It may seem surprising that bacteria can leave fossils at all. However, one particular group of bacteria, the cyanobacteria or “blue-green algae,” have left a fossils that extend far back into the Precambrian.
The cyanobacteria have an extensive fossil record. The oldest known fossils, in fact, are cyanobacteria from Archaean rocks of western Australia, dated 3.5 billion years old. This may be somewhat surprising, since the oldest rocks are only a little older: 3.8 billion years old!