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From Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Now, scientists propose that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were closely connected. In earlier research, scientists have discovered humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were engaging in intimate contact," she said, adding that the idea chimed with research that has revealed humans of non-African ancestry have bits of ancient genetic material in their genome, revealing interbreeding was at play.

Intimate Interpretation

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

Writing in the journal Evolution and Human Behavior, the researcher and her team detail how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how humans smooch.

Describing Intimate Contact

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

However, she noted some behaviors that looked like kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in aquatic species called French grunts.

As a result the research group came up with a definition of kissing based on social behaviors involving directed oral interaction with a member of the identical group, with some movement of the oral area but no transfer of food.

Research Methods

The lead researcher explained they concentrated on reports of intimate behavior in primates from Africa and Asian regions, including bonobos, apes and orangutans, and used digital recordings to confirm the reports.

The researchers then combined this data with details on the genetic connections between living and ancient types of such primates.

Historical Origins

Researchers propose the results indicate kissing developed somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is likely they, too, engaged in a kiss, the scientists say. But the behavior may not have been confined to their specific group.

"The fact that modern people kiss, the fact that we now have shown that Neanderthals probably engaged, indicates that the two [species] are probably did kissed," Brindle noted.

Evolutionary Significance

While the evolutionary explanation is debated, the expert said intimate contact could be used in reproductive situations to potentially increase mating outcomes or assist in selecting between partners, while it could assist reinforce bonding when used in a non-sexual manner.

A separate researcher in the activities of great apes commented that as kissing behavior was seen in a broad spectrum of primates it made sense its roots extend far into our ancient history, and an analysis of different forms of intimate behavior among a wider variety of species might extend its origins back further still.

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

Social Aspects

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

"Nonetheless, as people we succeed or struggle on the quality of our relationships, and ways of promoting confidence and closeness will have been significant for eons," the professor stated. "This could represent an image that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be no surprise that ancient hominins – and including Neanderthals and our human ancestors together – kissed."