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From Galápagos albatrosses to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance experts have proposed Neanderthals and early modern humans were intimately acquainted. Among previous studies, scientists have discovered modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea aligned with research that has revealed people of certain genetic backgrounds contain Neanderthal DNA in their genome, demonstrating interbreeding was occurring.

Romantic Interpretation

"This offers a more romantic spin on human-Neanderthal relations," the lead researcher commented.

Publishing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to investigate the historical roots of kissing, they first had to develop a definition that was not restricted by how humans smooch.

Defining Kissing

"Previously there were some efforts to define a intimate act, but it's very much been human-centric, which means that essentially non-human species don't kiss. Now we know that they likely engage, it might just not look from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she noted some behaviors that resembled kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", seen in fish known as certain marine animals.

As a result the research group came up with a definition of intimate contact centered around friendly interactions involving directed oral interaction with a individual of the same species, with some motion of the oral area but no transfer of nutrition.

Study Approach

Brindle said they focused on accounts of intimate behavior in non-human species from Africa and Asia, including bonobos, apes and orangutans, and employed online videos to verify the observations.

The researchers then combined this data with information on the evolutionary relationships between extant and ancient types of such animals.

Historical Origins

The team say the results suggest intimate contact developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of Neanderthals on this family tree suggests it is probable they, too, indulged in a intimate act, the scientists say. But the activity may not have been confined to their own species.

"The fact that humans kiss, the reality that we now have demonstrated that ancient relatives probably kissed, suggests that the both groups are probably did kissed," the researcher added.

Biological Importance

Although the evolutionary explanation is discussed, the expert explained kissing could be employed in sexual contexts to possibly increase mating outcomes or assist in selecting between mates, while it might help strengthen connections when practiced in a platonic way.

Another expert in the behavior of primates said that as kissing behavior was observed in a wide range of apes it made sense its roots lie deep in our evolutionary past, and an examination of various types of intimate behavior among a broader range of animals might extend its origins back even earlier still.

"Behaviors that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we examine carefully at different species," the expert noted.

Cultural Elements

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

"However, as humans we succeed or struggle on the quality of our relationships, and methods of promoting trust and closeness will have been important for millions of years," the professor stated. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be no surprise that Neanderthals – and including them and our human ancestors together – engaged intimately."