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From seabirds to Arctic mammals, primates to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins did it too – and might even have locked lips with modern humans.

Common Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were closely connected. In previous studies, researchers have found humans and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were kissing," the researcher noted, adding that the idea chimed with studies that has revealed people of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Intimate Spin

"It certainly puts a more romantic perspective on ancient interactions," Brindle said.

Publishing in the publication a scientific periodical, the researcher and her team report how, to explore the historical roots of intimate contact, they first had to come up with a description that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some efforts to define a kiss, but it's largely human-centric, which means that essentially other animals don't kiss. Now we understand that they probably do, it may appear different from what our intimate contact looks like," said Brindle.

Nonetheless, she said some actions that resembled kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in fish known as certain marine animals.

Consequently the research group developed a definition 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 oral area but no transfer of food.

Research Methods

Brindle said they focused on reports of kissing in non-human species from Africa and Asian regions, including bonobos, apes and great apes, and employed online videos to confirm the reports.

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

Evolutionary Origins

Researchers say the findings suggest intimate contact developed approximately 21.5m and 16.9m years ago in the ancestors of the large apes.

The position of Neanderthals on this evolutionary lineage means it is likely they, too, indulged in a kiss, the scientists conclude. But the behavior might not have been limited to their own species.

"Reality that humans engage intimately, the reality that we currently have shown that Neanderthals very likely kissed, indicates that the both groups are also likely to have kissed," Brindle noted.

Biological Significance

While the evolutionary explanation is discussed, Brindle explained kissing could be used in reproductive situations to possibly enhance reproductive success or assist in selecting between partners, while it could assist strengthen connections when used in a non-sexual manner.

A separate researcher in the behavior of primates commented that as intimate contact was observed in a broad spectrum of primates it was logical its origins extend far into our ancient history, and an analysis of various types of intimate behavior among a broader range of species might push its beginnings back further still.

"Things that we think of as signatures of our species, like kissing, are not unique to us if we look closely at other animals," he said.

Cultural Aspects

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

"However, as people we succeed or struggle on the quality of our relationships, and methods of promoting confidence and closeness will have been important for millions of years," the professor stated. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but really it ought to be no surprise that ancient hominins – and even Neanderthals and our human ancestors collectively – engaged intimately."