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Period: to
Precambrian Era
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First crustal rocks form.
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Earth's surface is cool enough for liquid water.
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First known unicellular life appears.
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Earth's atmosphere starts to become oxygen rich.
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First known multicellular life appears.
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First shelled organisms, such as arthropods and mollusks, appear.
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Period: to
Palezoic Era
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First possible vertebrates appear.
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Modern, oxygen-rich atmosphere has formed.
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First arthropods appear on land.
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First amphibians appear.
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First reptiles appear.
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Formation of Pangaea is complete.
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Permian mass extinction results from major environmental changes.
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Period: to
Mesozoic Era
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Pangaea begins to break apart.
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First mammals appear.
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First primitive birds appear.
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First flowering plants (angiospersm) appear.
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First modern birds appear.
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Most modern mammal families, hoofed animals, and primates have appeared.
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Period: to
Cenozoic Era
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First grasses appear.
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India collides with Asia, and Antartica drifts over south pole.
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First early human ancestors appear.
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Modern Ice Age begins.
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