do, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she noted some actions that resembled intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", observed in fish called French grunts.

As a result the team developed a description of intimate contact based on friendly interactions involving intentional oral interaction with a individual of the identical group, with some movement of the oral area but absence of nutrition.

Research Methods

Brindle said they concentrated on reports of intimate behavior in non-human species from the African continent and Asia, including bonobos, apes and orangutans, and employed online videos to verify the reports.

Scientists then integrated this information with information on the genetic connections between living and ancient types of such primates.

Evolutionary Timeline

The team say the results suggest intimate contact developed approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree means it is probable they, too, indulged in a intimate act, the scientists conclude. But the behavior may not have been confined to their own species.

"The fact that modern people kiss, the reality that we currently have shown that Neanderthals very likely engaged, suggests that the both groups are probably did engage," the researcher noted.

Biological Significance

While the scientific reasoning is debated, the expert said kissing could be employed in reproductive situations to potentially enhance reproductive success or assist in selecting between mates, while it might help reinforce bonding when used in a non-sexual manner.

A separate researcher in the activities of great apes commented that as intimate contact was seen in a wide range of apes it made sense its origins lie deep in our ancient history, and an analysis of different forms of kissing among a wider variety of species might extend its beginnings back further still.

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

Cultural Elements

Another professor explained that intimate contact had a social component as it was not universal to all societies.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and methods of encouraging trust and closeness will have been significant for millions of years," she said. "It might be an concept that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be no surprise that Neanderthals – and including them and our human ancestors collectively – kissed."