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Among seabirds to Arctic mammals, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that Neanderthals also engaged in this behavior – and possibly exchanged kisses with modern humans.

Shared Microbial Clues

It is not the first time scientists have proposed ancient relatives and early modern humans were closely connected. In previous studies, scientists have found modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the concept aligned with research that has found people of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Intimate Interpretation

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

Publishing in the journal a scientific periodical, the researcher and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to come up with a description 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 focused on humans, which means that basically other animals do not engage in this. Now we understand that they likely engage, it may appear different from what human kissing resembles," explained Brindle.

Nonetheless, she said some behaviors that looked like kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", seen in fish known as French grunts.

Consequently the research group came up with a definition of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but absence of nutrition.

Study Approach

Brindle explained they concentrated on reports of intimate behavior in non-human species from the African continent and Asian regions, including primates, chimpanzees and orangutans, and employed online videos to confirm the observations.

Scientists then integrated this information with information on the genetic connections between extant and ancient types of such animals.

Historical Origins

Researchers propose the results suggest kissing developed approximately 21.5 million and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree suggests it is likely they, too, indulged in a kiss, the scientists say. But the activity may not have been confined to their own species.

"The fact that humans kiss, the fact that we now have demonstrated that Neanderthals very likely kissed, suggests that the two [species] are also likely to have kissed," the researcher noted.

Biological Importance

While the evolutionary explanation is discussed, the expert said kissing could be used in reproductive situations to possibly enhance mating outcomes or help choose between mates, while it might help reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of great apes commented that as intimate contact was seen in a broad spectrum of primates it made sense its origins lie deep in our evolutionary past, and an examination of various types of kissing among a broader range of animals might push its origins back further still.

"Things that we consider as signatures of our species, like intimate contact, are not unique to us if we look closely at different species," he said.

Social Elements

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

"However, as humans we thrive or fail on the strength of our relationships, and ways of promoting confidence and closeness will have been important for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it should be no surprise that ancient hominins – and even them and our own species together – engaged intimately."