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Among seabirds to Arctic mammals, primates to orangutans, various animals appear to kiss. Now, scientists suggest that ancient hominins also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Shared Oral Evidence

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, researchers have found humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the idea chimed with studies that has found people of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, revealing genetic mixing was at play.

Romantic Spin

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle commented.

Writing in the journal Evolution and Human Behavior, Brindle and her team report how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how people smooch.

Describing Intimate Contact

"Previously there were some previous attempts to describe a intimate act, but it's largely focused on humans, which means that basically other animals don't kiss. Now we know that they likely engage, it might just not look from what our intimate contact resembles," explained the evolutionary biologist.

However, she said some behaviors that looked like kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", seen in fish called French grunts.

As a result the team came up with a description of intimate contact centered around social behaviors involving directed oral interaction with a individual of the same species, with some motion of the mouth but absence of nutrition.

Study Approach

Brindle explained they concentrated on reports of intimate behavior in primates from the African continent and Asian regions, including primates, apes and great apes, and used online videos to confirm the observations.

Scientists then integrated this data with details on the evolutionary relationships between extant and ancient species of such primates.

Evolutionary Origins

Researchers propose the results suggest intimate contact evolved approximately 21.5m and 16.9m years ago in the predecessors of the great primates.

The position of Neanderthals on this evolutionary lineage means it is probable they, too, indulged in a kiss, the researchers say. But the behavior might not have been limited to their own species.

"The fact that humans kiss, the reality that we currently have shown that Neanderthals probably kissed, suggests that the both groups are also likely to have engage," Brindle noted.

Evolutionary Importance

Although the scientific reasoning is debated, Brindle said intimate contact could be used in sexual contexts to potentially increase reproductive success or help choose between mates, while it could assist strengthen connections when practiced in a platonic way.

Another expert in the activities of primates commented that as kissing behavior was observed in a broad spectrum of primates it made sense its roots extend far into our ancient history, and an examination of different forms of intimate behavior among a broader range of animals might extend its beginnings back further still.

"Things that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Elements

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

"However, as people we thrive or fail on the quality of our relationships, and methods of promoting confidence and intimacy will have been significant for eons," she said. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be no surprise that ancient hominins – and even Neanderthals and our human ancestors together – kissed."