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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers suggest 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 early modern humans were intimately acquainted. Among previous studies, scientists have found humans and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea chimed with research that has found people of non-African ancestry contain Neanderthal DNA in their genome, revealing 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, the researcher 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 kiss.

Describing Kissing

"Previously there were some efforts to define a intimate act, but it's very much been human-centric, which means that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that looked like intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in aquatic species called French grunts.

Consequently the research group developed a definition of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but absence of nutrition.

Research Approach

The lead researcher said they focused on reports of kissing in primates from Africa and Asia, including bonobos, apes and great apes, and employed online videos to confirm the reports.

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

Historical Timeline

Researchers propose the results indicate intimate contact evolved approximately 21.5m and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree suggests it is likely they, too, indulged in a kiss, the scientists say. But the behavior might not have been confined to their specific group.

"Reality that humans engage intimately, the reality that we currently have shown that Neanderthals very likely engaged, suggests that the two [species] are probably did kissed," Brindle added.

Biological Significance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be used in sexual contexts to potentially increase reproductive success or help choose between mates, while it could assist reinforce bonding when used in a platonic way.

Another expert in the behavior of primates said that as kissing behavior was seen in a broad spectrum of primates it made sense its origins extend far into our ancient history, and an examination of various types of intimate behavior among a wider variety of species might push its beginnings back even earlier still.

"Things that we consider as characteristics of human life, like intimate contact, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Aspects

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

"However, as people we succeed or struggle on the quality of our emotional bonds, and methods of promoting confidence and intimacy will have been important for eons," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it should be no surprise that ancient hominins – and including Neanderthals and our own species together – kissed."