Do the risks of extinction vary through time? This question involves two variables: (1) Do the environmental conditions that affect life on a given planet change through time? (2) Does the susceptibility of life to extinction change as its evolution progresses? Just as we have had to modify the concept of the "habitable zone," this question cannot be answered without some qualification related to degree of complexity, for the extinction rates of microbes are far different from those of more complex forms. If, as we surmise, extinction risk or rate varies with an organism's complexity, we might expect rather low extinctions during the long period prior to the evolution of animal life, followed by increasing extinction rates (the percentage of the total biodiversity or species richness going extinct during any interval of time) as animal life evolves. Yet, as we have said, this question involves two variables, the second being the possibility that the frequency or intensity of mass extinctions may change as well. There is much evidence that the proximal causes of extinction—the planetary catastrophes that cause the mass extinctions in the first place—vary through time as well.
The history of major mass extinctions on Earth suggests that only two causes have operated in Earth history: impact and global climate change. There are other phenomena that may also have caused extinctions, such as nearby supernovae, but we have no credible evidence that this latter mechanism has indeed occurred. With regard to impact, there is good evidence that the frequency of impact has changed through time, as we saw earlier in this chapter. The most obvious of these changes was the cessation of major impacts during the period of "heavy bombardment" that lasted from approximately 4.3 to 3.8 billion years ago. But even after this rain of major comets ceased, there is evidence of a long, slow decline in impact rates, as documented by Richard Grieve and others. This decrease would have reduced the overall extinction rate during the same period in which the vulnerability of emerging animal groups was increasing. One could argue that even in the last 500 million years, the time of complex animals, there should have been enough comet or asteroid strikes to exterminate animal life on this planet. That has obviously not happened.
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