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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, certain species appear to kiss. Currently, researchers propose that ancient hominins did it too – and might even have locked lips with modern humans.

Shared Oral Clues

It is not the first time scientists have suggested ancient relatives and early modern humans were intimately acquainted. Among earlier research, scientists have found modern people and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the concept chimed with research that has found people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Intimate Interpretation

"This offers a more romantic spin on human-Neanderthal relations," Brindle commented.

Writing in the journal Evolution and Human Behavior, Brindle and her team detail how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how humans kiss.

Describing Kissing

"Previously there were some previous attempts to describe a intimate act, but it's very much been focused on humans, which implies that basically other animals do not engage in this. Now we know that they probably do, it might just not look from what human kissing resembles," said the evolutionary biologist.

However, she said some actions that resembled kissing were something rather different – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species called French grunts.

As a result the research group came up with a description of kissing centered around social behaviors involving intentional oral interaction with a individual of the identical group, with some movement of the oral area but absence of food.

Study Methods

The lead researcher explained they concentrated on accounts of kissing in primates from Africa and Asian regions, including primates, chimpanzees and orangutans, and employed digital recordings to confirm the reports.

Scientists then combined this data with details on the genetic connections between living and ancient species of such animals.

Historical Timeline

Researchers say the results suggest kissing developed approximately 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree suggests it is probable they, too, engaged in a intimate act, the scientists say. But the behavior may not have been limited to their specific group.

"The fact that modern people kiss, the reality that we currently have shown that ancient relatives very likely engaged, suggests that the two [species] are probably did engage," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is debated, Brindle said kissing could be employed in sexual contexts to potentially enhance reproductive success or help choose between partners, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the activities of great apes said that as intimate contact was observed in a broad spectrum of apes it made sense its origins lie deep in our evolutionary past, and an examination of various types of intimate behavior among a wider variety of species might extend its origins back further still.

"Behaviors that we think of as signatures of human life, like intimate contact, are not unique to us if we examine carefully at other animals," the expert noted.

Cultural Elements

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

"However, as humans we thrive or fail on the strength of our relationships, and methods of encouraging trust and closeness will have been significant for millions of years," the professor stated. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be expected that ancient hominins – and including Neanderthals and our own species together – kissed."