Rocky road

Geologists have just added a new chapter to Earth's lengthy time line, called the geologic time scale.

This scale divides Earth's history into eras (long units of geologic time) and periods (subdivisions of eras). These divisions are based on major changes in the types of life forms that inhabited the planet, explains Andrew Knoll, a geologist at Harvard University. Geologists track the changes using fossils (traces of ancient organisms found in rocks) in different rock layers.

The scale's newcomer--dubbed the Ediacaran (eedee-AH-kah-ren) Period--represents the time when scientists believe the first soft-bodied animals appeared on Earth.

GEOLOGIC TIME SCALE

MILLIONS OF
YEARS AGO     ERA           PERIOD

              Cenozoic      Quaternary      UP TO
                            Tertiary        DATE: You
                                            live during
                                            this period.

         65   Mesozoic      Createous
                            Jurassic
                            Triassic

        248   Paleozoic     Permian
                            Carboniferous
                            Devonian
                            Silurian
                            Ordovician
                            Cambrian

        543   Late          Ediacaran       NEWCOMER:
              Proterozoic   Cryogenian      The new
                                            period
                                            stretches
                                            from 600
                                            to 544
                                            million
                                            years ago.
                                            *

      4,600   Earth was formed

SOURCE: THE GEOLOGICAL SOCIETY OF AMERICA, 1999.

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