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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, researchers suggest that ancient hominins also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Shared Oral Evidence

It is not the first time experts have suggested Neanderthals and early modern humans were closely connected. In earlier research, scientists have discovered modern people and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were kissing," she said, explaining that the concept aligned with studies that has revealed humans of certain genetic backgrounds contain Neanderthal DNA in their genome, revealing genetic mixing was occurring.

Romantic Interpretation

"It certainly puts a different spin on ancient interactions," Brindle commented.

Publishing in the journal Evolution and Human Behavior, Brindle and her team detail how, to explore the evolutionary origins of kissing, they first had to develop a description that was not restricted by how people smooch.

Defining Intimate Contact

"Previously there were some previous attempts to describe a intimate act, but it's very much been focused on humans, which means that essentially other animals don't kiss. Currently we understand that they likely engage, it may appear different from what human kissing looks like," said Brindle.

Nonetheless, she noted some actions that resembled kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in fish known as certain marine animals.

As a result the research group came up with a definition of intimate contact centered around friendly interactions involving directed oral interaction with a member of the same species, with some movement of the oral area but no transfer of food.

Research Methods

The lead researcher said they concentrated on accounts of intimate behavior in primates from Africa and Asian regions, including bonobos, chimpanzees and great apes, and used online videos to verify the observations.

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

Historical Timeline

Researchers say the results suggest kissing evolved somewhere between 21.5 million and 16.9 million years ago in the predecessors of the large apes.

Placement of Neanderthals on this evolutionary lineage suggests it is likely 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 now have demonstrated that ancient relatives probably engaged, indicates that the both groups are also likely to have kissed," Brindle added.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle explained intimate contact could be used in sexual contexts to potentially enhance mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when used in a platonic way.

Another expert in the activities of great apes said that as kissing behavior was observed in a broad spectrum of primates it made sense its origins extend far into our ancient history, and an examination of different forms of kissing among a wider variety of species might push its beginnings back further still.

"Things that we consider as characteristics of our species, like kissing, are not exclusive to us if we look closely at different species," he said.

Cultural Elements

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

"Nonetheless, as people we succeed or struggle on the strength of our relationships, and methods of promoting trust and intimacy will have been significant for millions of years," she said. "This could represent an concept that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it should be no surprise that ancient hominins – and including Neanderthals and our human ancestors collectively – engaged intimately."