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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, 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 experts have proposed Neanderthals and early modern humans 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 exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the concept aligned with studies that has found humans of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating interbreeding was at play.

Intimate Interpretation

"This offers a different spin on human-Neanderthal relations," Brindle said.

Publishing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how humans smooch.

Defining Kissing

"Previously there were some previous attempts to define a kiss, but it's largely focused on humans, which implies that basically non-human species don't kiss. Now we understand that they probably do, it might just not look from what our intimate contact resembles," said Brindle.

Nonetheless, she said some behaviors that resembled intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", seen in fish known as certain marine animals.

Consequently the research group came up with a description of intimate contact based on social behaviors involving directed oral interaction with a member of the identical group, with some movement of the mouth but absence of nutrition.

Research Methods

Brindle said they concentrated on accounts of intimate behavior in non-human species from the African continent and Asian regions, including primates, apes and great apes, and used digital recordings to verify the observations.

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

Historical Timeline

Researchers propose the findings indicate intimate contact developed approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

The position of Neanderthals on this family tree means it is probable they, too, engaged in a kiss, the scientists say. But the activity might not have been limited to their specific group.

"Reality that modern people engage intimately, the reality that we now have demonstrated that ancient relatives very likely engaged, indicates that the both groups are also likely to have engage," Brindle added.

Evolutionary Importance

Although the scientific reasoning is debated, Brindle said intimate contact could be used in sexual contexts to potentially enhance mating outcomes or help choose between mates, while it could assist reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of great apes commented that as kissing behavior was observed in a wide range of apes it was logical its origins extend far into our evolutionary past, and an examination of various types of intimate behavior among a wider variety of animals might push its beginnings back even earlier still.

"Things that we think of as signatures of human life, like kissing, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Elements

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

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and methods of encouraging confidence and closeness will have been important for millions of years," she said. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it should be expected that Neanderthals – and even Neanderthals and our human ancestors together – engaged intimately."