key concept 24.2 Changes in Earth’s Physical Environment Have Affected the Evolution of Life

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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.

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Figure 24.2 Periodic Mass Extinctions Mark Many Geologic Boundaries Five sharp rises (marked by red dots) above the background extinction rate have occurred throughout the Phanerozoic. The most sweeping of these events, the end-Permian extinction, was associated with dramatic drops in sea level (see Figure 24.4), global temperature, and atmospheric oxygen level (see Figure 24.8).

focus your learning

  • The evolution of life has changed the physical nature of Earth.

  • The evolution of life and the physical nature of Earth are mutually related.

  • Research on living organisms can help us understand the evolution of ancient organisms.