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Among seabirds to polar bears, chimpanzees to great apes, various animals appear to kiss. Now, researchers suggest that ancient hominins did it too – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among previous studies, 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, explaining that the idea aligned with research that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Romantic Spin

"This offers a more romantic perspective on human-Neanderthal relations," the lead researcher commented.

Writing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to investigate the evolutionary origins of kissing, they first had to develop a description that was not limited to how humans kiss.

Defining Intimate Contact

"There have been some efforts to describe a intimate act, but it's very much been human-centric, which implies that essentially other animals do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing resembles," said the evolutionary biologist.

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

Consequently the research group came up with a definition of kissing centered around social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but absence of food.

Research Approach

The lead researcher explained they concentrated on accounts of kissing in non-human species from Africa and Asia, including primates, chimpanzees and orangutans, and used digital recordings to confirm the reports.

Scientists then integrated this information with information on the evolutionary relationships between extant and ancient types of such animals.

Historical Timeline

Researchers propose the findings indicate intimate contact developed approximately 21.5m and 16.9m years ago in the predecessors of the large apes.

The position of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the researchers say. But the behavior may not have been confined to their specific group.

"Reality that humans engage intimately, the reality that we currently have shown that ancient relatives very likely engaged, indicates that the both groups are also likely to have engage," the researcher added.

Evolutionary Significance

While the scientific reasoning is debated, Brindle said intimate contact could be used in sexual contexts to possibly increase reproductive success or help choose between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of great apes commented that as kissing behavior was seen in a broad spectrum of apes it made sense its roots lie deep in our ancient history, and an analysis of various types of intimate behavior among a wider variety of animals might extend its origins back further still.

"Behaviors that we think of as signatures of our species, like intimate contact, are not exclusive to us if we look closely at other animals," the expert noted.

Cultural Elements

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 quality of our emotional bonds, and ways of promoting trust and closeness will have been significant for millions of years," the professor stated. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be no surprise that ancient hominins – and including them and our own species together – engaged intimately."