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From seabirds to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Oral Evidence

This isn't the initial instance scientists have suggested Neanderthals and early modern humans were closely connected. In previous studies, researchers have discovered modern people and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the idea chimed with research that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Interpretation

"This offers a different perspective on human-Neanderthal relations," Brindle said.

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues detail how, to investigate the historical roots of intimate contact, they first had to develop a definition that was not limited to how people kiss.

Describing Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's largely focused on humans, which means 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," explained the evolutionary biologist.

However, she noted some actions that resembled intimate contact were something rather different – such as the processing and transfer of food, or "mouth contact", observed in fish known as French grunts.

As a result the research group came up with a description of intimate contact centered around social behaviors involving directed oral interaction with a member of the identical group, with some movement of the oral area but absence of nutrition.

Research Approach

The lead researcher explained they focused on accounts of intimate behavior in non-human species from the African continent and Asia, including primates, apes and orangutans, and used online videos to confirm the reports.

The researchers then combined this data with details on the genetic connections between living and ancient species of such primates.

Evolutionary Timeline

The team say the findings indicate kissing developed somewhere between 21.5 million and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree suggests it is likely they, too, indulged in a kiss, the researchers conclude. But the behavior might not have been limited to their specific group.

"Reality that humans kiss, the reality that we currently have shown that Neanderthals very likely kissed, suggests that the both groups are also likely to have kissed," Brindle noted.

Biological Importance

While the evolutionary explanation is discussed, the expert said intimate contact could be used in reproductive situations to potentially enhance mating outcomes or assist in selecting between partners, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was observed in a wide range of apes it made sense its origins extend far into our ancient history, and an analysis of various types of kissing among a broader range of animals might push its beginnings back even earlier still.

"Behaviors that we consider as characteristics of our species, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Social Aspects

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

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and ways of promoting confidence and closeness will have been significant for eons," she said. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and even them and our human ancestors together – kissed."