Fossils provide evidence in favour of evolution or establish evolutionary relationships by providing missing links. So, it is considered important in the study of evolution.
The age of the fossils can be determined by the following way:
1) Radiometric dating
In this method, the age of the fossil can be determined by tracing the radioactive elements present in the rocks and examining it chemically.
2) Relative dating
The fossils are found in the sedimentary rocks in the form of layers accumulated over the large span of time. So, using this fact, the geologist find the age of fossils in the way that the fossil found at the bottom of those layers are found to be older than that found above those.