Scientists have argued for a long time that there are more than four tastes1 which we can experience. A fifth taste, named 'umami', was identified in 1908 by Kikunae Ikeda of Tokyo Imperial University. The taste, also known as the monosodium glutamate taste, appears in a lot of protein, so biologically it would have been an important sense to develop. Oriental cuisine has used monosodium glutamate for centuries to add a little kick to food.
It was not, however, until early in 2000 that a team at the Miami School of Medicine led by Nirupa Chaudhari was able to confirm the umami taste and identify the causes of the it.
The flavour works like this: the body is very sensitive to glutamate, which is detected by a protein molecule called mGluR4. This protein is so sensitive to glutamate that all the other flavours are rendered undetectable. The taste buds use a taste receptor that is basically mGluR4 minus its tip (it has been truncated). This version of mGluR4 is not quite as sensitive as the complete version, and does not mask other tastes.
1 Sweet, sour, salty, bitter.