I don't remember where I heard this. Of American origin, native peoples have grown them as a valued condiment for thousands of years. The heat engendered by chilies proved so popular that they rapidly became an integral part of many Old World cuisines, including those of India, Southeast Asia, southern China and parts of Africa.
All wild chilies contain varying amounts of the chemical capsaicin and related compounds.
Capsaicin is not a protein, but a nitrogen-containing lipid related to vanillin, the active principle in vanilla. The key receptor molecule, a protein on the outer surface of the cell, was identified in 1997. When capsaicin comes into contact with it, a cascade of intracellular reactions is triggered that is perceived by the brain as pain. These reactions are very similar to those produced by damaging heat, so it is no coincidence that we sense chilies as being hot. The effect can be so overpowering that sprays containing capsaicin are used to repel grizzly bears and even elephants.