The history of life on Earth is deciphered through the examination of fossils. Fossils are inorganic remains of plants and animals that reveal the structure of certain parts of the organism. By examining the way structures change in certain organisms over time, the natural history of a particular group can be reconstructed. Many organisms do not have structures suitable for fossilization, like the soft body of a snail. However, some portions of the snail's body (the shell) are easily turned into fossils over time. Snail shells, millions of years old, are discovered when they are exposed near the surface of Earth. By studying these fossils it is possible to partially deduce what snails, or any other fossilized lineage of organisms, looked like over millions of years of change. Living fossils are divided into two categories. The first includes organisms that are believed to have changed very...