Ooph. If vision is needed for utility, taste is needed for life. Food is my lifeblood. Huge fan of taste to be honest.
Your tongue is the instrument for tasting things. On your tongue, you have taste buds specially designed to taste one of the five basic tastes: sweet, salty, bitter, umami, and sour. These taste buds have receptors on them that translate chemical stimuli into the subjective experience that is the five tastes.
Sweetness has been evolutionarily beneficial because sweetness corresponds to simple sugars. Simple sugars can be converted into energy by the body. This meant sweetness taste buds were a way to quickly detect how much convenient energy was in a morsel of food. When simple sugars came into contact with cell receptors, the cell receptors would cause channels to open, activating the cell and sending a signal to the brain.
We need salt to survive. Salt balances our fluid levels in our body, and without it, we would die. This is why taste buds have developed to taste salt, and why we like salty foods. The cell receptor that detects salt is simply a channel that lets sodium through. When sodium enters a cell, it causes the cell to send a signal to the brain.
We tend not to like foods that are too bitter. This is because historically, bitter foods tended to be toxic. For example, ricin tastes bitter, although I would never experimentally confirm this. Bitterness is the presence of certain chemicals. When these chemicals touch a bitter taste bud, the taste bud activates, sending the signal for bitter.
Umami is a more specific and scientific description of savory. Tasting umami involves tasting certain amino acids, such as glutamate, or ribonucleotides. We need glutamate to survive, so why umami developed makes sense. This is why MSG, or monosodium glutamate, makes things taste better. The cellular mechanism of umami is similar to sweet, with the exception of what stimuli is detected.
Sour is the ability to detect acidic foods. Sour is also more complicated than the previous tastes. We dislike extremely sour foods, like straight lemon juice, but we like mildly sour foods, like a lemon bar. Theories suggest that extremely sour foods were caused by bacteria or fungi spoiling foods, releasing lactic acid. Another thought is that Vitamin C (ascorbic acid) is necessary for humans, and needs to be supplemented by sour foods such as limes or oranges. There’s no clear explanation. Anyways, sour taste buds have channels that let hydrogen ions into the cell. Acidity is characterized by low pH, and a low pH means there are a lot of hydrogen ions floating in a solution. Therefore, the more acidic a substance is, the more hydrogen ions would enter sour taste buds, activating them and sending a signal to the brain.