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From Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Shared Oral Evidence

It is not the first time experts have proposed ancient relatives and early modern humans were closely connected. In earlier research, researchers have discovered modern people and their Neanderthal relatives possessed 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 studies that has revealed humans of non-African ancestry contain Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Spin

"This offers a different perspective on human-Neanderthal relations," the lead researcher commented.

Publishing in the publication a scientific periodical, the researcher and her team detail how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how humans kiss.

Describing Kissing

"Previously there were some efforts to describe a intimate act, but it's very much been human-centric, which implies that essentially other animals do not engage in this. Now we know that they probably do, it may appear different from what our intimate contact looks like," explained Brindle.

Nonetheless, she noted some actions that looked like intimate contact were something rather different – such as the processing and food sharing, or "kiss-fighting", seen in aquatic species known as certain marine animals.

As a result the team came up with a definition of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the mouth but absence of food.

Study Approach

The lead researcher said they focused on reports of intimate behavior in non-human species from the African continent and Asia, including primates, apes and orangutans, and used digital recordings to confirm the observations.

Scientists then combined this data with information on the genetic connections between living and extinct types of such animals.

Evolutionary Origins

The team say the findings suggest intimate contact developed approximately 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of Neanderthals on this family tree means it is likely they, too, indulged in a intimate act, the scientists say. But the behavior may not have been confined to their specific group.

"The fact that modern people kiss, the fact that we currently have demonstrated that ancient relatives probably engaged, indicates that the both groups are probably did kissed," Brindle noted.

Evolutionary Significance

While the evolutionary explanation is discussed, the expert explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or assist in selecting between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

A separate researcher in the activities of primates commented that as intimate contact was seen in a broad spectrum of apes it made sense its roots extend far into our ancient history, and an examination of various types of intimate behavior among a broader range of animals might push its beginnings back even earlier still.

"Behaviors that we consider as signatures of human life, like intimate contact, are not unique to us if we examine carefully at other animals," he said.

Social Elements

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

"Nonetheless, as people we succeed or struggle on the quality of our relationships, and methods of encouraging trust and intimacy will have been important for eons," she said. "It might be an image that appears a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be expected that ancient hominins – and including them and our own species collectively – engaged intimately."