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From seabirds to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Common Oral Clues

This isn't the initial instance experts have proposed ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, scientists have found modern people and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the concept chimed with studies that has revealed humans of non-African ancestry have bits of ancient genetic material in their genome, demonstrating genetic mixing was at play.

Intimate Spin

"It certainly puts a different spin on ancient interactions," Brindle commented.

Publishing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how people smooch.

Describing Kissing

"There have been some efforts to define a intimate act, 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 our intimate contact looks like," said Brindle.

Nonetheless, she noted some behaviors that resembled kissing were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in fish known as certain marine animals.

Consequently the research group developed a description of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the identical group, with some motion of the oral area but absence of food.

Research Approach

The lead researcher explained they concentrated on accounts of kissing in primates from the African continent and Asia, including bonobos, apes and great apes, and used online videos to verify the reports.

Scientists then integrated this information with information on the genetic connections between extant and ancient species of such primates.

Historical Origins

The team say the findings indicate intimate contact developed somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree suggests it is likely they, too, engaged in a kiss, the scientists conclude. But the activity might not have been limited to their specific group.

"Reality that modern people kiss, the fact that we currently have demonstrated that Neanderthals very likely kissed, indicates that the both groups are probably did engage," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is debated, the expert said kissing could be employed in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates said that as intimate contact was seen in a broad spectrum of primates it made sense its roots extend far into our evolutionary past, and an analysis of various types of intimate behavior among a broader range of animals might push its origins back further still.

"Things that we think of as signatures of our species, like kissing, are not unique to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

An archaeology expert explained that kissing had a cultural element as it was not common to all human groups.

"However, as humans we thrive or fail on the quality of our relationships, and ways of encouraging trust and closeness will have been important for eons," she said. "This could represent an image that appears a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and including Neanderthals and our own species collectively – kissed."