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From seabirds to Arctic mammals, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals did it too – and possibly locked lips with early Homo sapiens.

Common Oral Evidence

It is not the first time experts have proposed ancient relatives and early modern humans were closely connected. Among previous studies, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, adding that the idea chimed with studies that has found humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Interpretation

"It certainly puts a different spin on ancient interactions," the lead researcher said.

Publishing in the publication a scientific periodical, the researcher and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how humans smooch.

Describing Kissing

"There have been some efforts to describe a kiss, but it's largely human-centric, which implies that essentially non-human species don't kiss. Now we understand that they likely engage, it may appear different from what human kissing resembles," said the evolutionary biologist.

Nonetheless, she noted some behaviors that looked like kissing were something rather different – such as the processing and transfer of food, or "mouth contact", observed in fish called certain marine animals.

As a result the team came up with a definition of kissing based on social behaviors 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 Methods

Brindle explained they focused on accounts of intimate behavior in primates from Africa and Asian regions, including primates, apes and orangutans, and employed digital recordings to confirm the reports.

Scientists then integrated this information with information on the genetic connections between living and ancient species of such primates.

Evolutionary Timeline

The team say the results indicate intimate contact evolved approximately 21.5 million and 16.9m 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 scientists conclude. But the activity might not have been confined to their own species.

"The fact that modern people kiss, the fact that we now have shown that Neanderthals probably kissed, indicates that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle explained kissing could be employed in sexual contexts to potentially enhance reproductive success or help choose between partners, while it might help reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of great apes said that as intimate contact was seen in a wide range of primates it made sense its roots lie deep in our ancient history, and an analysis of different forms of intimate behavior among a broader range of animals might extend its beginnings back even earlier still.

"Behaviors that we consider as signatures of our species, like kissing, are not unique to us if we look closely at other animals," he said.

Cultural Aspects

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

"However, as humans we thrive or fail on the quality of our emotional bonds, and ways of promoting trust and closeness will have been significant for millions of years," she said. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be expected that ancient hominins – and including Neanderthals and our human ancestors collectively – engaged intimately."