Geologic Timescale

This chart divides the last 545 million years of earth history into named intervals of geological time, many of which are referred to in this book. The three major eras (Paleozoic, Mesozoic, and Cenozoic) are subdivided into the periods shown on the left (Cambrian, Ordovician, and so on), from the oldest at the top to the present time at the bottom. Note that this consists of only the last 12% of earth's 4.6-billion-year history.

Time is given in millions of years ago (mya), with the boundaries of each period on the left and a graduated scale on the right. There are three different scales: 50-million-year markers for the Paleozoic and Mesozoic, 10-million-year markers for the Cenozoic, and .5-million-year markers for the Quaternary.

Notable milestones in the history of life are listed on the right.

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