Paleontology (Gr. palaios, old — on, existing + logos, to study), which is the study of the fossil record, provides some of the most direct evidence for evolution. Fossils (L. fossilis. to dig) are evidence of plants and animals that existed in the past and have become incorporated into the earth’s crust (e.g., as rock or mineral). For fossilization to occur, sediments must quickly cover an organism to prevent scavenging and in a way that seals out oxygen and slows decomposition. Fossilization is most likely to occur in aquatic or semi-aquatic environments. The fossil record is, therefore, more complete for those groups of organisms living in or around water and for organisms with hard parts. This documentation provides some of the most convincing evidence for evolution.
In spite of gaps in the fossil record, paleontology has resulted in nearly complete understanding of many evolutionary lineages (figure 4.4). Paleontologists estimate that the earth is about 4.6 billion years old. They have also used the fossil record to describe the history of life on the earth.