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From seabirds to Arctic mammals, primates to great apes, certain species appear to kiss. Currently, researchers suggest that Neanderthals did it too – and might even have locked lips with early Homo sapiens.

Common Microbial Clues

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, scientists have found modern people and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the concept aligned with research that has revealed people of non-African ancestry contain ancient genetic material in their genome, demonstrating genetic mixing was at play.

Intimate Spin

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

Writing in the journal Evolution and Human Behavior, Brindle and her team 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.

Describing Kissing

"There have been some previous attempts to describe a kiss, but it's very much been human-centric, which means that basically non-human species don't kiss. Currently we understand that they probably do, it may appear different from what human kissing looks like," said Brindle.

Nonetheless, she noted some behaviors that resembled kissing were distinct activities – such as the chewing and transfer of food, or "kiss-fighting", seen in aquatic species called French grunts.

As a result the team came up with a definition of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the mouth but no transfer of food.

Research Approach

Brindle explained they concentrated on reports of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and great apes, and employed online videos to verify the reports.

The researchers then combined this data with details on the evolutionary relationships between extant and ancient types of such animals.

Evolutionary Timeline

Researchers say the findings indicate intimate contact developed approximately 21.5 million and 16.9 million years ago in the ancestors of the great primates.

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

"The fact that modern people engage intimately, the reality that we now have shown that ancient relatives probably engaged, suggests that the two [species] are probably did engage," the researcher added.

Evolutionary Importance

While the scientific reasoning is debated, the expert said intimate contact could be used in reproductive situations to possibly enhance mating outcomes or help choose between mates, while it might help reinforce bonding when used in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was seen in a wide range of primates it was logical its roots extend far into our evolutionary past, and an examination of different forms of intimate behavior among a wider variety of animals might extend its beginnings back even earlier still.

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

Cultural Elements

An archaeology expert said that intimate contact had a social component as it was not universal to all human groups.

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and ways of encouraging confidence and intimacy will have been significant for eons," the professor stated. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be expected that Neanderthals – and including Neanderthals and our human ancestors together – kissed."