The odds against any specific individual organism becoming a fossil are very great indeed. To become fossilized, a plant or animal must usually have hard parts, such as bone, shell, or wood. It also must be buried quickly to prevent decay or disintegration.

Even if the requirements for preservation have been met, there are other factors which might prevent fossils from being discovered. Many fossils have been destroyed by erosion, or the rock enclosing the fossil is subjected to temperatures and pressure which destroy those fossils. Much of the fossil record is in deeply buried deposits that are inaccessible to study.

Fossils are not only very rare but they are also give a limited view of life through the past (because of selective preservation of certain types of organisms). Those plants and animals living in or near water, where sediments accumulate quickly, are more likely to be preserved than those living on dry land.