To many, fossils are curiosities and are sometimes aesthetically pleasing. But what do fossils actually tell us?

First of all they are records of past environments. If we find sharks' teeth in a layer, we can reasonably assume that this layer formed in a marine environment. But the information is much more subtile than that.  For example, fossil pollen studies are a standard tool of climatalogy.  And cypress pollen will tell us a different story from that of oak or spruce pollen. Given enough data we can reconstruct entire ecosystems, past climates and infer geography.

Secondly fossils are the documents of evolution. The gene pool of any species changes only slowly. Because the human lifespan is short, there is little chance for watching evolution in action. Even if we could, changes are so gradual that we may not even be aware of them. Fossils give us a snapshot of what life was like when they were flourishing. Even though these pictures may be more blurred than we like, and lack much of the color and detail of the living things themselves, they nevertheless allow us to see how life has changed over time and to realize that we and all the other species alive now are but the latest in a long succession of ever changing forms.

Thirdly, fossils are time markers. Because evolution never repeats itself, if we know the succession of life forms, then we can create a time line based on this sequence. Much of the history of the earth is based on the powerful concept that every interval of time is characterized by its unique life forms.  Conversely, once we have established this sequence, we can fit any fossil we find into its proper time interval, based on its stage of evolution. Because all (nearly) fossils were entombed into the rock layer as it was forming, if we know the age of the fossil, we know the age of the rock layer.  Some fossils are of more help in this than others. We call such useful fossils index fossils. To be a good index fossil, a fossil must be common, easily recognized (even by geologists) geographically widespread, and rapidly evolving.

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