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Online Course for Teachers: Teaching Evolution

About this Course 


SESSION 3: What Is the Evidence for Evolution?

Explore Part B: Dating Fossils

Closeup of two hands holding a scientific instrument.

For Darwin's theory of evolution by natural selection to be viable, the Earth had to be very old. In Darwin's lifetime, people estimated the Earth's age by the thickness of sedimentary layers. These early estimates concluded that the sedimentary rocks in Great Britain, for example, were about 12 million years old -- old enough for evolution to have taken place. It wasn't until the twentieth century that more reliable methods of estimating the age of the Earth -- radioactive dating -- were developed. For information on relative and radiometric dating of fossils, watch the Evolution Library video segment "Radiometric Dating."

Radiometric Dating
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Taking it further

To learn more about fossilization and relative and radiometric dating of fossils, look at these Web sites:
The Record of Time
Zoom Dinosaurs at


Next: Explore Part C: Deep Time

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