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Among Galápagos albatrosses to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Microbial Clues

It is not the first time scientists have suggested ancient relatives and early modern humans were intimately acquainted. Among previous studies, scientists have discovered humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," she said, explaining that the idea chimed with studies that has revealed people of non-African ancestry have bits of ancient genetic material in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Interpretation

"This offers a different perspective on human-Neanderthal relations," the lead researcher commented.

Publishing in the journal a scientific periodical, the researcher and her team detail how, to explore the historical roots of kissing, they first had to come up with a description that was not limited to how people kiss.

Defining Kissing

"There have been some efforts to describe a intimate act, but it's very much been human-centric, which implies that essentially non-human species don't kiss. Currently we know that they probably do, it might just not look from what human kissing resembles," said Brindle.

Nonetheless, she noted some actions that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "mouth contact", observed in fish known as certain marine animals.

Consequently the research group developed a description of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the mouth but no transfer of food.

Study Approach

Brindle said they concentrated on accounts of kissing in primates from Africa and Asia, including bonobos, apes and great apes, and employed online videos to verify the reports.

The researchers then combined this data with information on the genetic connections between living and ancient types of such animals.

Historical Origins

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

The position of ancient hominins on this family tree suggests it is likely they, too, indulged in a intimate act, the researchers say. But the activity may not have been confined to their specific group.

"The fact that modern people kiss, the fact that we now have shown that Neanderthals very likely engaged, indicates that the both groups are probably did engage," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is debated, Brindle said intimate contact could be employed in reproductive situations to possibly increase mating outcomes or help choose between partners, while it could assist reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was seen in a broad spectrum of primates it made sense its origins extend far into our evolutionary past, and an examination of different forms of kissing among a wider variety of species might extend its origins back even earlier still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not unique to us if we examine carefully at different species," he said.

Cultural Aspects

Another professor explained that intimate contact had a cultural element as it was not universal to all societies.

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and methods of encouraging trust and intimacy will have been significant for eons," the professor stated. "It might be an image that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be expected that ancient hominins – and including them and our human ancestors collectively – kissed."