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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Currently, scientists propose that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Evidence

This isn't the initial instance experts have proposed Neanderthals and Homo sapiens were closely connected. Among previous studies, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were kissing," she said, explaining that the concept chimed with studies that has revealed people of non-African ancestry contain Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Interpretation

"This offers a more romantic perspective on human-Neanderthal relations," Brindle commented.

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues detail how, to explore the evolutionary origins of kissing, they first had to develop a description that was not restricted by how humans smooch.

Defining Kissing

"There have been some efforts to define a intimate act, but it's largely focused on humans, which implies that basically non-human species don't kiss. Now we know that they probably do, it might just not look from what our intimate contact looks like," said Brindle.

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

As a result the research group developed a definition of intimate contact based on social behaviors involving directed oral interaction with a individual of the same species, with some motion of the mouth but no transfer of nutrition.

Research Approach

Brindle said they focused on reports of kissing in non-human species from the African continent and Asian regions, including bonobos, chimpanzees and great apes, and employed online videos to verify the reports.

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

Historical Origins

Researchers propose the findings indicate kissing developed approximately 21.5m and 16.9m years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the scientists say. But the behavior might not have been limited to their own species.

"Reality that modern people kiss, the fact that we now have shown that ancient relatives very likely kissed, indicates that the two [species] are probably did engage," the researcher noted.

Biological Importance

While the evolutionary explanation is debated, the expert explained intimate contact could be employed in reproductive situations to potentially increase mating outcomes or assist in selecting between partners, while it could assist reinforce bonding when used in a non-sexual manner.

Another expert in the activities of primates commented that as kissing behavior was seen in a broad spectrum of primates it was logical its roots lie deep in our evolutionary past, and an examination of various types of intimate behavior among a wider variety of species might push its beginnings back further still.

"Behaviors that we consider as signatures of human life, like intimate contact, are not unique to us if we examine carefully at different species," he said.

Cultural Aspects

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

"However, as humans we succeed or struggle on the quality of our relationships, and ways of encouraging confidence and intimacy will have been important for eons," the professor stated. "This could represent an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be expected that ancient hominins – and including them and our own species together – kissed."