The study of how life evolved would be impossible if not for the history that is told in the fossilized remains going back billions of years. Scientists have described about 250,000 different fossil species, yet that is a small fraction of those that lived in the past.

The oldest fossils are remains of marine organisms that populated the planet's oceans. When they died, the plants and animals were buried by mud, sand, or silt on the sea floor. Land animals and plants usually decomposed or were eaten, and mainly the hard parts -- teeth, bones, shells, or wood -- were preserved.

Fossils can be formed in several ways. Buried bone and shell contain tiny air spaces into which water can seep, depositing minerals. Reinforced by these mineral deposits, bone and shell can survive for millions of years. Even if the bone or shell dissolves, the mineral deposits in the shape of the body structure remain.

Besides rock, fossils may be found as the result of an organism being entombed in ice, tar (like the famous La Brea Tar Pits in Los Angeles), or amber, in which ancient insects have been found, wonderfully preserved. Rare but highly informative are fossils created by a sudden event, like a volcanic eruption, that traps living things or, in the famous case in Laetoli, Ethiopia, footprints of human ancestors millions of years old.

Fossil remains come to the attention of scientists when they are exposed at Earth's surface. Erosion, land movements, or excavations often have revealed important fossil finds.

Especially rich fossil troves are called Lagerstatten, a German word meaning storehouse. These are localities where conditions were right to preserve even soft-bodied animals, and they allow scientists to read a key portion of life's history. In the famous Burgess Shale in Canada, for example, scientists have found dozens of bizarre, previously unknown animals.

Interpretation of fossils poses another set of challenges, and their age can only be estimated by radiometric dating of rocks they were found near or within. Discovery of fossils is only the beginning of mining their secrets.

Plot locations of important African finds in the search for early human remains in this NOVA classroom activity.