Why are fossils rare in precambrian rocks?
Because only the earliest forms of life (protozoa, algae, etc.) existed at that point in time three billion years ago, and those forms did not survive in the sediments that covered them at the bottom of the seas.
Here is a link to a chart of geologic time: http://anthro.palomar.edu/earlyprimates/time_scale_of_Earth_1.htm
In that chart, you can study how life forms evolved, changed, became extinct, etc.