Much like prewater. Prewater is the stuff in which fish lived before there was water. Prehistory is what people lived in before history. History is, essentially, anything that has to do with civilization or its establishment. Prehistory is, then, the history of humanity prior to the development of culture based upon civilization. Since we all know civilization is the end-all to human invention and something upon which it is not possible to improve, prehistory isn't that important and the reality of prehistoric life is fairly irrelevant.