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From Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals did it too – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Evidence

It is not the first time scientists have suggested Neanderthals and early modern humans were closely connected. Among earlier research, scientists have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," the researcher noted, explaining that the idea chimed with research that has found people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Spin

"It certainly puts a different spin on human-Neanderthal relations," Brindle said.

Writing in the publication a scientific periodical, Brindle and colleagues report how, to explore the historical roots of intimate contact, they first had to develop a definition that was not restricted by how humans kiss.

Describing Kissing

"There have been some previous attempts to describe a kiss, but it's largely human-centric, which implies that basically non-human species do not engage in this. Now we know that they probably do, it may appear different from what our intimate contact resembles," explained Brindle.

Nonetheless, she noted some actions that resembled kissing were distinct activities – such as the processing and food sharing, or "mouth contact", seen in aquatic species known as French grunts.

Consequently the research group developed a definition of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the mouth but absence of food.

Research Approach

Brindle said they focused on reports of intimate behavior in non-human species from the African continent and Asia, including bonobos, chimpanzees and great apes, and used digital recordings to confirm the observations.

The researchers then combined this information with information on the genetic connections between extant and extinct species of such animals.

Evolutionary Timeline

The team say the results suggest kissing developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree suggests it is probable they, too, indulged in a intimate act, the researchers conclude. But the behavior may not have been confined to their own species.

"Reality that humans engage intimately, the reality that we currently have shown that ancient relatives probably kissed, suggests that the two [species] are probably did kissed," Brindle noted.

Evolutionary Significance

Although the scientific reasoning is discussed, Brindle said kissing could be used in reproductive situations to possibly enhance reproductive success or help choose between partners, while it might help strengthen connections when practiced in a platonic way.

A separate researcher in the behavior of great apes said that as kissing behavior was observed in a wide range of primates it made sense its origins extend far into our ancient history, and an examination of different forms of kissing among a broader range of animals might extend its origins back further still.

"Things that we think of as characteristics of human life, like kissing, are not unique to us if we examine carefully at different species," he said.

Cultural Elements

Another professor said that intimate contact had a social component as it was not common to all societies.

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and methods of promoting confidence and closeness will have been significant for eons," the professor stated. "It might be an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that ancient hominins – and including them and our own species collectively – engaged intimately."