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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, certain species appear to kiss. Currently, scientists suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Oral Clues

This isn't the initial instance experts have suggested ancient relatives and Homo sapiens were intimately acquainted. In earlier research, researchers have found modern people and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the idea chimed with studies that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing genetic mixing was at play.

Romantic Spin

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

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

Describing Kissing

"There have been some efforts to define a kiss, but it's very much been human-centric, which implies that basically non-human species do not engage in this. Now we know that they likely engage, it might just not look from what human kissing resembles," said Brindle.

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

As a result the research group came up with a definition of intimate contact centered around friendly interactions involving directed oral interaction with a member of the identical group, with some motion of the oral area but no transfer of food.

Study Methods

Brindle said they concentrated on reports of intimate behavior in primates from the African continent and Asia, including bonobos, chimpanzees and great apes, and used online videos to verify the observations.

The researchers then integrated this information with information on the genetic connections between living and extinct species of such primates.

Historical Origins

Researchers say the results indicate intimate contact evolved approximately 21.5m and 16.9 million years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree suggests it is likely they, too, engaged in a intimate act, the scientists say. But the behavior may not have been limited to their own species.

"The fact that humans engage intimately, the reality that we now have demonstrated that Neanderthals very likely kissed, indicates that the both groups are probably did kissed," Brindle noted.

Evolutionary Importance

Although the evolutionary explanation is debated, Brindle said intimate contact could be employed in sexual contexts to possibly enhance reproductive success or help choose between partners, while it could assist strengthen connections when practiced in a platonic way.

A separate researcher in the activities of primates commented that as intimate contact was seen in a broad spectrum of apes it made sense its roots extend far into our ancient history, and an examination of different forms of kissing among a broader range of species might extend its beginnings back further still.

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

Social Elements

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

"However, as people we thrive or fail on the quality of our emotional bonds, and methods of promoting confidence and intimacy will have been important for millions of years," she said. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it should be no surprise that ancient hominins – and including Neanderthals and our human ancestors together – kissed."