A genetic variation seen worldwide in which people either taste or do not taste a bitter, synthetic compound called PTC has been preserved by natural selection, researchers have reported. Phenylthiocarbamide (PTC) is not found in nature, but the ability to taste it correlates strongly with the ability to taste other bitter substances that occur naturally, especially toxins. Eons ago, the ability to discern bitter tastes developed as an evolutionary mechanism to protect early humans from eating poisonous plants.