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From seabirds to polar bears, 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 modern humans.

Common Oral Clues

This isn't the initial instance scientists have suggested Neanderthals and early modern humans were closely connected. In earlier research, researchers have discovered humans and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with studies that has found people of non-African ancestry have bits of ancient genetic material in their genome, revealing interbreeding was at play.

Intimate Interpretation

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

Writing in the publication a scientific periodical, Brindle and her team report how, to investigate the evolutionary origins of kissing, they first had to develop a description that was not restricted by how people kiss.

Describing Intimate Contact

"Previously there were some efforts to describe a intimate act, but it's largely human-centric, which means that basically non-human species do not engage in this. Now we understand that they probably do, it may appear different from what our intimate contact resembles," said Brindle.

However, she said some behaviors that looked like kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in fish called certain marine animals.

As a result the research group developed a definition of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but no transfer of nutrition.

Research Methods

The lead researcher explained they concentrated on accounts of intimate behavior in primates from the African continent and Asian regions, including primates, apes and orangutans, and employed digital recordings to verify the observations.

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

Historical Origins

Researchers propose the findings indicate kissing evolved approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

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

"Reality that modern people kiss, the fact that we currently have demonstrated that Neanderthals probably kissed, suggests that the two [species] are probably did kissed," Brindle noted.

Evolutionary Importance

Although the evolutionary explanation is debated, Brindle explained kissing could be used in sexual contexts to potentially enhance mating outcomes or help choose between mates, while it might help reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of primates commented that as intimate contact was seen in a wide range of apes it was logical its roots extend far into our evolutionary past, and an examination of different forms of kissing among a wider variety of species might extend its beginnings back further still.

"Behaviors that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we look closely at different species," the expert noted.

Social Aspects

Another professor explained that intimate contact had a social component as it was not universal to all societies.

"However, as humans we succeed or struggle on the strength of our emotional bonds, and methods of promoting confidence and closeness will have been significant for eons," she said. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be expected that Neanderthals – and even Neanderthals and our own species together – kissed."