Researchers have delved into the ancient past to discover when romantic kissing likely originated and the pathogens, such as the herpes simplex virus, that have followed the practice from then until now.

Troels Pank Arbøll and Sophie Lund Rasmussen examined cuneiform writings from Mesopotamia, modern-day Iraq and Syria, looking for references to kissing.

The pair differentiate between two types of kiss: the “friendly-parental” kiss and the “romantic-sexual” kiss. The former is the kind of kiss a mother gives her child when she drops them off at school and, say the researchers, is seen in humans across time periods and the world.

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The second is not culturally universal. It’s thought that the sexy smooch evolved as a way of evaluating a potential mate through chemical cues communicated in saliva or breath, eventually leading to the sexual act.

Read more: New Atlas