When organisms die, their bodies decompose due to action of micro organisms. However, sometime the body or at least some parts of the body may be in such an environment that does not let it decompose completely. All such preserved traces of living organisms are called fossils.

The age of fossils can be estimated by the following two methods:

1. If we dig into the earth and start finding fossils, it can be assumed that the fossils closer to the surface are more recent to those found in deeper layers.
2. By detecting the ratios of different isotopes of the same element in the fossil material.