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From seabirds to polar bears, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Shared Microbial Clues

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

"Likely they were engaging in intimate contact," she said, explaining that the idea aligned with research that has revealed people of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing interbreeding was occurring.

Intimate Interpretation

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

Writing in the journal Evolution and Human Behavior, Brindle and colleagues detail how, to explore the evolutionary origins of kissing, 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 kiss, but it's largely focused on humans, which means that basically other animals do not engage in this. Currently we know that they probably do, it may appear different from what our intimate contact resembles," explained Brindle.

However, she said some actions that resembled kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species called French grunts.

Consequently the research group developed a description of kissing centered around social behaviors involving directed oral interaction with a member of the same species, with some movement of the mouth but absence of food.

Study Approach

The lead researcher said they concentrated on accounts of kissing in non-human species from the African continent and Asia, including primates, chimpanzees and orangutans, and used digital recordings to confirm the observations.

Scientists then combined this information with details on the genetic connections between living and extinct types of such primates.

Historical Timeline

Researchers propose the results suggest kissing developed somewhere between 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree means it is probable they, too, indulged in a kiss, the scientists say. But the activity may not have been confined to their specific group.

"Reality that modern people engage intimately, the fact that we now have shown that Neanderthals very likely engaged, indicates that the two [species] are also likely to have engage," Brindle noted.

Biological Importance

Although the scientific reasoning is discussed, the expert explained intimate contact could be employed in sexual contexts to possibly enhance reproductive success or help choose between partners, while it might help strengthen connections when practiced in a non-sexual manner.

A separate researcher in the behavior of great apes commented that as kissing behavior was seen in a broad spectrum of apes it made sense its origins extend far into our evolutionary past, and an examination of various types of intimate behavior among a wider variety of species might push its origins back further still.

"Things that we think of as signatures of our species, like intimate contact, are not unique to us if we look closely at other animals," the expert noted.

Social Elements

Another professor explained that intimate contact had a social component as it was not universal to all human groups.

"However, as humans we thrive or fail on the quality of our relationships, and ways of encouraging confidence and closeness will have been significant for millions of years," she said. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be expected that Neanderthals – and even them and our own species collectively – engaged intimately."