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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins did it too – and might even have locked lips with early Homo sapiens.

Common Microbial Clues

It is not the first time experts have suggested Neanderthals and early modern humans were closely connected. Among previous studies, scientists have discovered humans and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea aligned with studies that has revealed people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Intimate Spin

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

Publishing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to investigate the evolutionary origins of kissing, they first had to develop a description that was not limited to how humans kiss.

Defining Intimate Contact

"There have been some previous attempts to define a intimate act, but it's largely human-centric, which means that basically non-human species don't kiss. Currently we understand that they probably do, it may appear different from what our intimate contact resembles," said the evolutionary biologist.

However, she said some actions that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in aquatic species known as certain marine animals.

As a result the research group came up with a definition of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but absence of food.

Study Methods

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

The researchers then combined this information with information on the evolutionary relationships between extant and ancient species of such primates.

Evolutionary Timeline

Researchers say the results suggest intimate contact developed approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a intimate act, the scientists say. But the activity may not have been confined to their specific group.

"Reality that humans kiss, the fact that we now have shown that ancient relatives probably kissed, indicates that the both groups are also likely to have kissed," Brindle noted.

Biological Importance

While the evolutionary explanation is discussed, the expert said kissing could be used in sexual contexts to potentially increase reproductive success or help choose between mates, while it might help strengthen connections when used in a platonic way.

A separate researcher in the activities of primates commented that as intimate contact was observed in a broad spectrum of apes it was logical its roots extend far into our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of animals might push its origins back even earlier still.

"Behaviors that we consider as signatures of human life, like kissing, are not unique to us if we look closely at different species," he said.

Social Elements

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

"However, as humans 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 concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be expected that Neanderthals – and including Neanderthals and our human ancestors together – engaged intimately."