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Among seabirds to Arctic mammals, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.

Common Oral Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were intimately acquainted. In previous studies, scientists have found modern people and their thick-browed cousins possessed 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 idea aligned with studies that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Romantic Spin

"It certainly puts a different perspective on human-Neanderthal relations," the lead researcher said.

Publishing in the journal a scientific periodical, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to develop a description that was not restricted by how humans smooch.

Describing Intimate Contact

"There have been some previous attempts to describe a intimate act, but it's very much been human-centric, which implies that basically non-human species don't kiss. Currently we know that they probably do, it may appear different from what human kissing resembles," said the evolutionary biologist.

Nonetheless, she said some actions that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "mouth contact", seen in aquatic species called French grunts.

As a result the team developed a description of intimate contact based on friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but absence of food.

Research Methods

The lead researcher explained they concentrated on reports of intimate behavior in primates from Africa and Asia, including primates, apes and orangutans, and used online videos to verify the observations.

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

Historical Timeline

Researchers propose the results indicate kissing developed approximately 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals on this family tree means it is likely they, too, indulged in a kiss, the researchers say. But the behavior may not have been confined to their specific group.

"The fact that humans kiss, the reality that we currently have demonstrated that ancient relatives probably kissed, indicates that the two [species] are also likely to have engage," the researcher noted.

Evolutionary Significance

While the scientific reasoning is debated, Brindle said intimate contact could be employed in sexual contexts to possibly increase reproductive success or help choose between mates, while it might help strengthen connections when used in a non-sexual manner.

Another expert in the activities of great apes said that as kissing behavior was seen in a wide range of apes it was logical its origins extend far into our evolutionary past, and an analysis of various types of kissing among a broader range of animals might extend its origins back even earlier still.

"Things that we think of as characteristics of our species, like intimate contact, are not unique to us if we examine carefully at other animals," he said.

Social Aspects

An archaeology expert said that kissing had a cultural element as it was not universal to all societies.

"Nonetheless, as humans we succeed or struggle on the quality of our relationships, and ways of promoting confidence and closeness will have been important for eons," the professor stated. "It might be an concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but really it should be expected that ancient hominins – and including Neanderthals and our own species together – kissed."