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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:

		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.