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Among seabirds to polar bears, chimpanzees to orangutans, certain species appear to kiss. Currently, scientists propose that ancient hominins did it too – and might even have locked lips with modern humans.

Common Oral Evidence

It is not the first time scientists have proposed Neanderthals and early modern humans were closely connected. In previous studies, researchers have found humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, explaining that the idea chimed with research that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Romantic Interpretation

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

Writing in the publication a scientific periodical, Brindle and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not restricted by how humans smooch.

Defining Kissing

"There have been some efforts to describe a kiss, but it's largely human-centric, which implies that essentially other animals do not engage in this. Currently we know that they probably do, it might just not look from what human kissing looks like," explained Brindle.

Nonetheless, she said some behaviors that looked like intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", observed in aquatic species called French grunts.

As a result the research group developed a definition of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but absence of food.

Study Methods

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

The researchers then combined this information with information on the genetic connections between living and ancient species of such animals.

Historical Timeline

Researchers propose the results suggest intimate contact evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the large apes.

The position of Neanderthals on this evolutionary lineage suggests it is likely they, too, engaged in a intimate act, the scientists say. But the behavior may not have been limited to their own species.

"Reality that modern people kiss, the fact that we now have shown that ancient relatives very likely engaged, suggests that the two [species] are probably did engage," Brindle noted.

Evolutionary Significance

While the evolutionary explanation is debated, the expert explained kissing could be used in reproductive situations to potentially enhance reproductive success or assist in selecting between partners, while it could assist reinforce bonding when used in a platonic way.

Another expert in the behavior of great apes said that as intimate contact was seen in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an examination of various types of kissing among a broader range of species might push its beginnings back further still.

"Behaviors that we consider as characteristics of human life, like kissing, are not unique to us if we look closely at other animals," he said.

Social Elements

An archaeology expert said that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as humans we succeed or struggle on the quality of our relationships, and methods of promoting trust and intimacy will have been important for eons," she said. "This could represent an concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be no surprise that Neanderthals – and even Neanderthals and our human ancestors collectively – engaged intimately."