A kiss may be just a kiss, but when sweethearts pucker up on Valentine’s Day, they will be participating in one of the most bizarre and unlikely of human activities.

Experts say kissing evolved from sniffing, which people did centuries ago as a way of learning about each other.

“At some point, they slipped and ended up on the lips, and they thought that was a lot better,” said Vaughn Bryant, an anthropologist at Texas A&M University and an authority on the evolution of human kissing. “You got a lot more bang for your buck.”

For most of early human history, smell was more important than any other sense for human relationships, said Sheril Kirshenbaum, author of “The Science of Kissing.” People would use smell to determine a person’s mood, their health and their social status, she said.

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