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From seabirds to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have suggested ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have discovered modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were kissing," the researcher noted, adding that the concept chimed with studies that has found humans of non-African ancestry have bits of ancient genetic material in their genome, revealing interbreeding was at play.

Romantic Interpretation

"This offers a more romantic spin on human-Neanderthal relations," the lead researcher commented.

Writing in the publication a scientific periodical, Brindle and colleagues report how, to explore the historical roots of intimate contact, they first had to develop a definition that was not restricted by how humans kiss.

Defining Kissing

"Previously there were some efforts to describe a kiss, but it's very much been focused on humans, which means that essentially non-human species do not engage in this. Currently we understand that they probably do, it might just not look from what our intimate contact resembles," said Brindle.

Nonetheless, she said some behaviors that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "kiss-fighting", seen in aquatic species called certain marine animals.

As a result the research group developed a description of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a member of the identical group, with some motion of the mouth but absence of food.

Study Approach

The lead researcher said they concentrated on reports of kissing in primates from the African continent and Asia, including primates, apes and orangutans, and employed online videos to confirm the reports.

Scientists then combined this information with details on the genetic connections between extant and ancient species of such primates.

Evolutionary Origins

The team say the findings indicate kissing developed somewhere between 21.5 million and 16.9m years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the researchers conclude. But the activity may not have been confined to their specific group.

"Reality that humans kiss, the fact that we currently have shown that Neanderthals probably kissed, indicates that the both groups are also likely to have engage," Brindle added.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle explained kissing could be employed in reproductive situations to potentially increase reproductive success 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 said that as intimate contact was seen in a wide range of apes it made sense its origins extend far into our evolutionary past, and an analysis of different forms of kissing among a broader range of species might extend its beginnings back even earlier still.

"Behaviors that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we look closely at different species," he said.

Social Elements

Another professor said that kissing had a social component as it was not universal to all human groups.

"Nonetheless, as humans we succeed or struggle on the quality of our relationships, and ways of encouraging trust and closeness will have been significant for millions of years," the professor stated. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it should be expected that ancient hominins – and including Neanderthals and our human ancestors collectively – kissed."