Felipe Fernandez-Armesto answers:
For almost uniquely, in the repertoire of modern Western cuisine, the oyster is eaten uncooked and unkilled. It is the nearest thing we have to “natural” food—the only dish which deserves to be called “au naturel” without irony. Of course, when you eat it in a restaurant, its shell has been barbed and unclamped with all the panoply of civilization by a trained professional, wielding appropriate technology, an inviolable ritual and a stylish flourish. Before that, the oyster was reared underwater on a stone tile or wooden trellis, herded in an oyster bed, grown for years under expert eyes and harvested by practical hands—not plucked from a rock pool as a prize seized from nature. Still, it is the food that unites us with all our ancestors—the dish you consume in which is recognizably the way people have encountered their nourishment since the first emergence of our species.
–from Near A Thousand Tables: A History of Food.
More about oysters here.