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Among Galápagos albatrosses to polar bears, primates to great apes, certain species appear to kiss. Currently, researchers suggest that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.

Shared Microbial Evidence

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. Among earlier research, scientists have discovered modern people and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, explaining that the concept aligned with research that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing interbreeding was occurring.

Intimate Interpretation

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

Writing in the journal a scientific periodical, Brindle and her team report how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how people smooch.

Defining Intimate Contact

"There have been some previous attempts to describe a kiss, but it's largely human-centric, which means that basically non-human species do not engage in this. Currently we understand that they likely engage, it might just not look from what human kissing looks like," said Brindle.

Nonetheless, she noted some behaviors that looked like kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", observed in fish known as certain marine animals.

As a result the research group developed a definition of intimate contact based on social behaviors involving directed oral interaction with a member of the identical group, with some movement of the oral area but no transfer of nutrition.

Research Approach

Brindle said they concentrated on accounts of kissing in primates from Africa and Asia, including bonobos, chimpanzees and orangutans, and used online videos to verify the observations.

The researchers then integrated this information with details on the genetic connections between extant and extinct types of such animals.

Evolutionary Timeline

The team say the results indicate intimate contact evolved somewhere between 21.5 million and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree suggests it is probable they, too, engaged in a kiss, the researchers say. But the behavior may not have been limited to their specific group.

"The fact that modern people engage intimately, the reality that we currently have demonstrated that ancient relatives very likely kissed, indicates that the both groups are also likely to have engage," Brindle noted.

Evolutionary Importance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be used in sexual contexts to potentially enhance mating outcomes or assist in selecting between partners, while it could assist reinforce bonding when used in a platonic way.

Another expert in the behavior of primates said that as kissing behavior was seen in a broad spectrum of primates it made sense its roots extend far into our ancient history, and an analysis of various types of kissing among a broader range of animals might extend its origins back further still.

"Things that we consider as characteristics of our species, like intimate contact, are not unique to us if we examine carefully at other animals," he said.

Cultural Elements

An archaeology expert explained that kissing 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 methods of promoting confidence and closeness will have been significant for millions of years," the professor stated. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it should be expected that ancient hominins – and even them and our own species collectively – kissed."