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Among seabirds to Arctic mammals, primates to orangutans, various animals appear to kiss. Currently, researchers suggest that ancient hominins did it too – and possibly locked lips with modern humans.

Common Microbial Evidence

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were closely connected. In previous studies, researchers have discovered humans and their Neanderthal relatives 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," the researcher noted, explaining that the concept chimed with research that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Romantic Spin

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

Publishing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the evolutionary origins of kissing, they first had to come up with a description that was not restricted by how people smooch.

Defining Kissing

"Previously there were some efforts to define a kiss, but it's largely human-centric, which implies that basically other animals do not engage in this. Currently we understand that they likely engage, it may appear different from what human kissing resembles," explained the evolutionary biologist.

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

Consequently the team came up with a definition of kissing based on social behaviors involving directed oral interaction with a member of the same species, with some motion of the mouth but absence of nutrition.

Research Approach

Brindle explained they concentrated on accounts of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed digital recordings to confirm the reports.

Scientists then integrated this data with information on the genetic connections between extant and ancient types of such animals.

Evolutionary Origins

Researchers propose the findings indicate kissing developed somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of Neanderthals on this evolutionary lineage means it is likely they, too, indulged in a kiss, the scientists say. But the behavior may not have been limited to their own species.

"Reality that modern people engage intimately, the reality that we now have shown that ancient relatives very likely kissed, suggests that the two [species] are also likely to have kissed," the researcher added.

Biological Significance

While the scientific reasoning is discussed, the expert explained intimate contact could be employed in sexual contexts to potentially enhance reproductive success or assist in selecting between partners, while it might help reinforce bonding when used in a non-sexual manner.

A separate researcher in the activities of great apes commented that as kissing behavior was observed in a wide range of apes it made sense its roots lie deep in our ancient history, and an analysis of different forms of intimate behavior among a wider variety of species might extend its beginnings back even earlier still.

"Behaviors that we think of as characteristics of our species, like intimate contact, are not unique to us if we look closely at different species," the expert noted.

Cultural Aspects

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

"However, as humans we thrive or fail on the quality of our relationships, and methods of promoting confidence and intimacy will have been important for eons," the professor stated. "This could represent an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it should be no surprise that Neanderthals – and even Neanderthals and our own species together – engaged intimately."