Re: why has the tongue four different fields to taste(sweet, salty,...)

Our perception of food is a multisensory experience. It includes the feel of the food, i.e. the consistency, the fat content, the hotness (both in terms of temperature and spiciness), the smell, and the taste. Taste has traditionally been categorized into four basic types: sweet, sour, salty, and bitter. Different types of taste receptors (taste buds) recognize the four different types of taste, and these receptors tend to be grouped on different areas of the tongue. Your question can be interpreted two ways: Why do we have these four different tastes; and why are the four different taste receptors grouped together?

First off, the perception of taste is always associated with food, therefore, we must realize that the evolution of taste must have something to do with locating food and deciding what is good to eat (incidentally, only vertebrates have taste organs). Now, our bodies need glucose (sugars) and salts (sodium, chloride, calcium, magnesium, etc…) to function properly; therefore, evolutionarily speaking, it was advantageous to develop the ability to tell what foods contain sugars and salts. The ability to distinguish bitter and sour is probably a reverse of this. That is, bitter foods usually contain poisonous compounds and sour foods are usually spoiled. Again, it is evolutionarily advantageous to tell what potential foods are poisoned and which are spoiled.

Taste buds are grouped on the tongue so that we may perceive the four tastes better. As you know, the different tastes can mask one another, e.g., sweetness can counteract bitterness. If there were sweet taste buds next to bitter taste buds then we wouldn't be able to taste either, but by moving food around our mouth we get to perceive all of the tastes. I hope this answers your question.