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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, scientists suggest that Neanderthals did it too – and might even have locked lips with modern humans.

Common Microbial Evidence

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. In previous studies, researchers have discovered humans and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the concept aligned with studies that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing interbreeding was occurring.

Intimate Interpretation

"This offers a different spin on human-Neanderthal relations," the lead researcher said.

Publishing in the journal Evolution and Human Behavior, the researcher and her team detail how, to explore the historical roots of intimate contact, they first had to develop a definition that was not limited to how people kiss.

Defining Intimate Contact

"There have been some efforts to define a intimate act, but it's largely focused on humans, which means that basically non-human species don't kiss. Now we know that they likely engage, it may appear different from what human kissing looks like," said the evolutionary biologist.

However, she said some actions that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species called French grunts.

As a result the team came up with a definition of intimate contact centered around social behaviors involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but absence of nutrition.

Study Methods

Brindle explained they focused on reports of kissing in non-human species from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and employed online videos to confirm the reports.

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

Historical Timeline

The team say the findings suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, indulged in a kiss, the researchers say. But the behavior may not have been limited to their own species.

"Reality that humans engage intimately, the fact that we now have demonstrated that Neanderthals very likely engaged, indicates that the both groups are also likely to have engage," Brindle added.

Evolutionary Significance

Although the scientific reasoning is debated, Brindle said intimate contact could be used in sexual contexts 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 activities of great apes said that as kissing behavior was seen in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an examination of different forms of kissing among a broader range of animals might push its beginnings back even earlier still.

"Behaviors that we think of as characteristics of our species, like kissing, are not unique to us if we examine carefully at different species," he said.

Cultural Aspects

Another professor said that intimate contact had a cultural element as it was not universal to all human groups.

"However, as people we thrive or fail on the quality of our emotional bonds, and methods of encouraging confidence and intimacy will have been significant for eons," the professor stated. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it ought to be expected that ancient hominins – and including them and our human ancestors together – engaged intimately."