Scientists have found evidence suggesting that kissing dates back up to 21 million years and that our ape ancestors and Neanderthals likely locked lips, research published Wednesday said.
Kissing has long presented an evolutionary puzzle to scientists as it appears to carry high risks such as disease transmission while having no obvious reproductive or survival advantages.
But the new study found that the practice has evolved over millions of years, and that our extinct human relatives, the Neanderthals, could have shared a peck to stay warm in the ice age.
Related: Neanderthals May Never Have Truly Gone Extinct, Study Reveals
The research led by scientists at the University of Oxford was published in the journal Evolution and Human Behaviour.
"This is the first time anyone has taken a broad evolutionary lens to examine kissing," said co-author Matilda Brindle, an evolutionary biologist at the University of Oxford.
"Our findings add to a growing body of work highlighting the remarkable diversity of sexual behaviours exhibited by our primate cousins."
For the purpose of the study, researchers defined kissing as a "non-aggressive, mouth-to-mouth contact that did not involve food transfer".
"The results indicate that kissing is an ancient trait in the large apes," evolving in primate ancestors from 17 to 21 million years ago, according to an Oxford University statement.
"Kissing was retained over the course of evolution and is still present in most of the large apes," it added.
The researchers first collected data about modern primates that have been observed kissing, including chimpanzees, bonobos and orangutans.
Then, treating kissing as a "trait", they mapped it to the family tree of primates. By stimulating different evolutionary scenarios, they estimated the probability that ancient ancestors also engaged in kissing.
Related: Humans And Neanderthals Had a Longer, More Intertwined Relationship Than We Thought
Previous studies have found that humans and Neanderthals had shared oral microbes through saliva transfer, suggesting that the species had kissed each other.