Lesson Summary

  • Fossils are preserved remains or traces of organisms that lived in the past. Most fossils form in sedimentary rock. Fossils can also be preserved in other ways. Fossilization is rare. It’s very unlikely for any given organism to become a fossil.
  • Fossils are the best form of evidence about the history of life on Earth. Fossils also give us clues about major geological events and past climates. Index fossils are useful for determining the ages of rock layers.

Points to Consider

Fossils can help scientists estimate the ages of rocks. Some types of evidence show only that one rock is older or younger than another. Other types of evidence reveal a rock’s actual age in years.

  • What evidence might show that one rock is older or younger than another?
  • What evidence might reveal how long ago rocks formed?