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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 also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have proposed ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have found humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were kissing," the researcher noted, adding that the concept aligned with studies that has found people of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Romantic Interpretation

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

Publishing in the journal Evolution and Human Behavior, Brindle and her team report how, to investigate the evolutionary origins of intimate contact, they first had to develop a definition that was not restricted by how humans smooch.

Defining Kissing

"There have been some efforts to define a kiss, but it's very much been focused on humans, which means that essentially other animals don't kiss. Now we understand that they likely engage, it may appear different from what human kissing resembles," explained Brindle.

Nonetheless, she noted some actions that looked like intimate contact were distinct activities – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species called certain marine animals.

Consequently the team came up with a description of kissing centered around social behaviors involving intentional oral interaction with a individual of the identical group, with some motion of the oral area but absence of nutrition.

Study Methods

The lead researcher said they focused on accounts of kissing in primates from the African continent and Asia, including bonobos, apes and great apes, and employed online videos to verify the observations.

Scientists then integrated this information with information on the evolutionary relationships between living and extinct types of such primates.

Historical Origins

Researchers propose the findings indicate 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 evolutionary lineage suggests it is probable they, too, indulged in a kiss, the scientists conclude. But the activity may not have been limited to their specific group.

"Reality that humans kiss, the reality that we currently have demonstrated that Neanderthals probably engaged, suggests that the both groups are probably did kissed," the researcher noted.

Evolutionary Importance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be employed in sexual contexts to possibly enhance reproductive success or assist in selecting between partners, while it could assist strengthen connections when practiced in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was seen in a wide range of primates it was logical its roots lie deep in our ancient history, and an examination of various types of intimate behavior among a broader range of animals might push its beginnings back even earlier still.

"Things that we consider as characteristics of human life, like kissing, are not exclusive to us if we look closely at other animals," he said.

Social Elements

An archaeology expert said that intimate contact had a cultural element as it was not common to all human groups.

"However, as humans we succeed or struggle on the quality of our relationships, and methods of promoting trust and intimacy will have been important for eons," she said. "This could represent an image that appears a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but actually it ought to be no surprise that Neanderthals – and including them and our human ancestors together – kissed."