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Among seabirds to Arctic mammals, primates to orangutans, various animals 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

It is not the first time scientists have suggested ancient relatives and Homo sapiens were intimately acquainted. In previous studies, scientists have discovered modern people and their thick-browed cousins possessed the same mouth microbe for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," 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 genetic makeup, demonstrating genetic mixing was at play.

Romantic Spin

"This offers a different spin on human-Neanderthal relations," Brindle commented.

Writing in the publication a scientific periodical, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to come up with a definition that was not limited to how humans kiss.

Defining Kissing

"Previously there were some previous attempts to describe a kiss, but it's very much been focused on humans, which means that essentially non-human species don't kiss. Now we know that they probably do, it might just not look from what human kissing looks like," said Brindle.

Nonetheless, she said some behaviors that resembled intimate contact were distinct activities – such as the processing and food sharing, or "kiss-fighting", seen in aquatic species known as certain marine animals.

As a result the team came up with a description of intimate contact based on friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but no transfer of food.

Study Methods

Brindle said they concentrated on reports of kissing in primates from the African continent and Asian regions, including bonobos, apes and great apes, and employed digital recordings to verify the observations.

The researchers then combined this data with details on the genetic connections between extant and ancient species of such animals.

Evolutionary Timeline

Researchers say the results suggest intimate contact evolved approximately 21.5 million and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the scientists say. But the behavior might not have been confined to their own species.

"The fact that humans engage intimately, the reality that we currently have demonstrated that ancient relatives probably kissed, indicates that the both groups are probably did kissed," Brindle added.

Biological Importance

Although the scientific reasoning is debated, the expert explained intimate contact could be employed in sexual contexts to potentially increase mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of primates said that as kissing behavior was observed in a wide range of primates it made sense its origins lie deep in our ancient history, and an examination of different forms of kissing among a broader range of animals might push its beginnings back further still.

"Behaviors that we consider as characteristics of our species, like kissing, are not unique to us if we look closely at different species," the expert noted.

Social Aspects

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

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and ways of promoting confidence and closeness will have been important for millions of years," she said. "It might be an image that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it should be no surprise that Neanderthals – and including them and our own species together – engaged intimately."