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    THE GEOLOGIC TIME SCALE

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    Mohamed Selim
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    THE GEOLOGIC TIME SCALE

    Post by Mohamed Selim on Tue May 04, 2010 2:58 pm

    .






    Geologists have divided Earth history into units displayed in the geologic time scale (Table 1–1). The units are called eons, eras, periods, and epochs and are identified primarily by the types of life that existed at the various times. The two earliest eons, the Hadean and Archean, cover the first 2.5 billion years of Earth history.

    Life originated during Archean time. Living organisms then evolved and proliferated during the Proterozoic Eon (protero is from a Greek root meaning “earlier” or
    “before” and zoon is from the Greek word meaning “life”). However, most Proterozoic organisms had no hard parts such as shells and bones. Most were single
    celled, although some multicellular organisms existed.


    The Proterozoic Eon ended about 538 million years ago. Then, within an astonishingly short time—perhaps as little as 5 million years—many new species evolved.
    These organisms were biologically more complex than their Proterozoic ancestors, and many had shells and skeletons. The most recent 13 percent of geologic time,
    from 538 million years ago to the present, is called the Phanerozoic Eon (phaneros is Greek for “evident”). The Phanerozoic Eon is subdivided into the Paleozoic Era
    (“ancient life”), the Mesozoic Era (“middle life”), and
    the Cenozoic Era (“recent life”)






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