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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Currently, scientists propose that Neanderthals also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Common Microbial Clues

It is not the first time experts have proposed Neanderthals and Homo sapiens were closely connected. In earlier research, researchers have discovered humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the concept aligned with research that has revealed humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing genetic mixing was occurring.

Romantic Interpretation

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle commented.

Writing in the publication a scientific periodical, the researcher and her team detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how humans smooch.

Defining Intimate Contact

"Previously there were some efforts to describe a kiss, but it's very much been human-centric, which implies that essentially other animals do not engage in this. Now we know that they probably do, it might just not look from what our intimate contact resembles," explained the evolutionary biologist.

However, she noted some actions that resembled kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in fish known as certain marine animals.

Consequently the team came up with a description of kissing based on friendly interactions involving directed oral interaction with a member of the identical group, with some motion of the mouth but no transfer of food.

Research Approach

The lead researcher explained they focused on reports of intimate behavior in non-human species from the African continent and Asian regions, including primates, apes and orangutans, and used digital recordings to confirm the reports.

Scientists then combined this data with information on the genetic connections between living and ancient types of such animals.

Evolutionary Origins

Researchers say the findings indicate kissing evolved approximately 21.5 million and 16.9 million years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree suggests it is probable they, too, indulged in a kiss, the scientists say. But the behavior may not have been confined to their specific group.

"Reality that modern people kiss, the fact that we currently have demonstrated that ancient relatives very likely engaged, indicates that the both groups are probably did kissed," Brindle noted.

Biological Importance

While the evolutionary explanation is debated, the expert said kissing could be employed in reproductive situations to possibly increase reproductive success or assist in selecting between partners, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of primates commented that as kissing behavior was seen in a wide range of primates it was logical its roots extend far into our evolutionary past, and an analysis of different forms of kissing among a wider variety of animals might extend its beginnings back even earlier still.

"Behaviors that we think of as characteristics of our species, like kissing, are not unique to us if we examine carefully at different species," he said.

Cultural Elements

Another professor explained that intimate contact had a cultural element as it was not universal to all societies.

"Nonetheless, as people we succeed or struggle on the quality of our relationships, and methods of encouraging trust and closeness will have been important for millions of years," she said. "This could represent an concept that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be expected that ancient hominins – and even them and our human ancestors together – engaged intimately."