The normal color of human stool is brown due to the presence of heme-derived bile pigments, primarily stercobilin. These pigments are formed from the breakdown of hemoglobin in the liver and intestines. Variations in stool color can indicate different pathological conditions, but brown stool generally reflects normal digestion and absorption processes. Abnormal colors like black, red, or white suggest gastrointestinal bleeding, infection, or biliary obstruction, making brown stool an important clinical indicator of healthy gastrointestinal function.

Reference: Schwartz's Principles of Surgery, F. Charles Brunicardi, 11th Edition.