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Among Galápagos albatrosses to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals did it too – and might even have locked lips with modern humans.

Shared Microbial Evidence

It is not the first time scientists have proposed ancient relatives and Homo sapiens were closely connected. In earlier research, scientists have discovered humans and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, adding that the concept chimed with research that has revealed people of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating interbreeding was at play.

Intimate Interpretation

"This offers a more romantic perspective on ancient interactions," the lead researcher commented.

Writing in the publication a scientific periodical, the researcher and her team detail how, to investigate the historical roots of kissing, they first had to come up with a description that was not limited to how humans smooch.

Describing Intimate Contact

"There have been some previous attempts to define a kiss, but it's largely focused on humans, which implies that essentially non-human species don't kiss. Currently we understand that they probably do, it may appear different from what our intimate contact resembles," explained Brindle.

However, she noted some behaviors that looked like kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in fish known as French grunts.

As a result the research group developed a definition of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but absence of food.

Study Methods

Brindle said they focused on accounts of intimate behavior in non-human species from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and employed digital recordings to verify the reports.

Scientists then integrated this data with details on the genetic connections between living and extinct types of such animals.

Evolutionary Origins

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

Placement of ancient hominins on this family tree means it is probable they, too, engaged in a intimate act, the researchers conclude. But the behavior may not have been confined to their specific group.

"The fact that humans engage intimately, the fact that we currently have shown that ancient relatives probably kissed, suggests that the two [species] are probably did kissed," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, the expert explained kissing could be employed in reproductive situations to potentially enhance mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the activities of great apes commented that as kissing behavior was observed in a broad spectrum of apes it made sense its roots lie deep in our ancient history, and an analysis of various types of intimate behavior among a wider variety of species might push its beginnings back further still.

"Behaviors that we think of as signatures of human life, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Social Aspects

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

"Nonetheless, as people we succeed or struggle on the strength of our relationships, and ways of promoting trust and intimacy will have been significant for millions of years," she said. "It might be an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be expected that Neanderthals – and including them and our own species together – kissed."