The fossil record encompasses information in the form of biological remains preserved in rock to offer clues about the history of life throughout the planet. In many cases, entire organisms are found in geological strata. These organisms, which range from insects to mammals, act as snapshots in time, giving paleontologists and other scientists an idea of how life has changed over 4 billion years.
Scientists have gathered evidence of evolutionary change for the last 300 years. Much of this evidence exists in sedimentary rock layers. These rocks are the most common type of stone on the planet.
Fossils are formed in a variety of ways. For example, when shells, leaves and other objects collect on the bottoms of lakes, rivers and oceans, they are often buried in a variety of sediments. Over time these sediments compact to form shale, limestone, sandstone and other rock layers. Without being subjected to air, water and other eroding elements, the buried objects are then preserved as fossils.
The first recorded instance of a person discovering a fossil occurred in the 17th century when a man noted the similarity between what many had believed to be rocks called "tongue stones" and shark teeth. It turned out the rocks were fossilized shark teeth.