化石(fossil)普遍在挖掘岩石的过程中被发现，其石化的物体象征著生命遗留的痕迹（化石一词是源自拉丁语 fossilis）。有时候被石化的生物还保存著原有的实际形体，如骨骼。有时候生物的遗骸因被其周遭的岩石和矿物质渗入腐蚀而成为岩石和矿物，其岩石仍保存著遗骸的形状结构，所以化石可说是生物原有的遗骸和矿物化的遗骸两者之混合。也有化石形成的模式像是用模型复印在石头上，如脚印。最古老的化石是微化石 (microfossils)（蓝绿藻 cyanobacteria 的化石）大约出现在34亿5600万年前。
友谊网页连结
- Some information about the very earliest fossils provided by Westphalia Wilhelms University in Münster. Most of the links from this site are dead, but some still work.
- A fine article by Peter N. Spotts in the Christian Science Monitor about the earliest fossils from 3.4 billion years ago.
- One of the many fine websites devoted to fossils.
- The San Diego Natural History Museum’s web page about finding fossils.
- William L. Newman’s old USGS publication Geologic Time has a page showing some index fossils used to date rocks.
- A good explanation of the concept of fossil succession.