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Among Galápagos albatrosses to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Common Oral Evidence

It is not the first time experts have suggested Neanderthals and early modern humans were closely connected. Among earlier research, scientists have discovered modern people and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," she said, adding that the concept chimed with research that has found humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Spin

"It certainly puts a different spin on ancient interactions," Brindle said.

Publishing in the journal a scientific periodical, the researcher and her team report how, to explore the historical roots of kissing, they first had to come up with a definition that was not restricted by how people kiss.

Defining Kissing

"Previously there were some efforts to describe a kiss, but it's largely human-centric, which means that essentially non-human species do not engage in this. Now we know that they likely engage, it might just not look from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that looked like intimate contact were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species called French grunts.

As a result the research group came up with a description of kissing centered around friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the mouth but absence of food.

Research Approach

The lead researcher explained they concentrated on reports of kissing in non-human species from Africa and Asia, including primates, apes and orangutans, and used online videos to verify the observations.

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

Evolutionary Timeline

Researchers say the results indicate kissing evolved somewhere between 21.5 million and 16.9 million years ago in the predecessors of the great primates.

Placement of ancient hominins on this evolutionary lineage means it is likely they, too, engaged in a intimate act, the scientists say. But the activity may not have been limited to their own species.

"The fact that modern people kiss, the fact that we now have demonstrated that Neanderthals probably kissed, indicates that the two [species] are also likely to have kissed," the researcher noted.

Biological Importance

While the scientific reasoning is discussed, the expert explained kissing could be used in reproductive situations to possibly enhance reproductive success or assist in selecting between partners, while it might help strengthen connections when used in a platonic way.

Another expert in the activities of primates commented that as kissing behavior was seen in a wide range of primates it made sense its roots extend far into our evolutionary past, and an analysis of different forms of intimate behavior among a wider variety of species might extend its origins back further still.

"Things that we consider as signatures of our species, like intimate contact, are not exclusive to us if we look closely at different species," the expert noted.

Cultural Elements

Another professor explained that intimate contact had a social component as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the quality of our relationships, and methods of encouraging trust and closeness will have been significant for eons," she said. "It might be an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it should be expected that Neanderthals – and even Neanderthals and our own species collectively – engaged intimately."