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Among seabirds to polar bears, primates to great apes, various animals appear to kiss. Now, scientists propose that Neanderthals also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Shared Oral Clues

It is not the first time scientists have suggested ancient relatives and Homo sapiens were intimately acquainted. In earlier research, researchers have discovered humans and their thick-browed cousins possessed the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the concept aligned with studies that has found people of non-African ancestry have bits of ancient genetic material in their genome, revealing genetic mixing was occurring.

Intimate Interpretation

"This offers a different spin on human-Neanderthal relations," Brindle said.

Writing in the publication Evolution and Human Behavior, Brindle and her team report how, to investigate the historical roots of kissing, they first had to develop a description that was not limited to how humans kiss.

Defining Intimate Contact

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

Nonetheless, she said some actions that resembled intimate contact were distinct activities – such as the processing and food sharing, or "mouth contact", observed in aquatic species called French grunts.

As a result the team developed a description of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the mouth but absence of nutrition.

Research Methods

Brindle said they focused on accounts of kissing in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and used digital recordings to confirm the observations.

Scientists then integrated this data with information on the genetic connections between living and extinct species of such animals.

Evolutionary Origins

The team propose the findings indicate kissing evolved somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a kiss, the researchers say. But the activity might not have been limited to their specific group.

"The fact that humans engage intimately, the fact that we currently have shown that Neanderthals probably engaged, suggests that the two [species] are probably did kissed," Brindle noted.

Biological Importance

Although the evolutionary explanation is debated, Brindle said kissing could be employed in sexual contexts to possibly increase mating outcomes or help choose between partners, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates commented that as kissing behavior was observed in a broad spectrum of apes it made sense its roots lie deep in our evolutionary past, and an examination of different forms of kissing among a wider variety of species might extend its beginnings back further still.

"Things that we think of as signatures of our species, like kissing, are not unique to us if we look closely at different species," he said.

Social Elements

An archaeology expert explained that intimate contact had a cultural element as it was not universal to all societies.

"However, as humans we thrive or fail on the strength of our emotional bonds, and ways of encouraging trust and intimacy will have been significant for millions of years," she said. "This could represent an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it ought to be no surprise that ancient hominins – and even them and our human ancestors together – kissed."