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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, researchers suggest that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Shared Microbial Evidence

This isn't the initial instance experts have proposed ancient relatives and early modern humans were closely connected. Among earlier research, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea chimed with research that has found humans of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing interbreeding was at play.

Romantic Spin

"It certainly puts a different spin on human-Neanderthal relations," Brindle commented.

Publishing in the journal Evolution and Human Behavior, the researcher and her team detail how, to explore the evolutionary origins of intimate contact, they first had to develop a description that was not limited to how people kiss.

Defining Intimate Contact

"There have been some efforts to describe a kiss, but it's largely focused on humans, which means that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what human kissing resembles," explained the evolutionary biologist.

However, she said some actions that resembled intimate contact were something rather different – such as the processing and transfer of food, or "mouth contact", seen in aquatic species called French grunts.

Consequently the team came up with a description of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but absence of nutrition.

Research Methods

Brindle said they concentrated on accounts of intimate behavior in primates from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and used online videos to verify the reports.

Scientists then integrated this information with details on the genetic connections between extant and extinct types of such primates.

Evolutionary Timeline

Researchers propose the findings indicate intimate contact evolved approximately 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree means it is likely they, too, engaged in a intimate act, the researchers say. But the activity may not have been confined to their own species.

"Reality that modern people engage intimately, the reality that we currently have shown that Neanderthals probably engaged, indicates that the two [species] are also likely to have engage," the researcher added.

Biological Significance

Although the evolutionary explanation is discussed, Brindle explained kissing could be used in sexual contexts to potentially enhance reproductive success or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the behavior of great apes commented that as intimate contact was seen in a wide range of primates it made sense its roots extend far into our evolutionary past, and an examination of different forms of kissing among a broader range of species might push its beginnings back even earlier still.

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

Social Elements

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

"Nonetheless, as people we thrive or fail on the quality of our emotional bonds, and methods of encouraging trust and closeness will have been significant for millions of years," she said. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be no surprise that ancient hominins – and including Neanderthals and our own species together – kissed."