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From seabirds to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals did it too – and possibly exchanged kisses with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have found humans and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, implying they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the idea aligned with research that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Spin

"It certainly puts a more romantic spin on ancient interactions," the lead researcher commented.

Writing in the publication Evolution and Human Behavior, Brindle and colleagues detail how, to explore the historical roots of kissing, they first had to come up with 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 largely focused on humans, which means that essentially other animals don't kiss. Currently we know that they probably do, it might just not look from what our intimate contact looks like," explained the evolutionary biologist.

However, she said some behaviors that resembled intimate contact were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in aquatic species known as certain marine animals.

As a result the research group developed a definition of intimate contact based on friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but no transfer of food.

Research Approach

Brindle explained they focused on reports of intimate behavior in primates from the African continent and Asia, including primates, apes and orangutans, and used online videos to confirm the observations.

Scientists then integrated this data with information on the evolutionary relationships between living and extinct types of such primates.

Historical Timeline

Researchers propose the results indicate intimate contact evolved somewhere between 21.5m and 16.9m 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 scientists say. But the activity might not have been confined to their own species.

"The fact that modern people engage intimately, the reality that we now have demonstrated that Neanderthals very likely engaged, indicates that the both groups are also likely to have engage," Brindle noted.

Biological Importance

Although the evolutionary explanation is debated, the expert explained intimate contact could be used in reproductive situations to potentially increase reproductive success or help choose between mates, while it might help strengthen connections when used in a platonic way.

Another expert in the behavior of primates said that as intimate contact was seen in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an analysis of different forms of kissing among a wider variety of animals might extend its beginnings back further still.

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

Cultural Elements

Another professor explained that kissing had a social component as it was not universal to all human groups.

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and ways of promoting confidence and intimacy will have been important for eons," the professor stated. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be no surprise that ancient hominins – and even them and our human ancestors collectively – engaged intimately."