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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins did it too – and might even have locked lips with early Homo sapiens.

Common Microbial Evidence

It is not the first time scientists have proposed Neanderthals and Homo sapiens were closely connected. In previous studies, scientists have found modern people and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Probably they were kissing," she said, adding that the idea aligned with studies that has found humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Intimate Spin

"This offers a more romantic perspective on human-Neanderthal relations," Brindle said.

Writing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to explore the historical roots of intimate contact, they first had to come up with a definition that was not limited to how people smooch.

Describing Kissing

"Previously there were some efforts to describe a intimate act, but it's very much been focused on humans, which means that essentially other animals don't kiss. Now we understand that they probably do, it may appear different from what human kissing resembles," explained Brindle.

Nonetheless, she said some actions that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "kiss-fighting", seen in fish known as certain marine animals.

As a result the team developed a description of kissing centered around friendly interactions involving directed oral interaction with a individual of the identical group, with some movement of the oral area but no transfer of food.

Research Methods

Brindle said they concentrated on reports of kissing in primates from Africa and Asia, including bonobos, apes and great apes, and employed digital recordings to verify the reports.

The researchers then combined this information with information on the genetic connections between extant and ancient types of such primates.

Evolutionary Origins

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

The position of ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a intimate act, the researchers say. But the behavior may not have been confined to their specific group.

"Reality that humans engage intimately, the fact that we currently have demonstrated that Neanderthals very likely engaged, suggests that the two [species] are also likely to have engage," Brindle added.

Biological Significance

Although the scientific reasoning is discussed, the expert explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it could assist strengthen connections when practiced in a platonic way.

Another expert in the behavior of great apes said 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 analysis of various types of kissing among a wider variety of species might extend its beginnings back even earlier still.

"Things that we think of as characteristics of our species, like intimate contact, are not unique to us if we look closely at different species," he said.

Social Aspects

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

"However, as humans we thrive or fail on the quality of our emotional bonds, and ways of encouraging trust and closeness will have been significant for millions of years," the professor stated. "This could represent an image that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but actually it ought to be no surprise that ancient hominins – and including Neanderthals and our own species collectively – kissed."