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As we saw in the previous section, the Phanerozoic eon has been notable for the rapid diversification of multicellular eukaryotes. But the diversity of multicellular organisms has not simply increased steadily through time. New species have arisen, and species have gone extinct, throughout the history of life. In the absence of major, rapid environmental changes, the average rate of extinction is known as the background extinction rate. But there have been times during which extinction rates have increased dramatically over the background levels (Figure 24.2). These mass extinction events are the cause of some of the striking differences in fossil assemblages that geologists use to divide the Phanerozoic eon into eras and periods. After each mass extinction, the diversity of life rebounded, although recovery took millions of years. In this section we will discuss some of the physical changes on Earth that have resulted in dramatic changes in life’s diversity.