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From Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Common Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have discovered modern people and their Neanderthal relatives 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 concept chimed with research that has found humans of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating genetic mixing was occurring.

Romantic Spin

"It certainly puts a more romantic spin on ancient interactions," the lead researcher said.

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

Describing Kissing

"There have been some previous attempts to define a kiss, but it's very much been human-centric, which means that basically non-human species don't kiss. Currently we know that they probably do, it may appear different from what human kissing resembles," said Brindle.

Nonetheless, she said some actions that looked like kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", seen in fish known as French grunts.

Consequently the team developed a description of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the oral area but no transfer of food.

Research Methods

Brindle said they focused on accounts of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and used digital recordings to verify the observations.

The researchers then integrated this information with information on the genetic connections between living and ancient types of such animals.

Historical Timeline

Researchers say the findings indicate kissing evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the large apes.

Placement of Neanderthals on this family tree means it is likely they, too, engaged in a intimate act, the researchers say. But the activity may not have been limited to their own species.

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

Biological Significance

Although the evolutionary explanation is debated, the expert explained intimate contact could be used in reproductive situations to potentially 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 behavior of primates said that as intimate contact was observed in a broad spectrum of apes it made sense its roots lie deep in our evolutionary past, and an analysis of various types of kissing among a broader range of animals might extend its origins back even earlier still.

"Behaviors that we think of as signatures of human life, like intimate contact, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Elements

An archaeology expert explained that intimate contact had a cultural element as it was not universal to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and methods of encouraging trust and intimacy will have been significant for eons," she said. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be no surprise that Neanderthals – and including them and our own species together – engaged intimately."