• Adding Time
• How we know what happened when
  • Radiometric dating
  • Stratigraphy
  • Molecular clocks
• Important events in the history of life
  • Origins of life
  • How did life originate?
  • Looking for the oldest fossils
  • Studying the origin of life
  • Human evolution

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:

 
1. Radiometric dating relies on half-life decay of radioactive elements to allow scientists to date rocks and materials directly.

 
2. Stratigraphy provides a sequence of events from which relative dates can be extrapolated.

 
3. Molecular clocks allow scientists to use the amount of genetic divergence between organisms to extrapolate backwards to estimate dates.

	•	Radiometric dating
	•	Stratigraphy
	•	Molecular clocks

	Lesson plans for teaching about relative and absolute dating