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From Galápagos albatrosses to polar bears, primates to great apes, certain species appear to kiss. Now, researchers suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Oral Evidence

It is not the first time experts have suggested ancient relatives and Homo sapiens were closely connected. Among previous studies, scientists have discovered humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were kissing," she said, adding that the idea aligned with research that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genome, demonstrating interbreeding was occurring.

Intimate Spin

"It certainly puts a different spin on human-Neanderthal relations," the lead researcher said.

Writing in the publication Evolution and Human Behavior, the researcher 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 kiss.

Defining Kissing

"There have been some previous attempts to describe a intimate act, but it's very much been focused on humans, which implies that essentially other animals do not engage in this. Now we know that they likely engage, it may appear different from what human kissing looks like," explained the evolutionary biologist.

However, she said some actions that looked like kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species known as certain marine animals.

Consequently the research group came up with a definition of intimate contact based on friendly interactions involving intentional oral interaction with a individual of the same species, with some movement of the oral area but absence of nutrition.

Study Approach

The lead researcher said they focused on accounts of intimate behavior in non-human species from Africa and Asia, including bonobos, apes and orangutans, and employed online videos to confirm the observations.

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

Historical Origins

Researchers say the findings indicate intimate contact evolved approximately 21.5m and 16.9m years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a kiss, the scientists say. But the behavior might not have been confined to their own species.

"Reality that humans engage intimately, the fact that we now have shown that ancient relatives probably kissed, indicates that the two [species] are also likely to have engage," Brindle noted.

Biological Significance

Although the scientific reasoning is debated, Brindle explained intimate contact could be employed in sexual contexts to possibly enhance mating outcomes or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of great apes said that as kissing behavior was seen in a broad spectrum of primates it made sense its origins lie deep in our evolutionary past, and an examination of various types of intimate behavior among a broader range of animals might push its beginnings back further still.

"Things that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we examine carefully at different species," the expert noted.

Social Elements

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

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and methods of encouraging trust and closeness will have been important for eons," the professor stated. "This could represent an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be expected that ancient hominins – and including them and our human ancestors collectively – kissed."