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Among seabirds to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals did it too – and possibly locked lips with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, scientists have discovered humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," she said, adding that the idea aligned with studies that has found humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Interpretation

"This offers a more romantic spin on human-Neanderthal relations," Brindle said.

Publishing in the publication a scientific periodical, Brindle and colleagues report how, to investigate the historical roots of intimate contact, they first had to develop a description that was not restricted by how humans kiss.

Defining Kissing

"There have been some efforts to define a intimate act, but it's very much been focused on humans, which means that essentially non-human species do not engage in this. Currently we understand that they likely engage, it may appear different from what human kissing looks like," explained the evolutionary biologist.

However, she said some actions that resembled kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", observed in aquatic species known as French grunts.

As a result the research group developed a description of kissing centered around social behaviors involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but no transfer of nutrition.

Research Methods

Brindle said they focused on reports of kissing in primates from the African continent and Asian regions, including bonobos, apes and orangutans, and used online videos to confirm the observations.

The researchers then integrated this information with information on the evolutionary relationships between extant and extinct types of such animals.

Historical Timeline

The team propose the findings suggest intimate contact developed somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

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

"Reality that humans kiss, the fact that we now have shown that ancient relatives very likely engaged, indicates that the both groups are probably did kissed," the researcher added.

Evolutionary Significance

While the scientific reasoning is debated, Brindle explained kissing could be employed in reproductive situations to potentially increase reproductive success or assist in selecting between partners, while it might help strengthen connections when used in a platonic way.

Another expert 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 ancient history, and an examination of various types of kissing among a broader range of species might extend its origins back further still.

"Things that we consider as signatures of our species, like intimate contact, are not unique to us if we look closely at different species," he said.

Cultural Aspects

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

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and methods of promoting confidence and intimacy will have been significant for millions of years," she said. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it should be expected that Neanderthals – and even Neanderthals and our human ancestors collectively – kissed."