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From Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Currently, scientists propose that Neanderthals also engaged in this behavior – and might even have locked lips with modern humans.

Common Oral Evidence

It is not the first time scientists have suggested Neanderthals and Homo sapiens were closely connected. Among previous studies, scientists have discovered humans and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the concept chimed with studies that has revealed humans of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Interpretation

"It certainly puts a different spin on ancient interactions," the lead researcher said.

Writing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how humans smooch.

Defining Kissing

"Previously there were some efforts to describe a intimate act, but it's largely focused on humans, which means that essentially other animals do not engage in this. Now we understand that they probably do, it might just not look from what human kissing looks like," explained Brindle.

Nonetheless, she said some behaviors that resembled intimate contact were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species called French grunts.

Consequently the team came up with a description of intimate contact based on social behaviors involving intentional oral interaction with a member of the same species, with some movement of the oral area but no transfer of nutrition.

Study Methods

The lead researcher explained they focused on reports of intimate behavior in non-human species from the African continent and Asian regions, including primates, apes and great apes, and employed online videos to verify the reports.

The researchers then combined this data with details on the genetic connections between extant and ancient types of such animals.

Historical Timeline

Researchers say the findings suggest intimate contact evolved approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

The position of ancient hominins on this evolutionary lineage means it is likely they, too, indulged in a kiss, the researchers say. But the activity might not have been limited to their specific group.

"The fact that humans kiss, the reality that we currently have demonstrated that Neanderthals very likely engaged, indicates that the two [species] are probably did kissed," Brindle added.

Biological Significance

While the evolutionary explanation is discussed, Brindle explained kissing could be used in sexual contexts to possibly increase reproductive success or help choose between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

Another expert in the behavior of great apes commented that as kissing behavior was observed in a wide range of primates it was logical its origins lie deep in our evolutionary past, and an examination of various types of kissing among a wider variety of animals might extend its origins back further still.

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

Social Elements

Another professor said that kissing had a social component as it was not universal to all human groups.

"However, as humans we succeed or struggle on the quality of our relationships, and ways of promoting confidence and closeness will have been important for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be no surprise that ancient hominins – and even them and our own species together – engaged intimately."