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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Evidence

This isn't the initial instance experts have proposed ancient relatives and Homo sapiens were closely connected. Among previous studies, researchers have found modern people and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the concept chimed with studies that has found humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," the lead researcher commented.

Writing in the publication Evolution and Human Behavior, the researcher and colleagues detail how, to explore the historical roots of kissing, they first had to come up with a description that was not limited to how humans kiss.

Defining Intimate Contact

"Previously there were some efforts to describe a kiss, but it's largely human-centric, which implies that essentially non-human species don't kiss. Currently we understand that they likely engage, it might just not look from what human kissing looks like," explained Brindle.

However, she noted some actions that looked like intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in fish called French grunts.

Consequently the team came up with a description of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a member of the same species, with some motion of the oral area but absence of food.

Study Methods

The lead researcher explained they concentrated on reports of intimate behavior in non-human species from Africa and Asia, including bonobos, chimpanzees and orangutans, and employed digital recordings to confirm the reports.

The researchers then combined this data with details on the evolutionary relationships between extant and ancient species of such animals.

Historical Origins

The team propose the findings suggest intimate contact developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree suggests it is probable they, too, indulged in a kiss, the scientists conclude. But the behavior might not have been limited to their specific group.

"Reality that humans engage intimately, the reality that we now have shown that Neanderthals probably kissed, indicates that the two [species] are also likely to have engage," Brindle added.

Evolutionary Importance

While the evolutionary explanation is debated, the expert explained kissing could be employed in sexual contexts to possibly increase mating outcomes or help choose between mates, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was observed in a wide range of primates it was logical its origins lie deep in our evolutionary past, and an examination of different forms of intimate behavior among a broader range of species might extend its origins back further still.

"Things that we consider as characteristics of human life, like kissing, are not exclusive to us if we look closely at different species," he said.

Social Aspects

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

"However, as people we thrive or fail on the strength of our emotional bonds, and ways of promoting confidence and closeness will have been significant for millions of years," the professor stated. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it should be expected that Neanderthals – and including Neanderthals and our human ancestors collectively – engaged intimately."