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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Shared Microbial Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were intimately acquainted. In previous studies, researchers have discovered modern people and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were kissing," she said, adding that the idea chimed with research that has found humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Spin

"This offers a different perspective on human-Neanderthal relations," the lead researcher commented.

Publishing in the publication a scientific periodical, the researcher and colleagues detail how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how people kiss.

Defining Kissing

"There have been some previous attempts to describe a kiss, but it's largely focused on humans, which means that essentially other animals don't kiss. Currently we know that they probably do, it might just not look from what human kissing resembles," said the evolutionary biologist.

Nonetheless, she noted some actions that looked like kissing were something rather different – such as the processing and food sharing, or "kiss-fighting", observed in aquatic species called French grunts.

As a result the team came up with a description of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but absence of food.

Study Methods

Brindle explained they concentrated on reports of intimate behavior in non-human species from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed digital recordings to confirm the reports.

Scientists then integrated this data with information on the genetic connections between living and extinct species of such primates.

Historical Origins

The team propose the results indicate intimate contact developed somewhere between 21.5 million and 16.9 million years ago in the ancestors of the large apes.

The position of ancient hominins on this family tree suggests it is likely they, too, indulged in a kiss, the scientists conclude. But the activity may not have been confined to their specific group.

"Reality that modern people engage intimately, the reality that we now have shown that ancient relatives very likely engaged, suggests that the both groups are also likely to have kissed," the researcher added.

Evolutionary Importance

Although the scientific reasoning is debated, Brindle said intimate contact could be used in reproductive situations to potentially enhance reproductive success or help choose between mates, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates commented that as intimate contact was seen in a broad spectrum of apes it made sense its roots extend far into our ancient history, and an examination of different forms of intimate behavior among a wider variety of species might extend its origins back even earlier still.

"Behaviors that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Aspects

An archaeology expert said that intimate contact had a social component as it was not universal to all societies.

"However, as humans we thrive or fail on the strength of our relationships, and methods of promoting confidence and intimacy will have been significant for millions of years," she said. "It might be an concept that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but actually it ought to be expected that Neanderthals – and even them and our own species together – kissed."