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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins did it too – and possibly locked lips with modern humans.

Shared Microbial Clues

It is not the first time experts have suggested Neanderthals and early modern humans were intimately acquainted. In previous studies, researchers have discovered modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with studies that has revealed people of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Romantic Spin

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle commented.

Publishing in the publication a scientific periodical, the researcher and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a description that was not restricted by how humans smooch.

Defining Intimate Contact

"There have been some efforts to describe a kiss, but it's very much been focused on humans, which implies that essentially other animals don't kiss. Now we know that they probably do, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

Nonetheless, she said some behaviors that resembled intimate contact were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species called French grunts.

Consequently the research group came up with a definition of kissing based on social behaviors involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but no transfer of food.

Research Methods

The lead researcher explained they concentrated on accounts of intimate behavior in primates from the African continent and Asian regions, including bonobos, apes and great apes, and used online videos to verify the reports.

The researchers then integrated this information with details on the genetic connections between living and extinct species of such primates.

Evolutionary Timeline

Researchers propose the results suggest intimate contact developed somewhere between 21.5 million and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this family tree means it is likely they, too, engaged in a kiss, the scientists say. But the activity might not have been limited to their specific group.

"Reality that modern people kiss, the reality that we currently have shown that ancient relatives probably engaged, indicates that the two [species] are also likely to have engage," Brindle added.

Evolutionary Significance

Although the evolutionary explanation is debated, Brindle explained intimate contact could be used in sexual contexts to possibly increase mating outcomes or help choose between partners, while it could assist reinforce bonding when used in a platonic way.

Another expert in the behavior of great apes commented that as kissing behavior was observed in a wide range of primates it was logical its roots extend far into our evolutionary past, and an analysis of various types of intimate behavior among a broader range of animals might push its origins back further still.

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

Cultural Aspects

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

"Nonetheless, as people we thrive or fail on the quality of our emotional bonds, and ways of promoting confidence and closeness will have been significant for millions of years," she said. "This could represent an image that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be no surprise that Neanderthals – and including them and our human ancestors collectively – kissed."