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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Common Microbial Evidence

It is not the first time scientists have suggested Neanderthals and Homo sapiens were closely connected. In earlier research, scientists have found humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the idea chimed with studies that has revealed people of certain genetic backgrounds contain ancient genetic material in their genome, revealing interbreeding was occurring.

Romantic Interpretation

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

Publishing in the journal a scientific periodical, the researcher and her team report how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how humans kiss.

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 non-human species do not engage in this. Now we understand that they probably do, it might just not look from what human kissing looks like," explained Brindle.

However, she said some behaviors that looked like intimate contact were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in fish called certain marine animals.

Consequently the team developed a definition of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but absence of nutrition.

Research Approach

The lead researcher said they focused on accounts of intimate behavior in non-human species from Africa and Asia, including primates, chimpanzees and great apes, and used digital recordings to confirm the reports.

Scientists then combined this data with details on the genetic connections between extant and ancient species of such primates.

Evolutionary Timeline

The team propose the results suggest kissing developed somewhere between 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, indulged in a intimate act, the scientists conclude. But the behavior may not have been limited to their own species.

"The fact that modern people engage intimately, the reality that we now have demonstrated that ancient relatives probably engaged, indicates that the both groups are also likely to have kissed," the researcher added.

Biological Importance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be employed in sexual contexts to potentially increase reproductive success or help choose between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

A separate researcher in the activities of great apes commented that as intimate contact was seen in a wide range of apes it made sense its roots extend far into our ancient history, and an examination of various types of kissing among a wider variety of animals might extend its beginnings back even earlier still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not unique 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 societies.

"However, as humans we succeed or struggle on the quality of our emotional bonds, and ways of promoting confidence and intimacy will have been significant for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be expected that Neanderthals – and including Neanderthals and our own species together – kissed."