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Among Galápagos albatrosses to polar bears, primates to orangutans, various animals appear to kiss. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Shared Oral Clues

It is not the first time experts have proposed Neanderthals and early modern humans were closely connected. Among previous studies, scientists have found modern people and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the concept aligned with research that has revealed people of non-African ancestry contain Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Intimate Interpretation

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

Writing in the publication 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 description that was not restricted by how people kiss.

Defining Intimate Contact

"There have been some previous attempts to define a intimate act, but it's very much been human-centric, which means that basically non-human species don't kiss. Currently we know that they probably do, it may appear different from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she noted some actions that resembled kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in fish called certain marine animals.

As a result the research group developed a description of intimate contact based on social behaviors involving directed oral interaction with a individual of the identical group, with some motion of the oral area but no transfer of food.

Study Approach

The lead researcher explained they focused on reports of kissing in non-human species from the African continent and Asian regions, including bonobos, apes and orangutans, and employed online videos to confirm the reports.

Scientists then integrated this information with details on the evolutionary relationships between living and ancient types of such animals.

Historical Timeline

The team say the results suggest intimate contact developed somewhere between 21.5m and 16.9 million years ago in the predecessors of the great primates.

Placement of ancient hominins 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.

"Reality that modern people engage intimately, the reality that we now have shown that Neanderthals probably engaged, suggests that the both groups are probably did kissed," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is discussed, the expert explained kissing could be used in sexual contexts to potentially enhance mating outcomes or help choose between mates, while it might help reinforce bonding when used in a platonic way.

Another expert in the behavior of great apes commented that as kissing behavior was observed in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an analysis of various types of intimate behavior among a wider variety of animals might push its beginnings back even earlier still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," he said.

Cultural Elements

An archaeology expert explained that intimate contact had a cultural element as it was not universal to all societies.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and methods of encouraging trust and intimacy will have been important for millions of years," the professor stated. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it ought to be no surprise that Neanderthals – and including Neanderthals and our own species collectively – engaged intimately."