Fossils
They show how life has changed and the earth's past history.
http://www.nowpublic.com/world/fossils-1
How Fossils were Formed
A plant or an animal dies in the water and they quickly cover by layers upon layers of sediments. The pressure turns the sediments and the minerals into a fossil.
http://science.nationalgeographic.com/staticfiles/NGS/Shared/StaticFiles/Science/Images/Content/trilobite-fossil-422863-xl.jpg