Bacteria do not have hard skeletons or shells. If there were bacterial fossils, how would they form? Bacteria are so small that even if they were fossilized, how could we find them? In fact, bacteria can indeed be preserved as fossils. There is a special group of organisms in nature called cyanobacteria, which are among the earliest life forms on Earth. Scientists have found fossils similar to cyanobacteria in 3.5 billion-year-old ancient strata. Cyanobacteria are larger than most bacteria and can secrete a thin cell wall. Most importantly, cyanobacteria can form large layered structures called stromatolites. Stromatolites are biogenic sedimentary structures formed by cyanobacteria and other microorganisms during their growth and metabolic activities, by adhering and precipitating inorganic salts or capturing mineral particles. If these stromatolites are ground into thin slices, fine fossils of cyanobacteria and algae may be preserved within them. In the Zhangjiakou-Chicheng line of Hebei Province, China, strata from the late Precambrian period 1.8 billion years ago contain large amounts of iron ore, which constitutes the main body of Xuandong Iron Mine. In the iron ore and associated stromatolite iron ore of Xuandong Iron Mine, six genera of fossil cyanobacteria have been found.
Apart from cyanobacteria, the number of fossil bacteria is not very large. Under certain chemical conditions, after bacterial cells are replaced by inorganic salts, replicas of living cells or pseudocrystals are formed. Some bacteria can secrete iron outer sheaths and sometimes also form fossils. Some bacteria can burrow into animal shells or rocks, forming tiny tubes within the shells, thus indirectly recognizing them by analyzing these tubes in fossil shells of different periods. In addition, amber often preserves a large number of exquisite bacteria, and some bacteria can even survive after hundreds of millions of years. Disease-causing bacteria can also be found in some animal bone fossils, indicating that these animals suffered from diseases caused by bacteria during their lifetime. The most amazing fossil bacteria may be magnetotactic bacteria, which can form nanometer-sized magnetite crystals within cells. The magnetite crystals produced by this bacteria can be identified in 2 billion-year-old ancient rocks, with sizes of several micrometers, which may be the smallest fossils ever found.