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From Galápagos albatrosses to Arctic mammals, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins did it too – and possibly locked lips with modern humans.

Common Oral Clues

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, scientists have found humans and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, explaining that the concept aligned with research that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing interbreeding was occurring.

Romantic Spin

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle said.

Writing in the journal Evolution and Human Behavior, Brindle and her team detail how, to explore the historical roots of intimate contact, they first had to come up with a description that was not limited to how humans smooch.

Defining Kissing

"There have been 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 understand that they likely engage, it may appear different from what our intimate contact resembles," said the evolutionary biologist.

However, she said some behaviors that resembled kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in fish called certain marine animals.

Consequently the team developed a definition of intimate contact based on social behaviors involving intentional oral interaction with a member of the identical group, with some movement of the mouth but no transfer of nutrition.

Research Approach

The lead researcher said they focused on reports of intimate behavior in non-human species from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and used digital recordings to confirm the reports.

The researchers then combined this data with details on the genetic connections between living and extinct species of such primates.

Evolutionary Origins

Researchers say the results indicate kissing developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the researchers say. But the activity may not have been confined to their own species.

"Reality that humans engage intimately, the fact that we now have demonstrated that ancient relatives very likely kissed, suggests that the two [species] are probably did kissed," the researcher noted.

Biological Significance

While the scientific reasoning is debated, the expert said kissing could be used in reproductive situations to possibly increase mating outcomes or help choose between partners, while it could assist strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of great apes said that as intimate contact was seen in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an analysis of different forms of kissing among a wider variety of animals might push its origins back even earlier still.

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

Cultural Elements

An archaeology expert explained that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and ways of promoting confidence and intimacy will have been important for millions of years," she said. "It might be an concept that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be expected that Neanderthals – and even them and our human ancestors together – engaged intimately."