Final answer:
Fossils are the remains or traces of organisms that have been preserved by a natural chemical process that turns them partially or wholly into rock. They provide important evidence for the theory of evolution and help scientists understand life in the past.
Step-by-step explanation:
Fossils are the remains or traces of organisms that have been preserved by a natural chemical process that turns them partially or wholly into rock. When an organism dies, its soft parts decompose quickly, but the hard parts like bones and teeth can mineralize and form fossils. The conditions required for fossil formation are rare, so the chance of an organism being preserved as a fossil is very low. Fossils are important evidence for the theory of evolution and help scientists understand what life was like on Earth in the past.