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GEOLOGIC
TIME
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The earth is
approximately 4.6 billion years
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old and has
undergone large changes
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throughout
its lifetime. The earliest forms
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of life may
have appeared as early as
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about 4
billion years ago, with bacteria and
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algae-like
creatures appearing about 2.5
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billion
years ago. Evidence has emerged
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recently that
about 600-700 million years
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ago, the
earth was probably completely ice-
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covered more
than once (snowball
earth).
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Then, about
540 million years ago, the
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Cambrian Explosion occurred. This
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marks the
first appearance of creatures
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with hard
skeletons and is associated with
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an striking
increase in biological diversity.
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