Bananas like other fruits are ripened due to a hormone called ethylene.Ethylene breaks down complex sugars into simple sugars and breaks down pectin, a substance which keeps bananas hard.In addition there are hormones that break down green pigments which you see in un-ripe bananas and replace them with yellow pigments. However as the banana continues to ripen these yellow pigments are broken down and not replaced at all producing the brown color in a process much like that of leaves in deciduous trees (trees which lose their leaves annually) during fall and winter. So brown bananas are a result of over-ripening.