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From seabirds to polar bears, primates to great apes, certain species appear to kiss. Currently, researchers suggest that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Evidence

This isn't the initial instance experts have proposed Neanderthals and early modern humans were closely connected. Among previous studies, scientists have discovered modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they swapped saliva.

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

Intimate Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," the lead researcher said.

Writing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how humans kiss.

Describing Intimate Contact

"There have been some efforts to define a intimate act, but it's very much been human-centric, which implies that essentially non-human species do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing looks like," explained Brindle.

However, she noted some actions that looked like intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species called French grunts.

Consequently the team came up with a description of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but no transfer of nutrition.

Study Approach

Brindle explained they focused on reports of intimate behavior in primates from Africa and Asia, including bonobos, chimpanzees and orangutans, and used digital recordings to confirm the reports.

The researchers then combined this data with details on the evolutionary relationships between living and ancient types of such animals.

Evolutionary Origins

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

Placement of ancient hominins on this family tree means it is probable they, too, engaged in a intimate act, the scientists conclude. But the activity might not have been limited to their specific group.

"The fact that modern people engage intimately, the fact that we now have demonstrated that Neanderthals probably kissed, indicates that the both groups are probably did kissed," the researcher noted.

Biological Significance

While the evolutionary explanation is discussed, the expert explained kissing could be used in sexual contexts to potentially enhance reproductive success or help choose between partners, while it might help reinforce bonding when used in a platonic way.

Another expert in the behavior of great apes said that as intimate contact was seen in a wide range of primates 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 beginnings back further still.

"Things that we think of as signatures of our species, like kissing, are not exclusive to us if we look closely at other animals," he said.

Cultural Elements

Another professor said that intimate contact had a cultural element as it was not common to all societies.

"However, as people we succeed or struggle on the strength of our emotional bonds, and methods of encouraging confidence and closeness will have been important for eons," she said. "It might be an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it ought to be no surprise that Neanderthals – and even Neanderthals and our own species together – kissed."