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From Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals did it too – and might even have locked lips with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance scientists have suggested Neanderthals and early modern humans were closely connected. Among previous studies, scientists have discovered modern people 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 engaging in intimate contact," she said, explaining that the idea chimed with studies that has revealed humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing interbreeding was occurring.

Romantic Interpretation

"It certainly puts a different spin on ancient interactions," the lead researcher commented.

Writing in the publication Evolution and Human Behavior, Brindle and colleagues report how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how humans smooch.

Describing Intimate Contact

"There have been some previous attempts to describe 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 resembles," said the evolutionary biologist.

However, she noted some actions that looked like kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in fish known as French grunts.

Consequently the team came up with a description of intimate contact centered around social behaviors involving directed 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 reports of intimate behavior in primates from Africa and Asia, including primates, apes and great apes, and used digital recordings to confirm the observations.

Scientists then combined this data with information on the evolutionary relationships between extant and extinct species of such animals.

Historical Timeline

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

The position of ancient hominins on this family tree suggests it is probable they, too, indulged in a kiss, the scientists say. But the activity might not have been confined to their own species.

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

Biological Significance

Although the evolutionary explanation is debated, Brindle said kissing could be employed in sexual contexts to potentially increase mating outcomes or help choose between mates, while it might help reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was seen in a broad spectrum of apes it was logical its origins extend far into our ancient history, and an examination of various types of kissing among a broader range of animals might push its beginnings back even earlier still.

"Behaviors that we consider as characteristics of our species, like kissing, are not exclusive to us if we examine carefully at different species," he said.

Cultural Aspects

Another professor explained that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and ways of encouraging trust and closeness will have been significant for millions of years," the professor stated. "It might be an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it ought to be no surprise that ancient hominins – and including Neanderthals and our human ancestors collectively – kissed."