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From seabirds to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists propose that Neanderthals did it too – and possibly exchanged kisses with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were closely connected. Among previous studies, scientists have discovered 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," she said, adding that the idea chimed with studies that has found people of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing interbreeding was occurring.

Intimate Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle said.

Writing in the publication a scientific periodical, the researcher and her team detail how, to investigate the historical roots of kissing, they first had to develop a definition that was not restricted by how humans smooch.

Defining Kissing

"Previously there were some previous attempts to define a intimate act, but it's largely human-centric, which means that essentially non-human species don't kiss. Now we understand that they probably do, it might just not look from what human kissing resembles," said Brindle.

However, she noted some actions that looked like kissing were something rather different – such as the processing and food sharing, or "kiss-fighting", observed in fish called certain marine animals.

Consequently the research group came up with a description of kissing based on friendly interactions involving intentional oral interaction with a individual of the identical group, with some motion of the oral area but no transfer of nutrition.

Research Methods

The lead researcher said they concentrated on accounts of kissing in primates from the African continent and Asia, including primates, chimpanzees and great apes, and employed online videos to confirm the observations.

The researchers then combined this information with details on the genetic connections between extant and ancient types of such primates.

Historical Origins

The team propose the results suggest kissing developed somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the researchers conclude. But the behavior might not have been confined to their own species.

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

Biological Importance

Although the scientific reasoning is debated, Brindle said intimate contact could be used in reproductive situations to possibly increase mating outcomes or help choose between partners, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the activities of primates said that as kissing behavior was observed in a wide range of apes it was logical its roots extend far into our ancient history, and an examination of various types of intimate behavior among a broader range of species might extend its origins back further still.

"Things that we consider as signatures of our species, like kissing, 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 cultural element as it was not universal to all human groups.

"However, as people we succeed or struggle on the quality of our emotional bonds, and ways of encouraging trust and closeness will have been important for eons," the professor stated. "This could represent an image that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be expected that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."