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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were closely connected. Among previous studies, scientists have discovered humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with research that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Intimate Spin

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

Writing in the journal a scientific periodical, the researcher 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 limited to how humans smooch.

Describing Intimate Contact

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

Nonetheless, she said some actions that resembled kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in fish called certain marine animals.

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

Study Approach

Brindle explained they focused on accounts of intimate behavior in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and used digital recordings to confirm the reports.

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

Evolutionary Origins

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

The position of Neanderthals on this family tree means it is probable they, too, engaged in a kiss, the scientists say. But the activity may not have been limited to their own species.

"The fact that humans engage intimately, the reality that we currently have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have kissed," Brindle added.

Biological Significance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or assist in selecting between partners, while it could assist strengthen connections when practiced in a platonic way.

Another expert in the activities of primates commented that as intimate contact was seen in a broad spectrum of primates it was logical its roots extend far into our evolutionary past, and an examination of various types of kissing among a wider variety of species might extend its origins back even earlier still.

"Things that we think of as characteristics of human life, like kissing, are not unique to us if we look closely at different species," he said.

Social Elements

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

"Nonetheless, as people we thrive or fail on the quality of our relationships, and methods of promoting confidence and intimacy will have been important for eons," she said. "This could represent an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it should be expected that ancient hominins – and even Neanderthals and our human ancestors together – engaged intimately."