Pre-Cambrian: First life on earth, Mostly single cell bacteria, Earliest era: started 4.6 billion years ago, Rock layers found furthest down, Paleozoic: Cambrian explosion, Second earliest era, ends with Permian extinction (kills 90% of marine life), Pangea formed, Mesozoic: Age of reptiles, Pangea breaks apart, Third earliest era, End with Cretaceous extinction of dinosaurs, Cenozoic: Age of mammals, Humans appear, Present day, Rock layers found at the top,

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