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Among seabirds to Arctic mammals, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Shared Microbial Clues

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have found modern people and their thick-browed cousins possessed the identical oral bacteria for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," the researcher noted, explaining that the idea chimed with research that has found people of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating genetic mixing was at play.

Romantic Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle said.

Writing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how humans kiss.

Defining Kissing

"There have been some efforts to describe a kiss, but it's very much been focused on humans, which implies that basically other animals do not engage in this. Now we understand that they probably do, it might just not look from what human kissing looks like," said Brindle.

However, she said some behaviors that looked like intimate contact were distinct activities – such as the chewing and food sharing, or "mouth contact", observed in aquatic species known as certain marine animals.

As a result the research group developed a description of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but no transfer of food.

Research Approach

Brindle explained they focused on reports of intimate behavior in primates from the African continent and Asian regions, including bonobos, apes and great apes, and used digital recordings to verify the observations.

Scientists then combined this data with details on the genetic connections between living and ancient species of such animals.

Evolutionary Origins

The team propose the results indicate kissing developed approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the researchers say. But the activity may not have been confined to their own species.

"Reality that modern people engage intimately, the fact that we now have shown that Neanderthals very likely kissed, indicates that the two [species] are probably did kissed," the researcher added.

Evolutionary Importance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be employed in reproductive situations to potentially enhance mating outcomes or assist in selecting between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of great apes said that as kissing behavior was observed in a wide range of primates it was logical its origins extend far into our evolutionary past, and an analysis of different forms of kissing among a broader range of species might extend its beginnings back even earlier still.

"Behaviors that we think of as characteristics of our species, like kissing, are not exclusive to us if we examine carefully at different species," he said.

Social Elements

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of encouraging trust and intimacy will have been important for millions of years," she said. "It might be an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it ought to be expected that Neanderthals – and even them and our own species collectively – kissed."