Fossils are preserved evidence of once-living organisms—typically in sediment or rock. Body fossils are preserved remains of an organism or indications of remains; trace fossils are preserved indications of an organism's life activities, such as trails or burrows. Fossilization is the preservation of an organism as a fossil. Paleontology is the study of fossils, and the use of fossils for the study of Earth history.

As of 1998, the oldest body fossils discovered are 3.8 billion-year-old cyanobacteria, or blue-green algae. The oldest known trace fossils are 3.5 billion-year-old stromatolites--sediment layers formed by colonies of blue-green algae, known as algal mats, as a means of acquiring food from seawater.

The large and diverse collection of fossils that can be seen at a good science museum might indicate that fossils are common--and they are--but the likelihood that any single organism will be preserved, or fossilized, in the rock record is almost...