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Among seabirds to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins did it too – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have discovered modern people and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Probably they were kissing," the researcher noted, explaining that the concept chimed with research that has revealed people of certain genetic backgrounds contain Neanderthal DNA in their genome, revealing interbreeding was occurring.

Intimate Interpretation

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

Publishing in the publication Evolution and Human Behavior, the researcher and her team report how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how humans kiss.

Describing Intimate Contact

"There have been some efforts to define a kiss, but it's very much been human-centric, which means that essentially non-human species don't kiss. Currently we understand that they probably do, it might just not look from what human kissing looks like," said Brindle.

Nonetheless, she said some behaviors that resembled kissing were something rather different – such as the processing and food sharing, or "mouth contact", observed in aquatic species known as French grunts.

As a result the team came up with a definition of kissing centered around friendly interactions involving directed oral interaction with a individual of the same species, with some motion of the oral area but no transfer of food.

Study Methods

The lead researcher said they focused on reports of intimate behavior in primates from the African continent and Asia, including bonobos, chimpanzees and great apes, and used online videos to confirm the observations.

Scientists then combined this data with details on the genetic connections between living and extinct types of such animals.

Historical Origins

The team propose the findings suggest intimate contact evolved somewhere between 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is likely they, too, indulged in a intimate act, the scientists conclude. But the behavior might not have been confined to their own species.

"Reality that modern people engage intimately, the fact that we currently have demonstrated that ancient relatives probably kissed, suggests that the both groups are probably did engage," the researcher noted.

Biological Significance

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 used in a non-sexual manner.

A separate researcher in the behavior of great apes said that as kissing behavior was seen in a broad spectrum of apes it was logical its roots extend far into our ancient history, and an analysis of different forms of kissing among a wider variety of species might push its beginnings back further still.

"Behaviors that we think of as characteristics of human life, like kissing, are not exclusive to us if we look closely at different species," he said.

Cultural Aspects

Another professor said that intimate contact had a cultural element as it was not universal to all societies.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and ways of encouraging confidence and intimacy will have been important for eons," the professor stated. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but really it should be expected that Neanderthals – and including them and our human ancestors collectively – kissed."