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Among seabirds to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Now, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Shared Microbial Clues

It is not the first time scientists have suggested Neanderthals and early modern humans were closely connected. Among earlier research, scientists have found humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the idea chimed with research that has revealed humans of certain genetic backgrounds contain Neanderthal DNA in their genome, demonstrating genetic mixing was occurring.

Romantic Spin

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

Writing in the publication Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the historical roots of intimate contact, they first had to develop a definition that was not restricted by how people smooch.

Defining Intimate Contact

"There have been some efforts to define a intimate act, but it's very much been focused on humans, which means that essentially other animals do not engage in this. Currently we understand that they probably do, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she said some actions that looked like kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species called certain marine animals.

Consequently the team developed a description of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but no transfer of food.

Study Approach

Brindle said they concentrated on reports of intimate behavior in non-human species from the African continent and Asia, including primates, chimpanzees and great apes, and used online videos to verify the observations.

Scientists then combined this data with information on the genetic connections between extant and extinct species of such primates.

Evolutionary Timeline

Researchers propose the findings indicate intimate contact evolved somewhere between 21.5m and 16.9 million years ago in the predecessors of the large apes.

The position of ancient hominins on this family tree means it is likely 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 humans engage intimately, the fact that we now have shown that Neanderthals probably engaged, indicates that the two [species] are probably did kissed," Brindle added.

Evolutionary Importance

Although the evolutionary explanation is discussed, Brindle explained kissing could be used in sexual contexts to potentially enhance mating outcomes or assist in selecting between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of great apes commented that as intimate contact was observed in a broad spectrum of apes it made sense its roots lie deep in our ancient history, and an analysis of different forms of kissing among a broader range of animals might push its origins back even earlier still.

"Behaviors that we consider as characteristics of our species, like kissing, are not exclusive to us if we look closely at different species," the expert noted.

Social Elements

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

"Nonetheless, as humans we thrive or fail on the quality of our relationships, and ways of encouraging confidence and intimacy will have been significant for eons," she said. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but actually it should be expected that ancient hominins – and including them and our human ancestors collectively – engaged intimately."