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Among Galápagos albatrosses to Arctic mammals, primates to great apes, certain species appear to kiss. Now, scientists suggest that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Clues

It is not the first time experts have proposed Neanderthals and Homo sapiens were intimately acquainted. In previous studies, researchers have found humans and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the concept chimed with research that has found people of non-African ancestry contain ancient genetic material in their genome, demonstrating genetic mixing was at play.

Romantic Spin

"It certainly puts a more romantic spin on human-Neanderthal relations," the lead researcher commented.

Writing in the publication Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the historical roots of intimate contact, they first had to develop a definition that was not restricted by how people kiss.

Defining Kissing

"There have been some previous attempts to define a intimate act, but it's very much been focused on humans, which means that essentially other animals do not engage in this. Now we know that they probably do, it might just not look from what our intimate contact resembles," explained the evolutionary biologist.

However, she noted some actions that looked like kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in fish known as French grunts.

As a result the research group developed a description of kissing based on social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but no transfer of nutrition.

Study Approach

The lead researcher explained they concentrated on accounts of intimate behavior in primates from Africa and Asian regions, including primates, chimpanzees and orangutans, and used digital recordings to verify the reports.

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

Evolutionary Origins

Researchers propose the results indicate kissing evolved approximately 21.5 million and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is likely they, too, indulged in a kiss, the researchers say. But the activity may not have been confined to their specific group.

"Reality that modern people kiss, the fact that we now have shown that ancient relatives very likely engaged, suggests that the two [species] are probably did engage," the researcher added.

Evolutionary Importance

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

Another expert in the activities of great apes said that as kissing behavior was observed in a wide range of apes it was logical its origins lie deep in our ancient history, and an examination of various types of intimate behavior among a wider variety of species might push its beginnings back further still.

"Things that we consider as signatures of human life, like intimate contact, are not exclusive to us if we look closely at different species," the expert noted.

Social Elements

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

"However, as people we succeed or struggle on the quality of our emotional bonds, and methods of encouraging trust and intimacy will have been important for millions of years," she said. "It might be an concept that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be expected that ancient hominins – and including Neanderthals and our own species together – engaged intimately."