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From Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species appear to kiss. Currently, scientists suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Evidence

It is not the first time experts have suggested Neanderthals and Homo sapiens were closely connected. Among previous studies, scientists have found 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," she said, adding that the idea aligned with research that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, demonstrating interbreeding was at play.

Intimate Spin

"It certainly puts a more romantic spin on ancient interactions," Brindle said.

Writing in the journal a scientific periodical, Brindle and colleagues report how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how people smooch.

Defining Kissing

"Previously there were some previous attempts to describe a intimate act, but it's largely human-centric, which implies that essentially other animals do not engage in this. Now we know that they likely engage, it might just not look from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that looked like kissing were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species called French grunts.

As a result the team developed a description of intimate contact centered around social behaviors involving directed oral interaction with a member of the same species, with some movement of the oral area but absence of nutrition.

Research Approach

Brindle said they concentrated on reports of kissing in non-human species from the African continent and Asia, including primates, chimpanzees and great apes, and employed digital recordings to verify the reports.

Scientists then integrated this information with details on the genetic connections between extant and ancient types of such primates.

Evolutionary Origins

Researchers propose the results indicate kissing developed somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals on this evolutionary lineage suggests it is likely they, too, engaged in a intimate act, the researchers conclude. But the activity may not have been limited to their own species.

"The fact that modern people engage intimately, the reality that we currently have demonstrated that ancient relatives probably kissed, indicates that the both groups are probably did engage," the researcher noted.

Biological Significance

Although the evolutionary explanation is debated, the expert explained kissing could be employed in reproductive situations to potentially 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 said that as kissing behavior was observed in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a wider variety of animals might push its origins back even earlier still.

"Behaviors that we consider as signatures of human life, like kissing, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Elements

Another professor explained that intimate contact had a social component as it was not common to all societies.

"However, as humans we succeed or struggle on the strength of our emotional bonds, and ways of encouraging trust and intimacy will have been important for eons," she said. "It might be an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but actually it should be no surprise that Neanderthals – and even them and our own species together – engaged intimately."