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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals did it too – and possibly locked lips with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. Among previous studies, scientists have discovered humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the idea aligned with research that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Intimate Interpretation

"It certainly puts a more romantic perspective on ancient interactions," Brindle said.

Publishing in the journal a scientific periodical, the researcher and colleagues report how, to explore the evolutionary origins of kissing, they first had to come up with a description that was not restricted by how people smooch.

Defining Kissing

"Previously there were some efforts to describe a kiss, but it's largely human-centric, which means that essentially other animals do not engage in this. Currently we understand that they likely engage, it might just not look from what human kissing resembles," said the evolutionary biologist.

Nonetheless, she noted some actions that looked like intimate contact were something rather different – such as the processing and food sharing, or "mouth contact", seen in aquatic species called French grunts.

Consequently the research group came up with a definition of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the oral area but no transfer of food.

Study Methods

Brindle said they concentrated on reports of intimate behavior in non-human species from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and used digital recordings to verify the observations.

The researchers then integrated this data with details on the genetic connections between living and ancient species of such animals.

Historical Origins

Researchers propose the results indicate kissing evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, indulged in a intimate act, the scientists conclude. But the activity might not have been confined to their own species.

"Reality that modern people kiss, the reality that we now have shown that Neanderthals very likely engaged, indicates that the two [species] are also likely to have engage," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is discussed, Brindle explained intimate contact could be used in sexual contexts to possibly enhance mating outcomes or help choose between partners, while it might help strengthen connections when used in a non-sexual manner.

A separate researcher in the behavior of great apes commented that as kissing behavior was seen in a broad spectrum of primates it was logical its origins extend far into our evolutionary past, and an examination of different forms of kissing among a broader range of animals might push its origins back further still.

"Things that we consider 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 cultural element as it was not common to all societies.

"However, as humans we succeed or struggle on the quality of our relationships, and methods of promoting trust and intimacy will have been important for eons," she said. "It might be an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and ancient history, but really it should be expected that ancient hominins – and even Neanderthals and our human ancestors together – engaged intimately."