The question of microbial fossils is very interesting. Unfortunately, Schopf's early fossils are controversial in interpretation. However, many microfossils are now well documented. A good general reference is Andrew Knoll's book Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Also, the question of microfossil documentation is discussed in Chapter 17 of Slonczewski & Foster, Microbiology: An Evolving Science.