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From Galápagos albatrosses to Arctic mammals, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Common Microbial Evidence

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. In previous studies, researchers have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were kissing," she said, adding that the concept chimed with research that has revealed people of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Spin

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

Writing in the journal a scientific periodical, Brindle and her team report how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how people smooch.

Defining Kissing

"Previously there were some efforts to describe a intimate act, but it's largely focused on humans, which means that basically non-human species do not engage in this. Now we know that they likely engage, it might just not look from what human kissing resembles," said the evolutionary biologist.

However, she said some behaviors that resembled kissing were something rather different – such as the processing and transfer of food, or "kiss-fighting", observed in fish called certain marine animals.

As a result the research group developed a description of kissing based on social behaviors involving directed oral interaction with a individual of the same species, with some motion of the mouth but no transfer of food.

Study Methods

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

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

Evolutionary Timeline

Researchers propose the findings indicate intimate contact developed somewhere between 21.5 million and 16.9m years ago in the predecessors of the large apes.

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

"The fact that modern people kiss, the fact that we currently have shown that ancient relatives probably engaged, indicates that the two [species] are also likely to have kissed," the researcher added.

Biological Significance

Although the evolutionary explanation is discussed, the expert said kissing could be used in reproductive situations to potentially enhance mating outcomes or help choose between mates, while it might help reinforce bonding when used in a non-sexual manner.

A separate researcher in the activities of primates said that as kissing behavior was seen in a broad spectrum of apes it made sense its origins extend far into our evolutionary past, and an analysis of different forms of intimate behavior among a wider variety of animals might extend its beginnings back further still.

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

Social Elements

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting trust and intimacy will have been significant for eons," she said. "It might be an image that seems a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be no surprise that Neanderthals – and including Neanderthals and our human ancestors collectively – engaged intimately."