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

Shared Oral Clues

It is not the first time experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In earlier research, scientists have discovered humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," she said, adding that the concept aligned with studies that has found people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Intimate Spin

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

Writing in the publication a scientific periodical, Brindle and her team detail how, to explore the historical roots of kissing, they first had to develop a definition that was not restricted by how humans kiss.

Describing Kissing

"There have been some efforts to define a kiss, but it's largely focused on humans, which means that basically other animals don't kiss. Now we know that they probably do, it might just not look from what our intimate contact looks like," explained Brindle.

However, she noted some actions that resembled kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", seen in fish called French grunts.

As a result the team developed a description of kissing based on social behaviors involving directed oral interaction with a individual of the same species, with some movement of the oral area but absence of food.

Research Approach

Brindle explained they concentrated on accounts of kissing in non-human species from the African continent and Asian regions, including bonobos, apes and great apes, and used digital recordings to confirm the observations.

The researchers then combined this data with details on the evolutionary relationships between extant and ancient types of such primates.

Evolutionary Timeline

Researchers propose the findings suggest intimate contact evolved somewhere between 21.5m and 16.9m years ago in the predecessors of the great primates.

The position of ancient hominins on this evolutionary lineage means it is likely they, too, engaged in a intimate act, the scientists say. But the activity may not have been limited to their own species.

"Reality that humans engage intimately, the reality that we now have shown that Neanderthals probably kissed, indicates that the two [species] are also likely to have kissed," the researcher added.

Biological Significance

While the scientific reasoning is debated, Brindle said kissing could be used in sexual contexts to potentially increase reproductive success or assist in selecting between mates, while it might help strengthen connections when practiced in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was observed in a broad spectrum of primates it was logical its roots lie deep in our ancient history, and an analysis of different forms of kissing among a wider variety of animals might extend its origins back further still.

"Things that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we look closely at other animals," he said.

Cultural Aspects

Another professor explained that intimate contact had a social component as it was not universal to all human groups.

"However, as people we thrive or fail on the quality of our relationships, and methods of promoting confidence and closeness will have been important for millions of years," she said. "It might be an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be no surprise that ancient hominins – and including them and our human ancestors together – kissed."