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

Common Microbial Clues

It is not the first time scientists have suggested ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have found modern people and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," she said, explaining that the concept chimed with studies that has found people of certain genetic backgrounds contain ancient genetic material in their genome, revealing interbreeding was at play.

Romantic Interpretation

"It certainly puts a more romantic perspective on ancient interactions," the lead researcher said.

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to explore the historical roots of kissing, they first had to come up with a definition that was not restricted by how people kiss.

Defining Intimate Contact

"Previously there were some efforts to define a intimate act, but it's largely focused on humans, 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.

However, she noted some actions that looked like kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in fish known as certain marine animals.

As a result the research group came up with a description of kissing based on social behaviors involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but no transfer of nutrition.

Study Approach

The lead researcher explained they focused on accounts of intimate behavior in primates from Africa and Asia, including bonobos, chimpanzees and great apes, and employed online videos to confirm the observations.

The researchers then combined this information with details on the evolutionary relationships between living and ancient species of such animals.

Evolutionary Timeline

The team propose the findings suggest intimate contact evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals on this evolutionary lineage suggests it is likely they, too, engaged in a kiss, the researchers conclude. But the activity might not have been limited to their specific group.

"Reality that modern people engage intimately, the reality that we now have demonstrated that Neanderthals probably kissed, suggests that the both groups are probably did kissed," Brindle noted.

Biological Importance

Although the evolutionary explanation is discussed, the expert explained intimate contact could be employed in sexual contexts to possibly enhance reproductive success or help choose between mates, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates said that as kissing behavior was seen in a wide range of primates it made sense its roots lie deep in our ancient history, and an examination of different forms of intimate behavior among a broader range of species might extend its origins back further still.

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

Social Elements

Another professor said that kissing 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 ways of encouraging trust and closeness will have been important for millions of years," she said. "It might be an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be expected that Neanderthals – and even Neanderthals and our own species together – engaged intimately."