While kissing might feel like one of the most natural things in the world, this familiar behavior is quite mysterious—various animals also kiss, despite a lack of practical benefits and a real risk of disease transmission.

To shed light on the smooching enigma, researchers have attempted to reconstruct the evolutionary history of kissing in the primate family tree, which includes mammals such as monkeys, apes, and humans. The team’s findings suggest that kissing is an ancient trait, evolving in the ancestors of great apes (such as humans) 21.5 to 16.9 million years ago and sticking around to this day in most surviving great ape species.

“This is the first time anyone has taken a broad evolutionary lens to examine kissing,” Matilda Brindle, an evolutionary biologist at the University of O

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