Chapter 10 — Science & Nature
How old is the Earth?
About 4.54 billion years, dated from meteorites that formed at the same time as the solar system. Life appeared around 3.7 billion years ago; humans are the last few seconds on a 24-hour scale of Earth's history.
Follow-up questions
How do we know?
Radiometric dating — measuring how much of certain radioactive isotopes have decayed.
What existed before Earth?
The sun and a dust cloud from an earlier generation of stars whose supernovas made the heavy elements Earth is built from.
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