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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, researchers propose that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Shared Oral Clues

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

"Probably they were engaging in intimate contact," the researcher noted, adding that the idea chimed with studies that has found humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a different spin on human-Neanderthal relations," Brindle commented.

Publishing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to come up with a description that was not limited to how people smooch.

Defining Kissing

"Previously there were some efforts to define a kiss, but it's very much been focused on humans, which implies that basically other animals don't kiss. Now we understand that they likely engage, it may appear different from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that resembled intimate contact were something rather different – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species known as French grunts.

As a result the research group came up with a definition of kissing centered around social behaviors involving intentional oral interaction with a individual of the identical group, with some motion of the mouth but no transfer of food.

Study Approach

Brindle explained they focused on accounts of intimate behavior in primates from Africa and Asia, including primates, apes and great apes, and used digital recordings to verify the reports.

Scientists then combined this data with information on the genetic connections between extant and ancient species of such primates.

Evolutionary Origins

Researchers propose the findings indicate kissing evolved approximately 21.5 million and 16.9 million years ago in the predecessors of the great primates.

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

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

Biological Significance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in sexual contexts to potentially increase mating outcomes or assist in selecting between partners, while it might help strengthen connections when used in a platonic way.

Another expert in the activities of great apes commented that as kissing behavior was observed in a broad spectrum of primates it made sense its roots lie deep in our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of animals might push its origins back further still.

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

Social Elements

Another professor said that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as people we succeed or struggle on the strength of our relationships, and ways of promoting confidence and closeness will have been significant for millions of years," the professor stated. "This could represent an image that appears a bit contradictory 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 together – engaged intimately."