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From seabirds to polar bears, primates to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance experts have proposed Neanderthals and early modern humans were intimately acquainted. In previous studies, researchers have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with studies that has found humans of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing genetic mixing was at play.

Intimate Spin

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

Publishing in the journal a scientific periodical, the researcher and colleagues report how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how humans smooch.

Describing Kissing

"There have been some efforts to describe a intimate act, but it's largely focused on humans, which means that essentially other animals do not engage in this. Now we understand that they likely engage, it may appear different from what human kissing looks like," said Brindle.

However, she noted some actions that resembled kissing were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in fish called certain marine animals.

As a result the research group came up with a definition of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a member of the same species, with some movement of the oral area but no transfer of nutrition.

Research Methods

Brindle explained they focused on reports of kissing in non-human species from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and employed digital recordings to verify the reports.

The researchers then integrated this data with details on the genetic connections between living and extinct species of such animals.

Historical Origins

Researchers say the results suggest kissing developed somewhere between 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 probable they, too, engaged in a intimate act, the scientists say. But the activity might not have been confined to their specific group.

"Reality that humans engage intimately, the fact that we currently have shown that ancient relatives probably engaged, indicates that the both groups are probably did engage," the researcher noted.

Biological Importance

While the evolutionary explanation is discussed, Brindle explained kissing could be used in sexual contexts to possibly increase reproductive success or assist in selecting between mates, while it might help reinforce bonding when used in a non-sexual manner.

Another expert in the behavior of primates commented that as intimate contact was seen in a broad spectrum of primates it was logical its roots lie deep in our ancient history, and an analysis of various types of kissing among a broader range of animals might push its beginnings back further still.

"Things that we think of as signatures of human life, like kissing, are not unique to us if we examine carefully at different species," he said.

Social Aspects

Another professor explained that intimate contact had a social component as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the quality of our relationships, and methods of encouraging confidence and intimacy will have been significant for millions of years," she said. "It might be an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it should be no surprise that Neanderthals – and including Neanderthals and our own species collectively – kissed."