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From seabirds to polar bears, chimpanzees to great apes, certain species appear to kiss. Currently, scientists propose that ancient hominins also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Common Oral Clues

This isn't the initial instance experts have suggested ancient relatives and Homo sapiens were closely connected. Among earlier research, scientists have found humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, explaining that the idea chimed with studies that has found people of non-African ancestry contain ancient genetic material in their genome, demonstrating interbreeding was occurring.

Romantic Spin

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

Writing in the publication a scientific periodical, the researcher and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how humans kiss.

Describing Kissing

"Previously there were some efforts to define a kiss, but it's largely focused on humans, which implies that essentially non-human species do not engage in this. Currently we know that they probably do, it might just not look from what human kissing resembles," said Brindle.

However, she said some actions that looked like intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", observed in fish known as French grunts.

Consequently the team came up with a definition of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but absence of nutrition.

Research Methods

Brindle explained they focused on reports of intimate behavior in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and employed online videos to confirm the observations.

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

Historical Origins

Researchers propose the results 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 family tree means it is probable they, too, engaged in a intimate act, the scientists say. But the behavior might not have been confined to their own species.

"Reality that humans kiss, the reality that we currently have demonstrated that ancient relatives probably engaged, indicates that the both groups are probably did kissed," Brindle noted.

Evolutionary Importance

Although the scientific reasoning is debated, Brindle explained kissing could be employed in reproductive situations to possibly enhance reproductive success or help choose between partners, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of great apes said that as intimate contact was observed in a broad spectrum of apes it made sense its roots lie deep in our ancient history, and an examination of different forms of kissing among a broader range of species might extend its beginnings back further still.

"Behaviors that we consider as signatures of our species, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Social Aspects

Another professor said that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and ways of encouraging confidence and closeness will have been significant for eons," she said. "This could represent an concept that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it should be expected that Neanderthals – and including them and our own species together – engaged intimately."