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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

This isn't the initial instance scientists have proposed Neanderthals and Homo sapiens were intimately acquainted. Among earlier research, scientists have discovered humans and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, explaining that the concept aligned with research that has revealed people of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Spin

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

Publishing in the journal a scientific periodical, the researcher and colleagues report how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how humans kiss.

Describing Kissing

"Previously there were some previous attempts to define a kiss, but it's largely human-centric, which means that essentially non-human species don't kiss. Now we understand that they likely engage, it may appear different from what our intimate contact resembles," said the evolutionary biologist.

Nonetheless, she said some behaviors that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in fish called French grunts.

Consequently the research group developed a description of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but no transfer of nutrition.

Research Approach

Brindle said they concentrated on accounts of intimate behavior in primates from Africa and Asian regions, including primates, apes and great apes, and used digital recordings to confirm the reports.

Scientists then combined this data with information on the evolutionary relationships between extant and ancient species of such primates.

Evolutionary Timeline

Researchers propose the findings suggest kissing evolved approximately 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this family tree means it is likely they, too, indulged in a intimate act, the researchers say. But the behavior might not have been limited to their own species.

"Reality that modern people kiss, the reality that we currently have shown that ancient relatives probably engaged, suggests that the both groups are also likely to have engage," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle said intimate contact could be used in sexual contexts to possibly increase reproductive success or help choose between mates, while it could assist reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of primates commented that as intimate contact was seen in a broad spectrum of apes it was logical its origins lie deep in our ancient history, and an examination of different forms of kissing among a wider variety of animals might push its origins back even earlier still.

"Things that we think of as signatures of human life, like intimate contact, are not exclusive to us if we examine carefully at different species," the expert noted.

Cultural Aspects

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

"Nonetheless, as people we succeed or struggle on the strength of our relationships, and ways of promoting trust and closeness will have been important for eons," the professor stated. "This could represent an image that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but actually it should be expected that ancient hominins – and including Neanderthals and our human ancestors together – engaged intimately."