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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species appear to kiss. Currently, scientists suggest that Neanderthals did it too – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Evidence

This isn't the initial instance experts have proposed ancient relatives and Homo sapiens were closely connected. Among previous studies, scientists have found humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, explaining that the concept aligned with studies that has revealed people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a different perspective on ancient interactions," the lead researcher said.

Publishing in the journal a scientific periodical, the researcher and colleagues detail how, to investigate the historical roots of kissing, they first had to come up with a description that was not restricted by how people kiss.

Defining Intimate Contact

"There have been some previous attempts to describe a intimate act, but it's very much been focused on humans, which means that basically non-human species don't kiss. Now we know that they likely engage, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

Nonetheless, she said some actions that looked like kissing were distinct activities – such as the processing and food sharing, or "mouth contact", observed in fish called certain marine animals.

As a result the research group came up with a description of kissing based on social behaviors involving intentional oral interaction with a individual of the identical group, with some movement of the mouth but absence of food.

Study Approach

Brindle explained they focused on accounts of kissing in non-human species from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and employed digital recordings to verify the observations.

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

Historical Timeline

Researchers propose the results suggest kissing evolved approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of Neanderthals on this family tree means it is likely they, too, indulged in a intimate act, the scientists conclude. But the behavior may not have been limited to their specific group.

"Reality that humans engage intimately, the fact that we now have demonstrated that ancient relatives probably engaged, indicates that the two [species] are also likely to have engage," the researcher noted.

Biological Significance

Although the evolutionary explanation is debated, Brindle explained kissing could be used in sexual contexts to potentially increase mating outcomes or help choose between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the activities of great apes commented that as kissing behavior was seen in a wide range of apes it made sense its roots extend far into our evolutionary past, and an examination of different forms of intimate behavior among a broader range of animals might extend its origins back even earlier still.

"Behaviors that we consider as signatures of our species, like kissing, are not unique to us if we examine carefully at different species," he said.

Cultural Aspects

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

"However, as humans we thrive or fail on the strength of our relationships, and ways of encouraging trust and closeness will have been important for millions of years," she said. "It might be an concept that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be no surprise that Neanderthals – and including Neanderthals and our own species together – kissed."