Taste buds were the primary tool early humans used for testing foods  Sour taste warns of possible spoilage of food while bitterness is a warning of many poisons  Early humans also recognized sweet foods and salty foods as meeting nutritional needs
So the fact that people can now clearly distinguish these four tastes-sour, bitter, sweet, and salty-is completely explained by people' s use of taste to test for the healthfulness of foods
takes what might be only a partial explanation of a phenomenon to be the complete explanation