THE FOSSIL RECORD
The fossil record provides an historical account of the evolution of life. Fossils are parts or traces of ancient organisms (e.g., casts, molds, mineralized bones, footprints), usually preserved in sedimentary rock. One way the age of a fossil can be determined is by noting its placement within the layers of sedimentary rock.

Advances in our understanding of physics and the nature of atoms have given us new methods of dating fossils, including radioactive isotope dating. Carbon dating is useful for determining the age of relatively recent fossils, while potassium-argon dating is used for older fossils in proximity to igneous rock. Determining the age of rocks and fossils allows us to build a history of the Earth and its life forms.

Using the age of rocks and associated fossils, scientists have built a picture of apparent changes in groups of organisms from one geologic time period to another. In doing so, scientists have demonstrated linkages over time that support the idea that organisms have evolved and older species have given rise to more recent species.