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Among seabirds to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, scientists propose that ancient hominins did it too – and possibly locked lips with modern humans.

Common Oral Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In previous studies, scientists have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were kissing," she said, adding that the concept aligned with studies that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Romantic Interpretation

"This offers a different perspective on ancient interactions," Brindle said.

Writing in the journal a scientific periodical, Brindle and her team report how, to investigate the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how people kiss.

Defining Intimate Contact

"Previously there were some efforts to describe a intimate act, but it's very much been human-centric, which implies that basically other animals don't kiss. Currently we know that they likely engage, it may appear different from what our intimate contact looks like," explained Brindle.

However, she noted some behaviors that looked like intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", observed in aquatic species known as certain marine animals.

As a result the research group developed a definition of kissing based on social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the mouth but no transfer of food.

Study Methods

Brindle said they focused on reports of kissing in primates from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and used digital recordings to verify the reports.

The researchers then integrated this data with information on the evolutionary relationships between living and ancient types of such primates.

Evolutionary Timeline

Researchers propose the findings suggest intimate contact developed somewhere between 21.5 million and 16.9 million years ago in the predecessors of the large apes.

The position of ancient hominins on this family tree suggests it is likely they, too, engaged in a kiss, the scientists conclude. But the behavior might not have been limited to their specific group.

"Reality that modern people engage intimately, the fact that we currently have demonstrated that ancient relatives probably engaged, indicates that the both groups are also likely to have kissed," Brindle noted.

Evolutionary Significance

Although the scientific reasoning is discussed, the expert said kissing could be employed in reproductive situations to potentially increase mating outcomes or help choose between partners, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was seen in a broad spectrum of primates it was logical its origins extend far into our evolutionary past, and an examination of different forms of intimate behavior among a broader range of species might push its beginnings back further still.

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

Social Aspects

An archaeology expert said that kissing had a social component as it was not common to all human groups.

"However, as people we thrive or fail on the quality of our relationships, and methods of promoting trust and intimacy will have been significant for millions of years," she said. "It might be an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it should be no surprise that ancient hominins – and even them and our human ancestors collectively – kissed."