Which fact about fossils is most important to scientists who study evolution?

Knowing the age of the fossils is the most important factor for the scientists that study evolution.  

The age of the fossils is found out with the help of objective and radioactive dating. The fossils themselves generally cannot be dated directly, as the majority of them have no or less of the organic substance left in it, and dating with carbon 14 is only beneficial up to the age of 35000 to 50000 years.  

However, with the application of radiometric dating, the scientists are able to put an objective date. By determining the concentration of radioactive decay in specific elements since the solidification of the igneous rocks, one can find out the exact date of the fossil.  

The most generally used elements for radiometric dating of rocks are uranium, argon., potassium, and rubidium as they exhibit fairly long half lifes.  

not sure if this what   you'r looking for would be percent composition