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From Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, various animals appear to kiss. Currently, scientists propose that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. Among earlier research, researchers have found humans and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," the researcher noted, explaining that the idea chimed with research that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Romantic Spin

"This offers a different perspective on human-Neanderthal relations," Brindle said.

Writing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a description that was not limited to how people kiss.

Describing Kissing

"Previously there were some efforts to describe a kiss, but it's very much been focused on humans, which implies that essentially non-human species don't kiss. Currently we understand that they likely engage, it might just not look from what human kissing looks like," explained Brindle.

Nonetheless, she noted some actions that looked like kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", seen in fish known as French grunts.

As a result the team came up with a description of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but absence of food.

Study Approach

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

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

Historical Origins

The team propose the findings suggest intimate contact evolved approximately 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, engaged 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 also likely to have kissed," Brindle added.

Evolutionary Significance

Although the scientific reasoning is discussed, the expert explained intimate contact could be used in sexual contexts to possibly increase mating outcomes or assist in selecting between partners, while it might help reinforce bonding when used in a non-sexual manner.

Another expert in the behavior of primates said that as intimate contact was observed in a broad spectrum of primates it was logical its origins lie deep in our ancient history, and an examination of different forms of kissing among a broader range of animals might extend its origins back even earlier still.

"Things that we consider as signatures of human life, like kissing, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Elements

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

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and ways of encouraging confidence and intimacy will have been important for eons," the professor stated. "It might be an image that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it ought to be expected that ancient hominins – and including them and our own species together – engaged intimately."