How are scientists able to use fossils of extinct organisms as evidence?

Fossils show exactly how all extinct organisms are related to all organisms alive today.
Fossils provide weak evidence and are rarely used to show ancestry and relationships to present-day organisms.
Fossils can help scientists trace physical changes and compare how organisms looked over time.
Fossils that are deeper in the earth provide better evidence than fossils that are found closer to the surface.

Fossils can help scientists trace physical changes and compare how organisms looked over time.