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From seabirds to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Oral Clues

It is not the first time experts have suggested ancient relatives and early modern humans were intimately acquainted. In earlier research, scientists have found modern people and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

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

Romantic Interpretation

"It certainly puts a different spin on ancient interactions," Brindle commented.

Writing in the journal Evolution and Human Behavior, the researcher and her team detail how, to explore the historical roots of intimate contact, they first had to develop a description that was not limited to how humans kiss.

Describing Intimate Contact

"There have been some efforts to describe a intimate act, but it's largely human-centric, which implies that essentially other animals do not engage in this. Currently we know that they probably do, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that resembled intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", observed in fish called French grunts.

Consequently the research group developed a definition of kissing based on friendly interactions involving intentional oral interaction with a member of the identical group, with some movement of the mouth but no transfer of nutrition.

Research Methods

Brindle explained they focused on reports of intimate behavior in non-human species from the African continent and Asian regions, including bonobos, chimpanzees and great apes, and employed online videos to confirm the reports.

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

Historical Timeline

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

Placement of Neanderthals on this family tree means it is likely they, too, indulged in a kiss, the scientists conclude. But the activity might not have been limited to their own species.

"The fact that humans engage intimately, the reality that we currently have demonstrated that Neanderthals very likely kissed, suggests that the two [species] are probably did kissed," Brindle noted.

Evolutionary Significance

While the evolutionary explanation is discussed, Brindle explained kissing could be used in sexual contexts to potentially increase reproductive success or help choose between mates, while it could assist strengthen connections when practiced in a platonic way.

A separate researcher in the activities of primates said that as intimate contact was seen in a broad spectrum of primates it made sense its roots extend far into our ancient history, and an examination of different forms of intimate behavior among a wider variety of species might extend its beginnings back further still.

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

Cultural Aspects

An archaeology expert explained that kissing had a social component as it was not universal to all societies.

"Nonetheless, as humans we thrive or fail on the quality of our relationships, and ways of encouraging trust and intimacy will have been important for eons," the professor stated. "It might be an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be no surprise that ancient hominins – and including them and our human ancestors together – kissed."