Scientists use geological periods as the time scale that defines the history of the Earth. The surface of the Earth is made up of layers of rock, with the most recent layers on top. Fossils found in these rock layers can provide clues to what the planet was like far back in history.

• 3.8 to 2.6 billion years ago: Life on Earth began in the Archaean Era with bacteria
• 590 to 505 million years ago: First multi-celled Animals appeared in the Cambrian Period of the Paleozoic Era
• 248 to 66.4 million years ago: Dinosaurs lived in the Mesozoic Era
• 44 to 66.4 million years ago: First mammals lived in the Cretaceous Period of the Mesozoic Era
• 1.8 million to 10,000 years ago: Homo Sapiens appeared in the Pleistocene Period of the Cenozoic Era

• Palaeos: Proterozoic Eon (2500-600 MYA)

• • Wikipedia: Paleoproterozoic Era
  • Wikipedia: Siderian Period | Rhyacian Period
  • Wikipedia: Orosirian Period | Statherian Period

• • Palaeos: Mesoproterozoic Era
  • Wikipedia: Calymmian Period | Ectasian Period | Stenian Period

• • Wikipedia: Neoproterozoic Era
  • Wikipedia: Tonian Period | Ediacaran Period
  • Palaeos: Cryogenian Period

• Palaeos: Phanerozoic Eon (600-230 MYA)

• • Palaeos: Paleozoic Era
  • Wikipedia: Permian Period
  • Palaeos: Carboniferous Period | Devonian Period
  • Wikipedia: Silurian Period
  • Palaeos: Ordovician Period
  • Wikipedia: Cambrian Period

• • University of California Museum of Paleontology: Mesozoic Era (230-63 MYA)
  • Palaeos: The Triassic Period | The Jurassic Period
  • Wikipedia: Cretaceous Period

• • Paleos: Cenozoic Era (63-5 MYA)
  • Wikipedia: Tertiary Period
  • Quaternary Research Association: Quaternary Period

• Amazon.com: Geological Periods: Books

• Google Product Search: Geological Periods Merchandise

• Barnes & Noble: Geological Time Periods Books

• Amazon.com: A Geologic Time Scale 1989

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