Scientist Charles Zuker and his colleagues had just figured out how our tongues are able to taste any sour taste. He has zeroed in on the action of a receptor called PKD2L1, which seems to be the receptor which communicates the sour taste.
What remains baffling to the scientists is "why a sour receptor would come to be." They can explain 'bitter' as our way of avoiding poisonous substances, and 'sweet' as our way of knowing what to eat when we need a boost in energy. But sour??? They still don't know why we would need to detect sour food items.
Not all sour foods are bad. Most girls I know like the sour taste. They even like their mangoes green just to have that sour taste. Why do we need the sour taste? I can venture a guess: maybe we need to detect Vitamin C, too.
How about you? Any good guesses?