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The big issues

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How we know what happened when

Life began 3.8 billion years ago, and insects diversified 290 million years ago, but the human and chimpanzee lineages diverged only five million years ago. How have scientists figured out the dates of long past evolutionary events? Here are some of the methods and evidence that scientists use to put dates on events:

Atom icon 1. Radiometric dating relies on half-life decay of radioactive elements to allow scientists to date rocks and materials directly.
Stratigraphy icon 2. Stratigraphy provides a sequence of events from which relative dates can be extrapolated.
DNA icon 3. Molecular clocks allow scientists to use the amount of genetic divergence between organisms to extrapolate backwards to estimate dates.

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More details
Read more about radiometric dating, stratigraphy, and molecular clocks.

Evo examples
Learn more about molecular clocks in context: Evolutionary evidence takes the stand, a news brief with discussion questions.

Teach this
Teach your students about relative and absolute dating:

Find additional lessons, activities, videos, and articles that focus on relative and absolute dating.

Teach your students about molecular clocks: Solving the mystery of the Neanderthals, a web activity for grades 9-12.