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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species appear to kiss. Currently, researchers suggest that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Oral Evidence

This isn't the initial instance experts have suggested ancient relatives and early modern humans were intimately acquainted. In earlier research, researchers have discovered humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, explaining that the concept aligned with studies that has revealed people of certain genetic backgrounds have bits of ancient genetic material in their genome, demonstrating genetic mixing was at play.

Intimate Spin

"This offers a more romantic perspective on ancient interactions," the lead researcher said.

Publishing in the journal Evolution and Human Behavior, the researcher and her team detail how, to explore the historical roots of kissing, they first had to come up with a description that was not limited to how people smooch.

Describing Kissing

"There have been some efforts to define a kiss, but it's very much been human-centric, which implies that essentially non-human species don't kiss. Currently we know that they likely engage, it might just not look from what human kissing looks like," explained the evolutionary biologist.

However, she noted some actions that looked like intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in aquatic species known as certain marine animals.

As a result the team developed a definition of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the identical group, with some motion of the oral area but no transfer of food.

Study Approach

The lead researcher explained they concentrated on accounts of intimate behavior in primates from Africa and Asian regions, including bonobos, apes and great apes, and employed digital recordings to confirm the reports.

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

Historical Origins

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

Placement of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a intimate act, the researchers say. But the behavior may not have been limited to their own species.

"The fact that humans kiss, the reality that we now have demonstrated that Neanderthals probably kissed, indicates that the two [species] are probably did kissed," the researcher noted.

Evolutionary Importance

While the evolutionary explanation is debated, Brindle explained kissing could be used in reproductive situations to possibly enhance reproductive success or assist in selecting between partners, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the activities of primates said that as kissing behavior was seen in a broad spectrum of primates it made sense its origins extend far into our ancient history, and an examination of various types of kissing among a broader range of animals might extend its beginnings back even earlier still.

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

Social Elements

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

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and methods of promoting trust and closeness will have been important for millions of years," she said. "It might be an image that appears a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be no surprise that ancient hominins – and including them and our human ancestors collectively – engaged intimately."