When black pepper was first being consumed in large quantities in the early nineteenth century, it was considered hot, due to its piperine content. Chili peppers contain a structurally similar chemical called capsaicin -- both are considered chemical irritants.

Capsaicin bons to nerve cells in our mouth and nose; specifically to receptors called TRPV1. This receptor is designed to warn us about dangerous sensations such as acid, abrasions, and heat. Capsaicin triggers a "hot" response, which is why we feel warm when we eat it: our body interprets capsaicin as thermally warm, hence the use of the word heat when we refer to this particular type of spice.

— Eight Flavors by Sarah Lohman (Page 57)