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Among seabirds to polar bears, chimpanzees to orangutans, certain species appear to kiss. Currently, researchers suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

This isn't the initial instance experts have suggested ancient relatives and Homo sapiens were intimately acquainted. In earlier research, scientists have found humans and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea chimed with research that has found humans of non-African ancestry contain Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Spin

"It certainly puts a different spin on ancient interactions," the lead researcher commented.

Publishing in the journal a scientific periodical, Brindle and colleagues report how, to investigate the evolutionary origins of intimate contact, they first had to develop a definition that was not restricted by how humans kiss.

Describing Intimate Contact

"There have been some previous attempts to describe a intimate act, but it's very much been human-centric, which means that essentially other animals do not engage in this. Now we know that they likely engage, it may appear different from what our intimate contact looks like," explained Brindle.

Nonetheless, she said some behaviors that resembled kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in aquatic species known as French grunts.

As a result the research group came up with a definition of intimate contact centered around social behaviors involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but absence of food.

Research Approach

The lead researcher said they concentrated on reports of kissing in non-human species from the African continent and Asian regions, including primates, chimpanzees and great apes, and used online videos to confirm the reports.

Scientists then integrated this data with details on the genetic connections between living and ancient types of such primates.

Evolutionary Origins

The team propose the results indicate intimate contact evolved approximately 21.5m and 16.9m years ago in the ancestors of the great primates.

Placement of Neanderthals on this evolutionary lineage suggests it is probable they, too, engaged in a kiss, the researchers say. But the activity may not have been confined to their own species.

"Reality that modern people kiss, the fact that we now have shown that Neanderthals very likely kissed, suggests that the both groups are probably did engage," Brindle noted.

Evolutionary Significance

While the scientific reasoning is debated, the expert explained intimate contact could be employed in reproductive situations to potentially enhance reproductive success or help choose between mates, while it might help reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the activities of great apes commented that as intimate contact was seen in a broad spectrum of apes it was logical its roots extend far into our ancient history, and an analysis of various types of kissing among a broader range of animals might extend its beginnings back even earlier still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not unique to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

An archaeology expert said that intimate contact had a social component as it was not common to all societies.

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and methods of promoting confidence and intimacy will have been significant for eons," the professor stated. "This could represent an concept that appears a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be no surprise that ancient hominins – and even Neanderthals and our own species collectively – engaged intimately."