The remains of plants and animals that have been preserved in the earth, close to their original shape, are called fossils. This word comes from the Latin fossilis, meaning “something dug up.” The different types of fossils depend on the remains and conditions present at the time the organism died. Fossils may be formed from the hard parts of an organism, such as teeth, shells, bones, or wood; they may also be unchanged from their original features, the entire organism having been replaced by minerals such as calcite or pyrite. Animals and plants have also been preserved in other materials besides stone, including ice, tar, peat, and the resin of ancient trees.

Fossils of single-celled organisms have been recovered from rocks as old as 3.8 billion years. Animal fossils first appeared in rocks dating back over one billion years ago. The occurrence of fossils in unusual places, such as dinosaur fossils in Antarctica and fish fossils on the Siberian steppes, is due to the shifting of the continental plates that make up Earth’s crust, and environmental changes over time, such as an ice age. The best explanation of dinosaurs in Antarctica is not that they evolved there, but that Antarctica was once part of a much larger landmass with which it shared many life forms.