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From Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Now, researchers propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were intimately acquainted. Among previous studies, scientists have discovered modern people and their thick-browed cousins shared 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 idea aligned with research that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Romantic Spin

"This offers a more romantic spin on ancient interactions," Brindle said.

Publishing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how people smooch.

Describing Kissing

"Previously there were some efforts to describe a intimate act, but it's very much been focused on humans, which means that essentially non-human species don't kiss. Now we understand that they probably do, it may appear different from what human kissing resembles," said Brindle.

However, she noted some behaviors that looked like kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in aquatic species known as French grunts.

Consequently the research group developed a definition of kissing centered around social behaviors involving intentional oral interaction with a member of the same species, with some movement of the mouth but absence of nutrition.

Study Methods

Brindle said they focused on accounts of intimate behavior in non-human species from Africa and Asia, including primates, chimpanzees and orangutans, and employed online videos to verify the observations.

Scientists then integrated this information with information on the evolutionary relationships between living and ancient species of such primates.

Evolutionary Timeline

The team say the results indicate intimate contact evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this evolutionary lineage suggests it is likely they, too, engaged in a kiss, the scientists conclude. But the activity may not have been limited to their own species.

"The fact that modern people kiss, the reality that we currently have shown that Neanderthals very likely kissed, indicates that the both groups are also likely to have kissed," Brindle noted.

Evolutionary Significance

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

A separate researcher in the behavior of primates commented that as kissing behavior was seen in a broad spectrum of apes it was logical its origins extend far into our ancient history, and an examination of different forms of kissing among a wider variety of animals might extend its beginnings back further still.

"Behaviors that we think of as characteristics of human life, like kissing, are not unique to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

An archaeology expert explained that intimate contact had a social component as it was not common to all societies.

"Nonetheless, as humans we thrive or fail on the quality of our relationships, and methods of encouraging confidence and closeness will have been significant for millions of years," she said. "It might be an image that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it should be expected that ancient hominins – and even them and our human ancestors collectively – kissed."