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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species appear to kiss. Currently, researchers propose that Neanderthals did it too – and possibly locked lips with modern humans.

Common Oral Clues

It is not the first time scientists have proposed ancient relatives and early modern humans were closely connected. Among previous studies, researchers have found humans and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the concept aligned with studies that has revealed people of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing genetic mixing was occurring.

Romantic Interpretation

"This offers a different spin on ancient interactions," Brindle commented.

Writing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to investigate the historical roots of intimate contact, they first had to develop a description that was not limited to how humans kiss.

Describing Kissing

"There have been some efforts to describe a intimate act, but it's very much been focused on humans, which implies that essentially other animals do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing resembles," said the evolutionary biologist.

However, she said some behaviors that looked like kissing were something rather different – such as the processing and food sharing, or "kiss-fighting", seen in fish known as certain marine animals.

As a result the research group developed a definition of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the mouth but no transfer of nutrition.

Study Approach

Brindle explained they concentrated on accounts of intimate behavior in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and employed digital recordings to verify the reports.

Scientists then combined this data with details on the evolutionary relationships between living and extinct species of such animals.

Evolutionary Timeline

Researchers say the findings suggest intimate contact developed somewhere between 21.5m and 16.9 million years ago in the ancestors of the great primates.

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

"The fact that humans engage intimately, the reality that we now have demonstrated that ancient relatives probably kissed, suggests that the both groups are probably did kissed," Brindle added.

Biological Importance

While the evolutionary explanation is debated, Brindle explained kissing could be used in reproductive situations to possibly increase mating outcomes or help choose between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the activities of great apes commented that as intimate contact was observed in a wide range of primates it was logical its origins lie deep in our evolutionary past, and an examination of different forms of intimate behavior among a broader range of species might push its beginnings back even earlier still.

"Behaviors that we consider as signatures of our species, like kissing, are not exclusive to us if we look closely at other animals," he said.

Social Elements

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

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and methods of promoting confidence and intimacy will have been important for millions of years," she said. "It might be an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but actually it should be no surprise that ancient hominins – and even Neanderthals and our own species collectively – kissed."