What types of evidence can fossils provide about earth's history?

Fossils provide one body of evidence for evolution and an accurate theory of our phylogeny back to every living organism's common ancestor (because we are all related and so must have come from the same ancestor).

By performing radiometric dating of the surrounding geology where the fossil was found. There are several radiometric clocks that can be used and as many as are possible are used to get an accurate and confirmed age. This allows evolutionary scientists to better understand how we and all other living things have evolved, and allows for comparison with the other bodies of evolutionary evidence (such as genetic clocks and taxonomy).

That we evolved is a fact, the only question evolutionary scientists are asking now is "How?" And we're getting closer to the truth every day. A better explanation can be found on YouTube - search for DonExodusII.