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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, scientists propose that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.

Common Oral Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were closely connected. In previous studies, scientists have found humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the concept aligned with studies that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Spin

"This offers a more romantic perspective on human-Neanderthal relations," the lead researcher commented.

Publishing in the journal Evolution and Human Behavior, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to develop a definition that was not limited to how humans smooch.

Describing Kissing

"There have been some previous attempts to define a kiss, but it's largely focused on humans, which means that essentially other animals do not engage in this. Now we understand that they likely engage, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she noted some actions that looked like kissing were something rather different – such as the processing and transfer of food, or "mouth contact", observed in fish called certain marine animals.

As a result the research group came up with a description of kissing centered around friendly interactions involving directed oral interaction with a member of the identical group, with some movement of the mouth but no transfer of food.

Study Approach

The lead researcher said they focused on accounts of intimate behavior in non-human species from Africa and Asia, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the observations.

Scientists then combined this data with details on the genetic connections between extant and ancient types of such animals.

Historical Origins

Researchers say the results suggest kissing developed somewhere between 21.5 million and 16.9 million years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a kiss, the researchers say. But the behavior may not have been limited to their own species.

"Reality that humans engage intimately, the reality that we now have shown that Neanderthals very likely kissed, indicates that the two [species] are probably did engage," the researcher added.

Evolutionary Importance

Although the evolutionary explanation is debated, the expert explained intimate contact could be employed in reproductive situations to potentially enhance mating outcomes or help choose between partners, while it might help strengthen connections when practiced in a platonic way.

A separate researcher in the behavior of great apes commented that as intimate contact was seen in a wide range of primates it was logical its origins extend far into our ancient history, and an examination of various types of intimate behavior among a wider variety of animals might extend its beginnings back even earlier still.

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

Cultural Aspects

An archaeology expert explained that intimate contact had a social component as it was not common to all human groups.

"However, as humans we thrive or fail on the strength of our relationships, and ways of encouraging confidence and intimacy will have been important for millions of years," the professor stated. "It might be an concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it should be expected that ancient hominins – and even Neanderthals and our human ancestors together – engaged intimately."