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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals did it too – and possibly locked lips with modern humans.

Common Microbial Clues

This isn't the initial instance experts have suggested ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, scientists have found humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were kissing," she said, adding that the concept chimed with research that has found humans of certain genetic backgrounds contain ancient genetic material in their genetic makeup, revealing genetic mixing was at play.

Romantic Interpretation

"This offers a more romantic spin on ancient interactions," the lead researcher said.

Writing 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 limited to how people smooch.

Defining Kissing

"Previously there were some efforts to describe a kiss, but it's largely focused on humans, which means that essentially other animals do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing looks like," said Brindle.

Nonetheless, she noted some actions that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "kiss-fighting", seen in fish known as certain marine animals.

As a result the team developed a description of intimate contact centered around social behaviors involving directed oral interaction with a member of the same species, with some motion of the oral area but absence of nutrition.

Research Methods

The lead researcher explained they concentrated on accounts of kissing in primates from Africa and Asia, including bonobos, apes and orangutans, and used online videos to verify the reports.

The researchers then combined this information with information on the genetic connections between living and extinct types of such primates.

Evolutionary Timeline

Researchers say the findings suggest kissing developed somewhere between 21.5m and 16.9m years ago in the predecessors of the great primates.

Placement of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the scientists conclude. But the behavior might not have been limited to their specific group.

"Reality that modern people kiss, the reality that we currently have demonstrated that Neanderthals very likely kissed, indicates that the both groups are probably did engage," Brindle added.

Evolutionary Significance

While the scientific reasoning 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 strengthen connections when used in a platonic way.

Another expert in the activities of great apes commented that as kissing behavior was observed in a wide range of apes it was logical its roots extend far into our ancient history, and an analysis of different forms of intimate behavior among a broader range of species might extend its beginnings back even earlier still.

"Things that we consider as signatures of human life, like kissing, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Elements

An archaeology expert said that kissing had a social component as it was not universal to all societies.

"However, as humans we succeed or struggle on the strength of our emotional bonds, and methods of encouraging trust and closeness will have been significant for millions of years," she said. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be no surprise that Neanderthals – and even Neanderthals and our human ancestors collectively – kissed."