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Among seabirds to Arctic mammals, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Clues

This isn't the initial instance experts have proposed ancient relatives and early modern humans were intimately acquainted. In earlier research, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were kissing," the researcher noted, explaining that the idea chimed with research that has found people of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, demonstrating interbreeding was at play.

Romantic Spin

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

Writing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to explore the evolutionary origins of kissing, they first had to develop a description that was not limited to how people smooch.

Describing Kissing

"Previously there were some efforts to describe a intimate act, but it's largely focused on humans, which implies that essentially non-human species do not engage in this. Now we know that they likely engage, it may appear different from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she noted some behaviors that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species called French grunts.

As a result the team developed a definition of intimate contact based on friendly interactions involving directed oral interaction with a member of the identical group, with some motion of the mouth but absence of nutrition.

Study Methods

The lead researcher said they focused on accounts of kissing in non-human species from the African continent and Asian regions, including primates, apes and great apes, and employed online videos to confirm the observations.

The researchers then integrated this information with details on the evolutionary relationships between living and ancient species of such primates.

Historical Origins

The team say the results indicate intimate contact developed approximately 21.5 million 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, indulged in a kiss, the scientists conclude. But the behavior may not have been limited to their specific group.

"Reality that modern people kiss, the reality that we now have shown that ancient relatives very likely kissed, suggests that the both groups are also likely to have engage," the researcher noted.

Evolutionary Significance

While the scientific reasoning is discussed, the expert said kissing could be employed in reproductive situations to possibly increase reproductive success or assist in selecting between partners, while it might help strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates commented that as intimate contact was observed in a broad spectrum of primates it was logical its roots extend far into our ancient history, and an examination of various types of kissing among a wider variety of species might push its origins back further still.

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

Cultural Elements

Another professor said that intimate contact had a cultural element as it was not common to all human groups.

"However, as humans we thrive or fail on the strength of our relationships, and methods of encouraging trust and closeness will have been significant for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it ought to be no surprise that Neanderthals – and including them and our own species together – kissed."