Fossils are the dead remains of plants and animals which undergo several physical and chemical processes after they are dead.

Fossils help us in determining how a presently living species evolved or developed from the old species.

Example- Archaeopteryx lithographica is a fossil that resembles a bird (has feathers and wings) but has several features of reptiles (tail and teeth) . So Archaeopteryx is supposed to be the connecting link between aves and reptiles thus suggesting that birds have evolved from reptiles.