How old are fossils?
The oldest known fossils are of blue-green algae, which are single-celled organisms that do not have a nucleus. The algae fossils were found in 3.2-billion-year-old cherts, shales, and sandstone from Transvaal, South Africa. The oldest known fossils of multicellular organisms date back about 700 million years ago. The largest number of fossils are from the Cambrian period (570 to 505 million years ago), when living organisms began to develop bones and shells. An organism’s soft tissues will likely decompose before leaving an imprint in soil or clay. Organisms with hard body parts were more likely to leave an imprint in the ground and become fossilized. Sources: Fenton, Carroll L. The Fossil Book, pp. 91, 93; Trefil, James. 1001 Things Everyone Should Know About Science, pp. 59-61.
