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From seabirds to Arctic mammals, chimpanzees to great apes, certain species appear to kiss. Now, researchers suggest that Neanderthals did it too – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

This isn't the initial instance scientists have proposed Neanderthals and Homo sapiens were intimately acquainted. In earlier research, researchers have found humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," the researcher noted, explaining that the idea aligned with studies that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle said.

Publishing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to investigate the historical roots of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Describing Kissing

"There have been some previous attempts to define a intimate act, but it's largely focused on humans, which means that essentially non-human species do not engage in this. Currently we understand that they probably do, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

However, she said some behaviors that resembled intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", observed in fish known as French grunts.

As a result the team came up with a description of kissing based on friendly interactions involving intentional oral interaction with a member of the same species, with some movement of the mouth but absence of food.

Research Methods

The lead researcher explained they focused on accounts of intimate behavior in non-human species from Africa and Asian regions, including primates, apes and orangutans, and employed online videos to confirm the reports.

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

Historical Timeline

Researchers say the findings suggest kissing developed approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree suggests 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.

"The fact that humans kiss, the fact that we now have demonstrated that Neanderthals probably kissed, indicates that the both groups are also likely to have kissed," the researcher noted.

Evolutionary Importance

Although the evolutionary explanation is debated, the expert explained intimate contact could be employed in reproductive situations to possibly enhance reproductive success or assist in selecting between partners, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the behavior of primates commented that as kissing behavior was seen in a broad spectrum of apes it made sense its origins extend far into our evolutionary past, and an examination of various types of kissing among a broader range of animals might extend its beginnings back even earlier still.

"Things that we think of as signatures of our species, like intimate contact, are not exclusive to us if we look closely at different species," the expert noted.

Cultural Elements

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of encouraging trust and closeness will have been significant for millions of years," she said. "It might be an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it should be no surprise that Neanderthals – and even Neanderthals and our own species together – engaged intimately."