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From seabirds to polar bears, primates to orangutans, various animals appear to kiss. Now, researchers propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were closely connected. Among 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, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," she said, adding that the concept chimed with studies that has revealed people of non-African ancestry contain Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Spin

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

Publishing in the publication a scientific periodical, Brindle and colleagues report how, to investigate the historical roots of intimate contact, they first had to develop a description that was not restricted by how humans kiss.

Defining Kissing

"Previously there were some efforts to describe a kiss, but it's largely focused on humans, which means that essentially non-human species don't kiss. Now we understand that they likely engage, it might just not look from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in fish called French grunts.

As a result the research group developed a definition of kissing based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but no transfer of nutrition.

Research Approach

The lead researcher said they focused on reports of intimate behavior in primates from Africa and Asia, including bonobos, apes and orangutans, and employed online videos to confirm the reports.

Scientists then combined this data with information on the evolutionary relationships between extant and extinct species of such animals.

Evolutionary Timeline

Researchers say the findings suggest intimate contact developed approximately 21.5m and 16.9 million years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree means it is probable they, too, engaged in a intimate act, the scientists say. But the behavior might not have been confined to their own species.

"The fact that modern people kiss, the reality that we now have shown that ancient relatives probably engaged, suggests that the both groups are probably did kissed," Brindle added.

Biological Importance

Although the scientific reasoning is discussed, the expert said kissing could be used in reproductive situations to possibly increase reproductive success or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the behavior of great apes said that as kissing behavior was observed in a broad spectrum of primates it was logical its roots lie deep in our evolutionary past, and an examination of different forms of kissing among a wider variety of animals might extend its origins back further still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not exclusive to us if we look closely at other animals," he said.

Cultural Aspects

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

"Nonetheless, as people we thrive or fail on the quality of our relationships, and methods of encouraging confidence and intimacy will have been significant for millions of years," the professor stated. "It might be an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be no surprise that ancient hominins – and including Neanderthals and our own species collectively – kissed."