http://en.wikipedia.org/wiki/Sweetness
Studies indicate that responsiveness to sugars and sweetness has very ancient evolutionary beginnings, being manifest as chemotaxis even in motile bacteria such as E. coli.[1] Newborn human infants also demonstrate preferences for high sugar concentrations and prefer solutions that are sweeter than lactose, the sugar found in breast milk.[2][3] Sweetness appears to have the highest taste recognition threshold, being detectable at around 1 part in 200 of sucrose in solution. By comparison, bitterness appears to have the lowest detection threshold, at about 1 part in 2 million for quinine in solution.[4] In the natural settings that human primate ancestors evolved in, sweetness intensity should indicate energy density, while bitterness tends to indicate toxicity[5][6][7] The high sweetness detection threshold and low bitterness detection threshold would have predisposed our primate ancestors to seek out sweet-tasting (and energy-dense) foods and avoid bitter-tasting foods. Even amongst leaf-eating primates, there is a tendency to prefer immature leaves, which tend to be higher in protein and lower in fibre and poisons than mature leaves.[8] The 'sweet tooth' thus has an ancient evolutionary heritage.