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From Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists suggest that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Common Microbial Evidence

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were closely connected. In earlier research, scientists have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with research that has found people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Interpretation

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

Writing in the publication Evolution and Human Behavior, the researcher and her team report how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how people smooch.

Defining Kissing

"Previously there were some efforts to define a intimate act, but it's largely focused on humans, which means that basically other animals do not engage in this. Now we understand that they likely engage, it might just not look from what our intimate contact looks like," explained the evolutionary biologist.

However, she noted some actions that resembled kissing were something rather different – such as the chewing and food sharing, or "mouth contact", observed in aquatic species called certain marine animals.

Consequently the research group came up with a description of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the mouth but absence of nutrition.

Research Approach

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

Scientists then combined this data with details on the genetic connections between extant and ancient types of such animals.

Evolutionary Origins

The team propose the results indicate kissing evolved approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

The position of ancient hominins on this evolutionary lineage means it is probable they, too, indulged in a kiss, the researchers say. But the activity might not have been confined to their specific group.

"The fact that modern people engage intimately, the fact that we currently have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have engage," Brindle added.

Evolutionary Significance

While the evolutionary explanation is discussed, Brindle said kissing could be employed in sexual contexts to potentially increase reproductive success or help choose between mates, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of great apes commented that as kissing behavior was seen in a wide range of primates it was logical its origins extend far into our evolutionary past, and an examination of different forms of kissing among a wider variety of species might push its beginnings back even earlier still.

"Behaviors that we think of as characteristics of our species, like intimate contact, are not exclusive to us if we examine carefully at different species," 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.

"However, as humans we succeed or struggle on the quality of our emotional bonds, and ways of encouraging confidence and closeness will have been significant for millions of years," she said. "It might be an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be no surprise that Neanderthals – and including Neanderthals and our human ancestors collectively – engaged intimately."