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Among seabirds to polar bears, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists propose that Neanderthals did it too – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

It is not the first time experts have suggested ancient relatives and early modern humans were closely connected. In earlier research, scientists have discovered humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were kissing," she said, adding that the idea aligned with studies that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating interbreeding was at play.

Intimate Interpretation

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

Writing in the publication a scientific periodical, Brindle and colleagues detail how, to investigate the historical roots of kissing, they first had to come up with a definition that was not limited to how people kiss.

Describing Kissing

"There have been some previous attempts to describe a kiss, but it's very much been human-centric, which means that basically non-human species do not engage in this. Currently we know that they probably do, it may appear different from what human kissing resembles," said the evolutionary biologist.

Nonetheless, she noted some behaviors that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in fish known as certain marine animals.

Consequently the team developed a description of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the oral area but no transfer of food.

Study Methods

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

Scientists then combined this data with information on the genetic connections between extant and extinct types of such primates.

Evolutionary Origins

Researchers propose the findings suggest intimate contact developed approximately 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of Neanderthals on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the researchers say. But the behavior may not have been limited to their specific group.

"Reality that humans engage intimately, the reality that we currently have shown that ancient relatives very likely engaged, indicates that the two [species] are probably did kissed," Brindle added.

Biological Importance

Although the evolutionary explanation is debated, Brindle said kissing could be employed in reproductive situations to possibly increase mating outcomes or assist in selecting between partners, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the behavior of great apes commented that as intimate contact was seen in a broad spectrum of primates it made sense its roots lie deep in our ancient history, and an examination of different forms of intimate behavior among a broader range of species might push its beginnings back even earlier still.

"Things that we think of as characteristics of our species, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Cultural Elements

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

"However, as humans we succeed or struggle on the strength of our relationships, and methods of promoting confidence and closeness will have been significant for millions of years," the professor stated. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be no surprise that Neanderthals – and including them and our human ancestors together – kissed."