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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Oral Clues

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. In previous studies, researchers have discovered humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea chimed with studies that has found people of non-African ancestry have bits of ancient genetic material in their genome, demonstrating interbreeding was at play.

Intimate Spin

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

Writing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to investigate the historical roots of kissing, they first had to come up with a definition that was not limited to how humans smooch.

Defining Intimate Contact

"There have been some previous attempts to describe a kiss, but it's largely focused on humans, which means that essentially other animals don't kiss. Currently we understand that they probably do, it may appear different from what human kissing resembles," said the evolutionary biologist.

However, she said some behaviors that looked like intimate contact were distinct activities – such as the chewing and food sharing, or "mouth contact", observed in aquatic species known as certain marine animals.

Consequently the team came up with a definition of intimate contact based on social behaviors involving intentional oral interaction with a individual of the identical group, with some movement of the mouth but absence of nutrition.

Research Methods

Brindle said they concentrated on reports of intimate behavior in primates from Africa and Asia, including bonobos, apes and orangutans, and employed digital recordings to confirm the observations.

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

Evolutionary Origins

The team propose the results indicate intimate contact developed approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the scientists conclude. But the activity might not have been limited to their own species.

"The fact that humans kiss, the reality that we currently have demonstrated that Neanderthals very likely kissed, suggests that the both groups are also likely to have kissed," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is debated, Brindle said kissing could be employed in sexual contexts to possibly increase mating outcomes or help choose between partners, while it could assist strengthen connections when used in a platonic way.

A separate researcher in the activities of great apes commented that as intimate contact was observed in a broad spectrum of primates it was logical its origins extend far into our evolutionary past, and an examination of various types of kissing among a wider variety of animals might extend its origins back further still.

"Behaviors that we consider as signatures of human life, like kissing, are not unique to us if we look closely at other animals," the expert noted.

Social Elements

Another professor explained that intimate contact had a social component as it was not common to all societies.

"However, as people we thrive or fail on the strength of our emotional bonds, and ways of encouraging trust and intimacy will have been important for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but really it ought to be expected that ancient hominins – and including Neanderthals and our human ancestors collectively – engaged intimately."