When organisms die, their bodies decay and decompose. But every once in a while, the body or at least some parts may be in an environment that does not let it decompose completely. If a dead insect gets caught in hot mud, for example, it will not decompose quickly, and the mud will eventually harden and retain the impression of the body parts of the insect. All such preserved traces of living organisms are called fossils.
The fossils tell us the following:
(i) It helps to identify an evolutionary, relationship between apparently different species.
(ii) The fossils present in the bottom rocks are simple while the most recent fossil found in the upper strata are highly complex. This geographical succession completely agrees with the concept of evolution.
