Fossils are the remains of plants, animals or other organisms that once existed on earth. These are formed when plants or animals buried in mud and soil. The soft tissues decay easily by leaving the hard bones and shells. Over a period of time sediment builds over these bones and shells, then hardens to form rock.
Fossils help in tracing the organic evolution in the following ways:-

a. Fossils represent modes of preservation of ancient species.

b. Fossils help in establishing evolutionary traits among organisms and their ancestors.

c. Fossils help in establishing the time period in which organisms lived.

d. Fossils provide good evidence of the structure and lifestyle of animals (and even plants) from the past.

e. They provide a record of the remains of long-extinct species.

For ex:

i. A Fossil Bird called Archaeopteryx looks like a bird, but it has many other features which are found in reptiles. Hence it is a connecting link between the reptiles and birds and hence suggests that the birds have evolved from reptiles. Thus, Fossil provides evidence that the present animals and plants have originated from the previously existing ones through the process of continuous evolution.

ii. Earliest fossils are of Monera, followed by Protoctista and then fungi. Plants and animals appeared later. Even among the vertebrates, the oldest fossils are those of the fishes, followed by amphibians and progressively mammals are the latest organisms to appear on earth.

iii. Human fossils show a progressive increase in skull sizes to the present age while evolutionary stages of the horse have been reconstructed on the basis of increased complexity of fossil limbs.