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Among Galápagos albatrosses to Arctic mammals, primates to orangutans, various animals engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were closely connected. Among earlier research, scientists have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," the researcher noted, explaining that the concept chimed with research that has found people of certain genetic backgrounds contain Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Spin

"It certainly puts a different perspective on human-Neanderthal relations," Brindle said.

Writing in the journal a scientific periodical, the researcher and her team detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a description that was not limited to how people kiss.

Describing Kissing

"There have been some previous attempts to describe a intimate act, but it's very much been focused on humans, which means that basically non-human species don't kiss. Currently we know that they likely engage, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that looked like intimate contact were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in fish called certain marine animals.

Consequently the research group developed a definition of kissing centered around social behaviors involving intentional oral interaction with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Study Methods

Brindle said they concentrated on accounts of kissing in primates from Africa and Asian regions, including primates, chimpanzees and orangutans, and used digital recordings to verify the reports.

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

Evolutionary Origins

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

The position of Neanderthals on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the scientists conclude. But the behavior might not have been confined to their own species.

"The fact that modern people engage intimately, the reality that we currently have demonstrated that Neanderthals very likely kissed, indicates that the both groups are probably did kissed," Brindle added.

Biological Significance

Although the scientific reasoning is debated, Brindle explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates commented that as kissing behavior was observed in a wide range of apes it was logical its origins lie deep in our evolutionary past, and an analysis of various types of kissing among a broader range of species might extend its origins back even earlier still.

"Things that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

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

"However, as humans we thrive or fail on the quality of our emotional bonds, and ways of promoting confidence and intimacy will have been significant for millions of years," she said. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be expected that ancient hominins – and including Neanderthals and our human ancestors collectively – engaged intimately."