Discuss why scientists know so little about Precambrian Earth

Pre-Cambrian life was mostly soft-bodied - - there were very few if any creatures with bones, skeletons, or hard shells, so there were very few fossils left.

Besides, it was a long, long time ago - - and geological activity on the earth (volcanic action, subduction of continental plates, erosion, etc.) have wiped out a lot of what was the surface of the earth then.