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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Common Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were closely connected. Among earlier research, researchers have discovered modern people and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were kissing," she said, adding that the concept chimed with studies that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Intimate Spin

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

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how people smooch.

Defining Intimate Contact

"There have been some previous attempts to describe a kiss, but it's very much been human-centric, which means that essentially non-human species do not engage in this. Currently we understand that they likely engage, it might just not look from what human kissing resembles," explained Brindle.

Nonetheless, she said some actions that looked like intimate contact were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species called French grunts.

As a result the research group developed a description of kissing centered around social behaviors involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but absence of food.

Study Approach

The lead researcher explained they focused on accounts of kissing in primates from the African continent and Asia, including bonobos, apes and great apes, and used online videos to verify the observations.

Scientists then combined this data with information on the genetic connections between extant and extinct types of such animals.

Evolutionary Origins

Researchers propose the findings suggest kissing developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the researchers say. But the behavior may not have been confined to their own species.

"Reality that modern people kiss, the fact that we now have demonstrated that ancient relatives probably kissed, suggests that the two [species] are also likely to have kissed," Brindle noted.

Biological Significance

Although the scientific reasoning is debated, Brindle said intimate contact could be employed in reproductive situations to potentially increase mating outcomes or assist in selecting between mates, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the behavior of great apes commented that as intimate contact was seen in a wide range of primates it was logical its roots extend far into our evolutionary past, and an analysis of different forms of kissing among a wider variety of animals might extend its beginnings back even earlier still.

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

Cultural Aspects

An archaeology expert said 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 quality of our emotional bonds, and ways of promoting confidence and closeness will have been important for millions of years," the professor stated. "This could represent an image that appears a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it ought to be expected that ancient hominins – and even them and our human ancestors collectively – engaged intimately."