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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Oral Clues

It is not the first time scientists have suggested Neanderthals and early modern humans were intimately acquainted. In previous studies, scientists have discovered modern people and their thick-browed cousins possessed the same mouth microbe for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the idea aligned with research that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing interbreeding was occurring.

Intimate Interpretation

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

Publishing in the journal a scientific periodical, the researcher and colleagues detail how, to investigate the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how humans smooch.

Describing Intimate Contact

"There have been some efforts to define a intimate act, but it's largely focused on humans, which implies that basically other animals do not engage in this. Currently we know that they probably do, it might just not look from what human kissing resembles," said Brindle.

However, she noted some actions that resembled 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 team developed a definition of intimate contact centered around social behaviors involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but absence of nutrition.

Study Approach

Brindle said they focused on reports of intimate behavior in primates from the African continent and Asia, including bonobos, chimpanzees and great apes, and employed digital recordings to verify the reports.

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

Evolutionary Origins

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

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

"The fact that modern people engage intimately, the reality that we currently have shown that ancient relatives probably engaged, suggests that the two [species] are also likely to have kissed," the researcher added.

Evolutionary Importance

While the scientific reasoning is discussed, Brindle explained intimate contact could be used in reproductive situations to possibly enhance reproductive success or assist in selecting between mates, while it might help strengthen connections when practiced in a non-sexual manner.

A separate researcher in the activities of great apes commented that as intimate contact was observed in a broad spectrum of primates it made sense its roots extend far into our ancient history, and an analysis of different forms of kissing among a broader range of species might push its origins back even earlier still.

"Behaviors that we think of as characteristics of our species, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Social Elements

Another professor explained that intimate contact had a cultural element as it was not universal to all societies.

"However, as people we succeed or struggle on the strength of our emotional bonds, and methods of encouraging trust and closeness will have been significant for eons," she said. "It might be an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be no surprise that Neanderthals – and even them and our own species collectively – engaged intimately."