Using the simple dates marking the end of various periods (e.g., 145 MYA = end of Jurassic period),
what fraction of earth’s history occurred after
1. the “Cambrian explosion” (aka the start of the Phanerozoic eon and Cambrian period)
2. the end-Cretacious mass extinction (aka the start of the Cenozoic era and Paleogene period)
3. the emergence of our species Homo sapiens (at about 200,000 years ago)
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