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Among seabirds to Arctic mammals, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Now, researchers suggest that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Microbial Evidence

It is not the first time scientists have proposed Neanderthals and Homo sapiens were intimately acquainted. In earlier research, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the idea chimed with research that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Spin

"This offers a different perspective on human-Neanderthal relations," Brindle commented.

Publishing in the journal a scientific periodical, Brindle and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how people kiss.

Defining Kissing

"There have been some previous attempts to describe a kiss, but it's largely focused on humans, which implies that essentially other animals do not engage in this. Currently we know that they likely engage, it might just not look from what our intimate contact resembles," explained 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 "kiss-fighting", observed in fish known as certain marine animals.

Consequently the research group came up with a definition of kissing based on social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the mouth but no transfer of nutrition.

Study Approach

Brindle explained they focused on accounts of kissing in non-human species from Africa and Asia, including bonobos, chimpanzees and orangutans, and employed online videos to confirm the observations.

The researchers then combined this data with details on the genetic connections between living and ancient types of such animals.

Evolutionary Origins

Researchers propose the results indicate intimate contact developed 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, indulged in a kiss, the scientists conclude. But the activity might not have been limited to their own species.

"Reality that humans engage intimately, the fact that we now have shown that Neanderthals very likely kissed, indicates that the two [species] are also likely to have engage," the researcher noted.

Evolutionary Importance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be employed in sexual contexts to possibly enhance reproductive success or assist in selecting between mates, while it might help strengthen connections when practiced in a platonic way.

Another expert in the behavior of primates commented that as intimate contact was seen in a wide range of apes it was logical its roots lie deep in our ancient history, and an examination of different forms of intimate behavior among a broader range of species might push its origins back even earlier still.

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

Cultural Aspects

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

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and methods of encouraging trust and intimacy will have been important for millions of years," she said. "It might be an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it ought to be expected that Neanderthals – and even Neanderthals and our own species together – engaged intimately."