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From seabirds to polar bears, primates to orangutans, certain species appear to kiss. Now, scientists propose that Neanderthals did it too – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. In earlier research, researchers have found modern people and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, explaining that the concept chimed with studies that has found humans of non-African ancestry have bits of ancient genetic material in their genome, demonstrating interbreeding was occurring.

Romantic Spin

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

Writing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to investigate the historical roots of kissing, they first had to come up with a description that was not restricted by how humans kiss.

Defining Kissing

"Previously there were some efforts to define a kiss, but it's very much been human-centric, which means that basically other animals do not engage in this. Now we know that they probably do, it may appear different from what human kissing looks like," said Brindle.

However, she noted some actions that looked like kissing were distinct activities – such as the chewing and food sharing, or "mouth contact", observed in fish known as French grunts.

As a result the research group came up with a definition of kissing based on friendly interactions involving directed oral interaction with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Study Methods

Brindle said they concentrated on accounts of intimate behavior in non-human species from the African continent and Asian regions, including bonobos, apes and great apes, and used online videos to confirm the observations.

The researchers then combined this information with information on the evolutionary relationships between extant and ancient species of such primates.

Evolutionary Timeline

The team say the findings indicate intimate contact evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the large apes.

The position of ancient hominins on this family tree means it is probable they, too, indulged in a intimate act, the scientists say. But the activity might not have been limited to their specific group.

"The fact that humans kiss, the reality that we currently have shown that Neanderthals probably engaged, suggests that the two [species] are also likely to have engage," Brindle added.

Biological Significance

While the scientific reasoning is discussed, the expert said intimate contact could be employed in sexual contexts to possibly enhance reproductive success or help choose between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was observed in a broad spectrum of apes it made sense its roots lie deep in our evolutionary past, and an examination of various types of kissing among a wider variety of animals might extend its origins back further still.

"Behaviors that we consider as signatures of human life, like intimate contact, are not exclusive to us if we look closely at other animals," the expert noted.

Social Elements

An archaeology expert explained that kissing had a social component as it was not universal to all human groups.

"Nonetheless, as people we thrive or fail on the quality of our relationships, and methods of encouraging trust and closeness will have been important 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 ought to be expected that ancient hominins – and even them and our human ancestors collectively – kissed."