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From Galápagos albatrosses to polar bears, primates to great apes, various animals appear to kiss. Currently, researchers propose 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 experts have proposed ancient relatives and early modern humans were intimately acquainted. In earlier research, researchers have found humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," she said, adding that the idea chimed with studies that has found humans of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Interpretation

"This offers a more romantic spin on ancient interactions," the lead researcher commented.

Writing in the journal Evolution and Human Behavior, the researcher and her team report how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how people kiss.

Describing Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's largely focused on humans, which means that basically non-human species don't kiss. Now we understand that they probably do, it may appear different from what human kissing resembles," said Brindle.

Nonetheless, she noted some behaviors that looked like intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in aquatic species called French grunts.

Consequently the team developed a definition of intimate contact based on friendly interactions involving directed oral interaction with a member of the same species, with some motion of the oral area but absence of nutrition.

Research Approach

The lead researcher explained they concentrated on accounts of intimate behavior in non-human species from Africa and Asian regions, including bonobos, chimpanzees and great apes, and used online videos to verify the observations.

Scientists then integrated this data with details on the evolutionary relationships between living and extinct species of such primates.

Evolutionary Timeline

Researchers propose the results indicate intimate contact developed somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of Neanderthals on this evolutionary lineage suggests it is likely they, too, engaged in a kiss, the scientists conclude. But the behavior may not have been limited to their own species.

"Reality that humans engage intimately, the fact that we currently have demonstrated that Neanderthals probably engaged, indicates that the two [species] are also likely to have kissed," the researcher noted.

Biological Significance

While the scientific reasoning is discussed, the expert said kissing could be used in sexual contexts to potentially increase reproductive success or assist in selecting between partners, while it could assist strengthen connections when practiced in a platonic way.

A separate researcher in the behavior of primates said that as intimate contact was observed in a wide range of primates it made sense its origins extend far into our evolutionary past, and an analysis of different forms of kissing among a wider variety of species might push its origins back further still.

"Behaviors that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we look closely at different species," the expert noted.

Cultural Aspects

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

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and methods of encouraging trust and closeness will have been important for millions of years," the professor stated. "It might be an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be expected that Neanderthals – and even Neanderthals and our human ancestors together – engaged intimately."