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Among Galápagos albatrosses to polar bears, primates to great apes, certain species engage in mouth-to-mouth contact. Now, researchers suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

It is not the first time experts have proposed Neanderthals and early modern humans were closely connected. Among earlier research, researchers have found modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," she said, explaining that the concept aligned with research that has revealed humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Interpretation

"This offers a different spin on human-Neanderthal relations," Brindle commented.

Writing in the publication Evolution and Human Behavior, the researcher and her team report how, to investigate the historical roots of kissing, they first had to come up with a definition that was not restricted by how people kiss.

Describing Kissing

"There have been some efforts to define a kiss, but it's very much been focused on humans, which means that basically non-human species do not engage in this. Currently we know that they probably do, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

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

Consequently the research group came up with a definition of intimate contact based on friendly interactions involving directed oral interaction with a member of the same species, with some motion of the mouth but no transfer of food.

Research Methods

Brindle explained they focused on accounts of intimate behavior in primates from Africa and Asian regions, including bonobos, apes and orangutans, and employed online videos to verify the reports.

Scientists then integrated this information with details on the evolutionary relationships between living and ancient types of such animals.

Evolutionary Origins

The team propose the findings indicate intimate contact developed somewhere between 21.5m and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals on this family tree means it is likely they, too, engaged in a kiss, the researchers conclude. But the behavior might not have been confined to their own species.

"Reality that modern people engage intimately, the reality that we now have demonstrated that ancient relatives probably kissed, indicates that the two [species] are probably did kissed," Brindle added.

Evolutionary Significance

While the evolutionary explanation is discussed, Brindle said intimate contact could be used in sexual contexts to potentially enhance reproductive success or help choose between partners, while it could assist strengthen connections when used in a platonic way.

A separate researcher in the activities of primates commented that as kissing behavior was seen in a broad spectrum of primates it was logical its origins extend far into our ancient history, and an examination of different forms of kissing among a wider variety of species might extend its beginnings back even earlier still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not exclusive to us if we look closely at other animals," the expert noted.

Cultural Aspects

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

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and ways of promoting confidence and closeness will have been important for millions of years," she said. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be no surprise that ancient hominins – and including Neanderthals and our human ancestors collectively – kissed."