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From seabirds to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, researchers suggest that Neanderthals did it too – and possibly exchanged kisses with modern humans.

Shared Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were closely connected. In previous studies, scientists have discovered modern people and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the idea chimed with research that has found humans of non-African ancestry contain Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Intimate Interpretation

"It certainly puts a different perspective on human-Neanderthal relations," Brindle said.

Writing in the publication a scientific periodical, Brindle and her team detail how, to explore the historical roots of intimate contact, they first had to come up with a description that was not restricted by how people smooch.

Describing Intimate Contact

"There have been some efforts to define a intimate act, but it's largely human-centric, which implies that basically non-human species do not engage in this. Currently we understand that they probably do, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

However, she said some behaviors that resembled kissing were distinct activities – such as the chewing and food sharing, or "mouth contact", observed in aquatic species known as French grunts.

As a result the research group came up with a description of kissing based on friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but no transfer of nutrition.

Study Approach

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

The researchers then combined this data with details on the genetic connections between living and extinct types of such animals.

Evolutionary Origins

Researchers say the results suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the scientists conclude. But the activity might not have been limited to their own species.

"The fact that humans kiss, the fact that we currently have demonstrated that Neanderthals very likely engaged, indicates that the two [species] are probably did engage," Brindle noted.

Evolutionary Importance

While the evolutionary explanation is debated, the expert 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 non-sexual manner.

A separate researcher in the behavior of primates commented that as intimate contact was observed in a broad spectrum of primates it was logical its roots lie deep in our ancient history, and an analysis of various types of kissing among a broader range of animals might extend its origins back even earlier still.

"Things that we think of as characteristics of human life, like kissing, are not unique to us if we look closely at other animals," he said.

Social Elements

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

"However, as humans we succeed or struggle on the strength of our emotional bonds, and methods of encouraging trust and closeness will have been important for millions of years," she said. "It might be an concept that appears a bit incongruous to our misplaced ideas 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 together – engaged intimately."