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From Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species appear to kiss. Now, researchers propose that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Oral Clues

This isn't the initial instance experts have suggested ancient relatives and early modern humans were closely connected. In earlier research, researchers have found modern people and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the idea chimed with studies that has found people of certain genetic backgrounds contain ancient genetic material in their genome, revealing genetic mixing was at play.

Romantic Interpretation

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

Writing in the journal a scientific periodical, Brindle and her team report how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some previous attempts to define a kiss, but it's largely human-centric, which implies that essentially other animals don't kiss. Now we understand that they probably do, it might just not look from what our intimate contact resembles," explained Brindle.

However, she said some behaviors that looked like kissing were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in fish known as French grunts.

As a result the research group came up with a description of intimate contact centered around social behaviors involving directed oral interaction with a individual of the same species, with some motion of the mouth but absence of food.

Study Approach

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

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

Evolutionary Timeline

Researchers say the results suggest intimate contact evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a intimate act, the scientists conclude. But the behavior may not have been confined to their own species.

"The fact that humans kiss, the reality that we now have shown that Neanderthals very likely engaged, indicates that the both groups are probably did kissed," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle said intimate contact could be used in sexual contexts to possibly enhance mating outcomes or assist in selecting between mates, while it could assist strengthen connections when used in a platonic way.

Another expert in the activities 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 ancient history, and an analysis of different forms of intimate behavior among a broader range of species might extend its origins back further still.

"Things that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we look closely at other animals," he said.

Cultural Aspects

Another professor explained that intimate contact had a cultural element as it was not common to all societies.

"However, as humans we thrive or fail on the strength of our emotional bonds, and ways of promoting trust and intimacy will have been significant for eons," the professor stated. "This could represent an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be no surprise that Neanderthals – and even Neanderthals and our human ancestors together – kissed."