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Among seabirds to Arctic mammals, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

It is not the first time scientists have suggested ancient relatives and early modern humans were closely connected. Among earlier research, researchers have discovered humans and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the concept aligned with studies that has found people of non-African ancestry contain ancient genetic material in their genome, revealing genetic mixing was occurring.

Romantic Spin

"This offers a more romantic spin on ancient interactions," Brindle commented.

Writing in the publication a scientific periodical, Brindle and her team detail how, to investigate the evolutionary origins of kissing, they first had to come up with a description that was not restricted by how people smooch.

Defining Intimate Contact

"There have been some previous attempts to describe a kiss, but it's largely focused on humans, which implies that essentially non-human species don't kiss. Now we understand that they probably do, it might just not look from what our intimate contact looks like," said Brindle.

However, she noted some actions that looked like kissing were something rather different – such as the processing and transfer of food, or "mouth contact", observed in aquatic species called certain marine animals.

As a result the team came up with a description of intimate contact based on friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the oral area but absence of nutrition.

Study Methods

The lead researcher explained they concentrated on reports of intimate behavior in non-human species from Africa and Asian regions, including bonobos, apes and great apes, and employed online videos to confirm the observations.

Scientists then combined this data with information on the genetic connections between living and ancient species of such animals.

Evolutionary Origins

The team propose the results indicate intimate contact developed somewhere between 21.5m and 16.9m years ago in the ancestors of the large apes.

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

"Reality that modern people engage intimately, the fact that we currently have shown that ancient relatives very likely kissed, indicates that the both groups are probably did kissed," Brindle added.

Biological Significance

Although the evolutionary explanation is discussed, Brindle said intimate contact could be used in reproductive situations to possibly increase reproductive success or help choose between partners, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the behavior of great apes said that as intimate contact was seen in a broad spectrum of apes it was logical its roots extend far into our evolutionary past, and an examination of various types of intimate behavior among a wider variety of species might extend its beginnings back even earlier still.

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

Cultural Aspects

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

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and ways of encouraging trust and intimacy will have been significant for eons," she said. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be expected that ancient hominins – and including Neanderthals and our human ancestors collectively – engaged intimately."