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From Galápagos albatrosses to Arctic mammals, primates to orangutans, various animals appear to kiss. Now, scientists suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Microbial Evidence

It is not the first time experts have suggested ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have found modern people and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," she said, adding that the idea chimed with research that has found humans of non-African ancestry have bits of Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Intimate Spin

"It certainly puts a different perspective on human-Neanderthal relations," Brindle commented.

Publishing in the publication Evolution and Human Behavior, 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.

Describing Intimate Contact

"Previously there were some efforts to describe a intimate act, but it's very much been focused on humans, which means that essentially non-human species don't kiss. Now we understand that they likely engage, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she noted some actions that looked like kissing were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in fish known as French grunts.

As a result the team came up with a definition of kissing based on friendly interactions involving intentional oral interaction with a member of the same species, with some movement of the mouth but no transfer of nutrition.

Study Methods

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 integrated this data with information on the evolutionary relationships between extant and extinct species of such animals.

Evolutionary Origins

The team propose the results suggest kissing evolved approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the scientists say. But the behavior might not have been limited to their specific group.

"The fact that humans engage intimately, the fact that we now have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Significance

While the evolutionary explanation is debated, Brindle said intimate contact could be employed in sexual contexts 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 behavior of primates commented that as intimate contact was seen in a wide range of apes it made sense its roots lie deep in our evolutionary past, and an analysis of different forms of kissing among a broader range of species might extend its origins back further still.

"Things that we consider as characteristics of human life, like kissing, are not unique to us if we look closely at other animals," he said.

Cultural Elements

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

"However, as people we succeed or struggle on the quality of our relationships, and methods of promoting confidence and intimacy will have been significant for millions of years," she said. "It might be an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it should be expected that Neanderthals – and even them and our human ancestors collectively – engaged intimately."