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Among seabirds to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Microbial Evidence

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, researchers have discovered humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, explaining that the concept chimed with research that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Romantic Interpretation

"This offers a different spin on ancient interactions," Brindle commented.

Writing in the journal Evolution and Human Behavior, Brindle and her team detail how, to explore the historical roots of kissing, they first had to come up with a description that was not limited to how humans kiss.

Describing Kissing

"There have been some previous attempts to define a intimate act, but it's very much been focused on humans, which means that basically other animals don't kiss. Currently we understand that they likely engage, it may appear different from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she noted some behaviors that resembled kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species called certain marine animals.

As a result the research group developed a description of intimate contact centered around friendly interactions involving intentional oral interaction with a individual of the same species, with some motion of the oral area but no transfer of nutrition.

Research Approach

The lead researcher explained they concentrated on reports of intimate behavior in non-human species from the African continent and Asia, including bonobos, apes and orangutans, and employed online videos to confirm the observations.

The researchers then combined this information with details on the evolutionary relationships between extant and extinct species of such primates.

Evolutionary Origins

Researchers say the findings suggest intimate contact developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of Neanderthals on this family tree suggests it is likely they, too, indulged in a intimate act, the researchers say. But the activity may not have been limited to their own species.

"The fact that modern people kiss, the fact that we now have shown that Neanderthals probably engaged, indicates that the both groups are probably did kissed," Brindle added.

Evolutionary Importance

Although the scientific reasoning is debated, the expert said kissing could be used in sexual contexts to possibly increase mating outcomes or help choose between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

Another expert 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 wider variety of animals might extend its origins back further still.

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

Social Elements

An archaeology expert said that intimate contact had a social component as it was not universal to all human groups.

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting trust and closeness will have been important for eons," the professor stated. "It might be an concept that seems a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and even Neanderthals and our human ancestors collectively – kissed."