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Among seabirds to Arctic mammals, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Oral Evidence

It is not the first time scientists have suggested ancient relatives and early modern humans were closely connected. Among earlier research, scientists have found humans and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," she said, adding that the idea aligned with studies that has found humans of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Romantic Spin

"It certainly puts a more romantic perspective on ancient interactions," the lead researcher commented.

Publishing in the publication Evolution and Human Behavior, the researcher and her team detail how, to explore the historical roots of kissing, they first had to come up with a definition that was not restricted by how humans kiss.

Defining Kissing

"Previously there were some previous attempts to define a kiss, but it's very much been focused on humans, which implies that basically non-human species do not engage in this. Now we know that they probably do, it might just not look from what our intimate contact looks like," said the evolutionary biologist.

However, she said some behaviors that looked like intimate contact were distinct activities – such as the processing and food sharing, or "kiss-fighting", seen in aquatic species known as French grunts.

As a result the team came up with a definition of kissing based on social behaviors involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but no transfer of food.

Study Methods

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

Scientists then integrated this data with details on the genetic connections between extant and extinct species of such primates.

Evolutionary Timeline

Researchers say the results suggest intimate contact evolved approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

Placement of Neanderthals on this family tree means it is likely they, too, indulged in a intimate act, the scientists say. But the activity may not have been confined to their specific group.

"The fact that modern people engage intimately, the reality that we currently have shown that Neanderthals very likely engaged, suggests that the two [species] are probably did kissed," the researcher added.

Biological Importance

While the evolutionary explanation is debated, Brindle said intimate contact could be used in sexual contexts to potentially increase mating outcomes or assist in selecting 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 seen in a wide range of primates it made sense its roots lie deep in our ancient history, and an examination of different forms of kissing among a wider variety of species might push its origins back further still.

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

Cultural Aspects

An archaeology expert said that kissing had a cultural element as it was not universal to all societies.

"However, as people we succeed or struggle on the quality of our relationships, and ways of encouraging trust and intimacy will have been important for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be no surprise that ancient hominins – and including Neanderthals and our human ancestors together – engaged intimately."