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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that Neanderthals did it too – and possibly exchanged kisses with modern humans.

Shared Oral Clues

It is not the first time scientists have suggested ancient relatives and early modern humans were closely connected. In earlier research, scientists have found modern people and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with research that has revealed humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Romantic Spin

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

Writing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to investigate the historical roots of kissing, they first had to develop a description that was not limited to how humans smooch.

Defining Intimate Contact

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

However, she said some behaviors that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in fish called certain marine animals.

As a result the research group came up with a description of intimate contact based on friendly interactions involving directed oral interaction with a member of the same species, with some motion of the mouth but no transfer of food.

Research Approach

Brindle said they focused on reports of kissing in primates from the African continent and Asian regions, including bonobos, apes and great apes, and used online videos to confirm the reports.

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

Evolutionary Origins

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

The position of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a kiss, the researchers conclude. But the activity may not have been limited to their own species.

"The fact that modern people kiss, the fact that we currently have demonstrated that Neanderthals probably engaged, suggests that the both groups are probably did kissed," Brindle noted.

Biological Significance

While the scientific reasoning is debated, the expert said kissing could be employed in sexual contexts to possibly increase mating outcomes or help choose between mates, 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 wide range of apes it made sense its roots extend far into our evolutionary past, and an analysis of various types of intimate behavior among a broader range of animals might extend its origins back even earlier still.

"Things that we think of as characteristics of our species, like kissing, are not exclusive to us if we examine carefully at different species," he said.

Cultural Elements

An archaeology expert explained that kissing had a cultural element as it was not common to all human groups.

"However, as humans we thrive or fail on the strength of our emotional bonds, and ways of promoting confidence and closeness will have been important for millions of years," she said. "It might be an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it should be no surprise that Neanderthals – and even them and our human ancestors together – engaged intimately."