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Among seabirds to Arctic mammals, primates to great apes, certain species appear to kiss. Now, researchers propose that ancient hominins did it too – and possibly locked lips with modern humans.

Common Oral Evidence

It is not the first time scientists have suggested Neanderthals and early modern humans were intimately acquainted. Among previous studies, scientists have found humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with studies that has found humans of certain genetic backgrounds contain ancient genetic material in their genome, revealing genetic mixing was occurring.

Romantic Spin

"It certainly puts a more romantic spin on human-Neanderthal relations," Brindle said.

Publishing in the publication Evolution and Human Behavior, Brindle and her team detail how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how humans smooch.

Defining Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's very much been focused on humans, which implies that essentially other animals do not engage in this. Now we understand 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 looked like kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species known as certain marine animals.

Consequently the research group developed a definition of kissing centered around friendly interactions involving intentional oral interaction with a individual of the identical group, with some motion of the mouth but absence of nutrition.

Research Approach

Brindle explained they concentrated on accounts of intimate behavior in non-human species from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and employed online videos to confirm the reports.

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

Historical Timeline

Researchers say the findings indicate kissing evolved approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage means it is likely they, too, engaged in a kiss, the researchers conclude. But the activity might not have been limited to their specific group.

"The fact that modern people kiss, the reality that we now have demonstrated that Neanderthals probably kissed, suggests that the two [species] are probably did engage," the researcher noted.

Biological Importance

Although the evolutionary explanation is discussed, Brindle said kissing could be used in reproductive situations to potentially enhance mating outcomes or assist in selecting between mates, while it might help strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates said that as kissing behavior was seen in a broad spectrum of apes it was logical its roots extend far into our ancient history, and an examination of various types of kissing among a wider variety of species might extend its origins back even earlier still.

"Behaviors that we think of as characteristics of our species, like intimate contact, are not exclusive to us if we examine carefully at other animals," he said.

Cultural Aspects

Another professor said that kissing had a social component as it was not universal to all societies.

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting trust and intimacy will have been important for millions of years," she said. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be no surprise that Neanderthals – and even Neanderthals and our human ancestors collectively – engaged intimately."