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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Microbial Clues

It is not the first time scientists have proposed Neanderthals and early modern humans were closely connected. Among earlier research, researchers have found humans and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, adding that the concept aligned with research that has found people of non-African ancestry contain Neanderthal DNA in their genome, demonstrating interbreeding was occurring.

Intimate Interpretation

"It certainly puts a more romantic perspective on ancient interactions," Brindle said.

Publishing in the journal Evolution and Human Behavior, Brindle and her team detail how, to explore the historical roots of intimate contact, they first had to develop a description that was not restricted by how humans kiss.

Describing Kissing

"There have been some previous attempts to define a intimate act, but it's largely human-centric, which means that basically non-human species do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing resembles," explained Brindle.

Nonetheless, she noted some actions that looked like kissing were something rather different – such as the processing and food sharing, or "kiss-fighting", seen in aquatic species called French grunts.

As a result the team came up with a definition of intimate contact based on friendly interactions involving intentional oral interaction with a individual of the identical group, with some movement of the oral area but absence of nutrition.

Research Approach

Brindle explained they focused on reports of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed online videos to confirm the reports.

Scientists then combined this data with details on the evolutionary relationships between living and ancient types of such primates.

Evolutionary Timeline

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

The position of ancient hominins on this family tree suggests it is probable they, too, indulged in a kiss, the researchers say. But the behavior might not have been confined to their specific group.

"The fact that humans kiss, the fact that we now have demonstrated that Neanderthals probably kissed, indicates that the both groups are also likely to have engage," the researcher added.

Evolutionary Importance

While the scientific reasoning is debated, the expert explained intimate contact could be employed in reproductive situations to possibly increase mating outcomes or help choose between mates, while it might help strengthen connections when used in a platonic way.

Another expert in the behavior of great apes said that as intimate contact was seen in a broad spectrum of apes it was logical its roots lie deep in our ancient history, and an examination of different forms of intimate behavior among a broader range of species might extend its origins back further still.

"Things that we consider as signatures of human life, like kissing, are not unique to us if we look closely at different species," the expert noted.

Cultural Aspects

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

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and methods of promoting trust and intimacy will have been significant for eons," the professor stated. "It might be an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be no surprise that Neanderthals – and even Neanderthals and our human ancestors together – kissed."