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Among seabirds to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Currently, researchers propose that Neanderthals did it too – and possibly locked lips with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were intimately acquainted. Among previous studies, researchers 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 swapped saliva.

"Probably they were kissing," she said, explaining that the idea chimed with studies that has found people of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Intimate Interpretation

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

Publishing in the publication a scientific periodical, the researcher and her team report how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how people smooch.

Describing Intimate Contact

"Previously there were some efforts to define a intimate act, but it's largely focused on humans, which implies that basically non-human species do not engage in this. Currently we know that they likely engage, it might just not look from what human kissing resembles," said the evolutionary biologist.

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

As a result the team came up with a definition of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but no transfer of food.

Research Approach

Brindle explained they focused on reports of intimate behavior in non-human species from the African continent and Asian regions, including primates, chimpanzees and great apes, and used digital recordings to verify the reports.

The researchers then integrated this information with details on the genetic connections between living and ancient species of such animals.

Evolutionary Timeline

The team say the results indicate kissing developed approximately 21.5 million and 16.9 million years ago in the ancestors of the large apes.

The position of Neanderthals on this evolutionary lineage means it is probable they, too, engaged in a intimate act, the researchers say. 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 ancient relatives probably kissed, suggests that the both groups are also likely to have engage," Brindle added.

Biological Importance

While the evolutionary explanation is discussed, Brindle said kissing could be employed in reproductive situations to possibly increase reproductive success or assist in selecting between partners, while it might help strengthen connections when used in a platonic way.

A separate researcher in the activities of primates commented that as kissing behavior was seen in a wide range of apes it was logical its origins extend far into our evolutionary past, and an analysis of various types of kissing among a wider variety of species might push its origins back even earlier still.

"Behaviors that we consider as signatures of our species, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Social Aspects

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

"Nonetheless, as people we thrive or fail on the strength of our relationships, and ways of promoting trust and closeness will have been significant for millions of years," she said. "It might be an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it should be expected that Neanderthals – and including Neanderthals and our own species collectively – engaged intimately."