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From seabirds to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Common Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have found humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea chimed with studies that has found people of certain genetic backgrounds contain ancient genetic material in their genome, demonstrating genetic mixing was occurring.

Intimate Spin

"It certainly puts a different spin on ancient interactions," Brindle said.

Writing in the publication a scientific periodical, the researcher and her team report how, to investigate the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Describing Kissing

"There have been some previous attempts to define a intimate act, but it's very much been focused on humans, which means that essentially other animals don't kiss. Currently we understand that they probably do, it may appear different from what our intimate contact looks like," said Brindle.

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

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

Study Methods

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

Scientists then combined this information with information on the evolutionary relationships between living and ancient species of such animals.

Evolutionary Timeline

Researchers say the findings suggest intimate contact 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 suggests it is probable they, too, engaged in a intimate act, the scientists conclude. But the behavior may not have been limited to their own species.

"The fact that modern people kiss, the fact that we now have shown that ancient relatives very likely engaged, indicates that the two [species] are also likely to have engage," the researcher added.

Evolutionary Significance

Although the evolutionary explanation is debated, the expert said intimate contact could be employed in sexual contexts to possibly increase reproductive success or help choose between partners, while it might help reinforce bonding when used in a platonic way.

Another expert in the activities of great apes said that as intimate contact was observed in a wide range of apes it was logical its roots extend far into our evolutionary past, and an examination of various types of kissing among a wider variety of animals might push its origins back even earlier still.

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

Cultural Elements

An archaeology expert said that kissing had a cultural element as it was not common to all societies.

"Nonetheless, as people we thrive or fail on the strength of our emotional bonds, and ways of encouraging confidence and closeness will have been important for millions of years," the professor stated. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be expected that ancient hominins – and including them and our human ancestors collectively – engaged intimately."