• How do most fossils form?
• How can scientists determine a fossil’s age?
• What is the Geologic Time Scale?
• What are some unanswered questions about evolution?

• How do we know so much about dinosaurs if none of them are alive now?

• The formation of a fossil is a rare event.
• Usually on the hard parts of an organism form fossils.
– bones
– shells
• Most fossils form when organisms that die become buried in sediments .
– Particles of soil and rock
• Layers of sediments from rivers and streams may cover dead organisms.
• Over millions of years, the layers may harden to become sedimentary rock .

• Remains buried in sediment change to rock.
– Dissolved minerals in water soak into the buried remains.
– Gradually, the minerals replace the remains.

• Shells or other hard parts buried in sediment gradually dissolve.
• An empty space remains in the hard part once occupied.
• Mold
– hollow space
• Cast
– mold filled with hardened minerals

•Organisms can be preserved in substances other than sediments.
–Resin
–Ice

• By determining the age of a fossil, scientists can . . .
– understand how living things have changed through time.
– determine the order in which past events occurred.
– construct and reconstruct the history of life on Earth.

• Used to determine which of two fossils is older.
• Relative dating cannot tell a fossil’s actual age.
• Layers of sedimentary rock pile up one on top of another over time.
– top layers – youngest
– bottom layers – oldest

• Used to determine the actual age of fossils.
• Fossil rock contains radioactive elements .
– Elements that decay and break down into different elements.
• The half-life of the radioactive element is measured.
– Time it takes for half of the atoms to decay.
• Scientists use this information to calculate the age of the rock, and thus the age of the fossil.

• Fossils help scientists piece together information about
Earth’s past.
• Scientists have learned about the history of life on
Earth.

• Almost all of the species preserved as fossils are now extinct .
– No members of that species are still alive.
• Most of what is known about extinct species is based on the fossil record.

• A “calendar” of
Earth’s history.
• History of life on
Earth dating back
4.6 billion years.
• Time span divided into smaller units called eras and periods.

• The fossil record is incomplete.
– Most organisms die without leaving fossils behind.
• There are gaps in the fossil record.
– What causes mass extinction?
• Major climate changes?
– An asteroid hitting Earth?
– Dust clouds blocking the sun preventing plant growth?
– Volcanic eruptions?
– At what rate does evolution occur?
• Gradualism?
• Punctuated equilibrium?

Gives us a glimpse into the future!