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Among Galápagos albatrosses to Arctic mammals, primates to great apes, certain species appear to kiss. Currently, scientists suggest that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Oral Clues

It is not the first time scientists have suggested ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, scientists have found humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were kissing," she said, adding that the concept chimed with research that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing interbreeding was at play.

Romantic Spin

"This offers a different perspective on ancient interactions," the lead researcher said.

Writing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how people kiss.

Defining Kissing

"Previously there were some previous attempts to define a kiss, but it's very much been focused on humans, which implies that essentially non-human species don't kiss. Now we understand that they probably do, it might just not look from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she said some behaviors that resembled intimate contact were something rather different – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species known as French grunts.

Consequently the research group came up with a description of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but no transfer of food.

Study Methods

Brindle said they concentrated on reports of kissing in non-human species from the African continent and Asian regions, including bonobos, chimpanzees and great apes, and used online videos to verify the reports.

The researchers then combined this information with information on the genetic connections between extant and extinct types of such primates.

Historical Origins

Researchers propose the results suggest intimate contact evolved approximately 21.5 million and 16.9 million years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree means it is probable they, too, indulged in a intimate act, the researchers conclude. But the activity may not have been confined to their own species.

"Reality that humans engage intimately, the fact that we currently have demonstrated that ancient relatives very likely kissed, indicates that the two [species] are also likely to have engage," the researcher noted.

Biological Significance

While the scientific reasoning is debated, the expert said kissing could be employed in sexual contexts to potentially increase reproductive success or assist in selecting between partners, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the activities of primates said that as kissing behavior was observed in a broad spectrum of apes it made sense its roots lie deep in our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of animals might extend its beginnings back further still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not exclusive to us if we examine carefully at different species," he said.

Social Elements

Another professor said that intimate contact had a social component as it was not universal to all societies.

"However, as humans we succeed or struggle on the strength of our emotional bonds, and ways of promoting trust and intimacy will have been important for eons," she said. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be no surprise that Neanderthals – and including Neanderthals and our human ancestors collectively – kissed."