We can summarize the implications of Earth's history of mass extinctions with regard to the Rare Earth Hypothesis as follows. Mass extinctions probably occurred rarely during the long period in Earth history when life was only of a bacterial grade. With the evolution of more complex creatures, such as eukaryotic cells, however, susceptibility to extinction increased. With the advent of abundant complex animals in the Cambrian, vulnerability to mass extinction may have reached a peak, because diversity was very low. As more and more species evolved within the various body plans, susceptibility to extinction decreased again.
On any planet, the number of mass extinctions may be one of the most important determinants of where animal life arises and, if so, how long it lasts. In planetary systems with large amounts of space debris—and thus a high impact record—the chance that animal life will arise and persist will surely be much lower than in systems where impacts are few. In similar fashion, inhabiting a cosmic neighborhood where large amounts of celestial collisions, supernovae, gamma ray bursts, or other cosmic catastrophes occur will also reduce a planet's likelihood of attaining and maintaining animal life.
It appears that the best "life insurance" is diversity. In the next chapter we will document our view that plate tectonics, also known as continental drift, has been the main process promoting high diversity among animals on Earth.
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