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From Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, scientists suggest that ancient hominins did it too – and might even have locked lips with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the concept chimed with research that has found humans of non-African ancestry contain ancient genetic material in their genome, demonstrating genetic mixing was occurring.

Romantic Interpretation

"It certainly puts a different perspective on human-Neanderthal relations," the lead researcher commented.

Writing in the journal a scientific periodical, Brindle and her team detail how, to explore the historical roots of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Defining Kissing

"There have been some efforts to define a intimate act, but it's largely human-centric, which implies that basically other animals do not engage in this. Now we understand that they probably do, it might just not look from what human kissing looks like," said the evolutionary biologist.

However, she noted some behaviors that looked like kissing were something rather different – such as the processing and food sharing, or "mouth contact", seen in fish called French grunts.

Consequently the research group came up with a definition 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 oral area but no transfer of nutrition.

Research Methods

Brindle said they concentrated on accounts of intimate behavior in non-human species from Africa and Asia, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the reports.

Scientists then combined this information with details on the genetic connections between living and extinct types of such animals.

Historical Timeline

The team propose the findings suggest kissing developed somewhere between 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the scientists say. But the activity might not have been limited to their own species.

"The fact that humans engage intimately, the fact that we now have demonstrated that ancient relatives probably engaged, suggests that the two [species] are also likely to have engage," the researcher noted.

Biological Significance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in sexual contexts to possibly enhance reproductive success or help choose between partners, while it could assist strengthen connections when practiced in a non-sexual manner.

Another expert in the activities of great apes commented that as intimate contact was seen in a wide range of primates it was logical its origins extend far into our ancient history, and an analysis of various types of kissing among a wider variety of animals might push its beginnings back further still.

"Behaviors that we think of as signatures of human life, like kissing, are not exclusive to us if we look closely at different species," he said.

Social Aspects

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

"However, as people we succeed or struggle on the strength of our relationships, and methods of promoting confidence and intimacy will have been important for millions of years," she said. "It might be an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be no surprise that Neanderthals – and including Neanderthals and our own species collectively – engaged intimately."