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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals also engaged in this behavior – and might even have locked lips with modern humans.

Common Microbial Evidence

It is not the first time experts have suggested Neanderthals and early modern humans were intimately acquainted. Among earlier research, scientists have found modern people and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea aligned with research that has found people of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Spin

"This offers a different perspective on human-Neanderthal relations," Brindle said.

Writing in the journal Evolution and Human Behavior, Brindle and colleagues detail 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 kiss.

Describing Kissing

"Previously there were some efforts to define a kiss, but it's very much been human-centric, which means that essentially non-human species do not engage in this. Currently we know that they probably do, it may appear different from what human kissing resembles," explained Brindle.

Nonetheless, she noted some actions that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in fish called French grunts.

Consequently the team developed a description of kissing centered around social behaviors involving directed oral interaction with a individual of the same species, with some motion of the mouth but no transfer of food.

Study Approach

Brindle explained they focused on accounts of kissing in non-human species from Africa and Asian regions, including bonobos, apes and orangutans, and employed online videos to verify the reports.

Scientists then combined this data with information on the evolutionary relationships between extant and extinct types of such primates.

Evolutionary Origins

The team propose the results indicate intimate contact evolved approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

The position of ancient hominins on this evolutionary lineage means it is likely they, too, indulged in a kiss, the scientists say. But the behavior may not have been limited to their own species.

"Reality that humans kiss, the reality that we now have shown that Neanderthals probably engaged, suggests that the both groups are probably did engage," Brindle added.

Biological Significance

Although the scientific reasoning is debated, Brindle said intimate contact could be employed in sexual contexts to possibly increase mating outcomes or help choose between partners, while it might help strengthen connections when used in a platonic way.

Another expert in the activities of primates commented that as kissing behavior was observed in a broad spectrum of primates it was logical its origins lie deep in our ancient history, and an examination of different forms of intimate behavior among a wider variety of animals might push its beginnings back even earlier still.

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

Social Aspects

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

"However, as humans we thrive or fail on the quality of our relationships, and ways of promoting confidence and closeness will have been significant for eons," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it should be no surprise that Neanderthals – and including Neanderthals and our own species collectively – engaged intimately."