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Among seabirds to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, scientists propose that Neanderthals also engaged in this behavior – and might even have locked lips with modern humans.

Common Microbial Clues

It is not the first time experts have suggested ancient relatives and Homo sapiens were intimately acquainted. In earlier research, scientists have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Likely they were kissing," she said, explaining that the concept chimed with studies that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Intimate Spin

"This offers a more romantic spin on ancient interactions," the lead researcher commented.

Writing in the publication Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how humans kiss.

Defining Kissing

"Previously there were some efforts to define a intimate act, but it's very much been human-centric, which implies that basically non-human species do not engage in this. Now we understand that they probably do, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she said some actions that looked like kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", observed in fish called French grunts.

As a result the team came up with a definition of intimate contact 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 no transfer of food.

Research Approach

The lead researcher 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 verify the reports.

The researchers then combined this information with details on the evolutionary relationships between extant and ancient species of such primates.

Historical Timeline

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

The position of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the scientists conclude. But the behavior might not have been limited to their own species.

"Reality that modern people engage intimately, the fact that we now have shown that ancient relatives probably kissed, suggests that the both groups are also likely to have engage," the researcher noted.

Evolutionary Significance

Although the scientific reasoning is debated, Brindle said kissing could be used in sexual contexts to possibly increase mating outcomes or assist in selecting between mates, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the behavior of primates commented that as intimate contact was seen in a broad spectrum of apes it was logical its roots extend far into our ancient history, and an examination of various types of kissing among a broader range of species might push its origins back further still.

"Things that we consider as signatures of human life, like intimate contact, are not unique to us if we look closely at other animals," the expert noted.

Social Aspects

An archaeology expert said that intimate contact had a cultural element as it was not common to all societies.

"However, as people we succeed or struggle on the quality of our emotional bonds, and methods of promoting trust and intimacy will have been significant for millions of years," she said. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but really it should be expected that ancient hominins – and even Neanderthals and our human ancestors collectively – engaged intimately."