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From seabirds to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Shared Oral Clues

It is not the first time experts have suggested ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, scientists have found modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding 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 commented.

Publishing in the publication a scientific periodical, the researcher and colleagues report how, to explore the evolutionary origins of kissing, they first had to develop a description that was not limited to how humans kiss.

Describing Kissing

"Previously there were some efforts to define a intimate act, but it's very much been focused on humans, which means that basically other animals don't kiss. Now we understand that they likely engage, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

However, she said some actions that resembled intimate contact were something rather different – such as the chewing and transfer of food, or "mouth contact", observed in fish known as French grunts.

As a result the research group came up with a definition of kissing based on social behaviors involving directed oral interaction with a member of the same species, with some movement of the oral area but absence of food.

Study Approach

The lead researcher explained they concentrated on reports of kissing in non-human species from the African continent and Asian regions, including primates, apes and great apes, and used digital recordings to verify the reports.

Scientists then combined this data with details on the genetic connections between extant and extinct species of such primates.

Historical Timeline

Researchers say the results suggest intimate contact evolved approximately 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals 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.

"The fact that humans kiss, the reality that we currently have demonstrated that ancient relatives very likely engaged, suggests that the two [species] are probably did engage," the researcher added.

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle explained kissing could be used in sexual contexts to possibly increase reproductive success or help choose between partners, while it could assist strengthen connections when used in a non-sexual manner.

A separate researcher in the activities of primates commented that as kissing behavior was observed in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an analysis of different forms of intimate behavior among a broader range of animals might push its origins back further still.

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

Cultural Elements

Another professor said that kissing had a social component as it was not common to all societies.

"Nonetheless, as people we succeed or struggle on the quality of our emotional bonds, and methods of promoting trust and closeness will have been important for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but really it should be no surprise that Neanderthals – and even them and our own species collectively – engaged intimately."