Studies of fossilization of bacteria provide an indication of the age of ancient bacteria and of the rate of geological and geochemical processes on ancient Earth. Fossils of cyanobacteria or "blue-green algae" have been recovered from rocks that are nearly 3.5 million years old. Bacteria known as magnetobacteria form very small crystals of a magnetic compound inside the cells. These crystals have been found inside rock that is two billion years old.

The fossilization process in cyanobacteria and other bacteria appears to depend on the ability of the bacteria to trap sediment and metals from the surrounding solution. Cyanobacteria tend to grow as mats in their aquatic environment. The mats can retain sediment. Over time and under pressure the sediment entraps the bacteria in rock. As with other living organisms, the internal structure of such bacteria is replaced by minerals, notably pyrite or siderite (iron...