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From seabirds to polar bears, chimpanzees to great apes, certain species appear to kiss. Now, scientists propose that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Microbial Evidence

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

"Probably they were engaging in intimate contact," she said, adding that the concept chimed with studies that has revealed humans of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Romantic Spin

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

Publishing in the journal a scientific periodical, Brindle and colleagues report how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how people smooch.

Defining Intimate Contact

"There have been some previous attempts to define a intimate act, but it's largely focused on humans, which implies that essentially 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 the evolutionary biologist.

Nonetheless, she said some behaviors that looked like kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species known as 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 identical group, with some motion of the mouth but absence of nutrition.

Research Approach

Brindle explained they focused on accounts of intimate behavior in primates from Africa and Asian regions, including primates, apes and orangutans, and employed online videos to confirm the observations.

Scientists then combined this information with details on the evolutionary relationships between extant and ancient types of such animals.

Evolutionary Timeline

The team say the findings indicate kissing developed approximately 21.5 million and 16.9 million years ago in the predecessors of the large apes.

Placement of Neanderthals on this family tree suggests it is likely they, too, indulged in a intimate act, the scientists conclude. But the behavior may not have been limited to their specific group.

"Reality that humans kiss, the reality that we currently have demonstrated that Neanderthals very likely kissed, suggests that the both groups are probably did kissed," Brindle added.

Evolutionary Importance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in sexual contexts to possibly increase reproductive success or help choose between partners, while it could assist reinforce bonding when used in a non-sexual manner.

Another expert in the activities of primates said that as intimate contact was seen in a broad spectrum of primates it made sense its origins extend far into our ancient history, and an examination of different forms of intimate behavior among a broader range of animals might extend its origins back even earlier still.

"Behaviors that we think of as signatures of human life, like kissing, are not unique to us if we examine carefully at other animals," the expert noted.

Cultural Elements

Another professor explained that intimate contact had a social component as it was not universal to all human groups.

"However, as people we thrive or fail on the strength of our emotional bonds, and methods of promoting confidence and closeness will have been significant for millions of years," the professor stated. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be no surprise that ancient hominins – and even them and our own species together – kissed."