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That would still happen in a catastrophe, of course, but it would not be a honing of traits best suited to particular niches. For a time chaos would serve those organisms that could thrive in a wide range of environmental conditions, and at first there would be little selection of the sort Darwin imagined, because as the damaged planet recovered, there would be plenty of room for the fit and unfit to prosper, as long as they weren’t too ecologically picky.
    Think of the Permian extinction, for instance. Before the extinction, in which 95 percent of species were wiped out, many changes in behavior or form prompted by mutations in genes would have been lost, because organisms had particular niches that they had evolved to exploit and too much deviation would probably diminish their fitness. After the extinction, however, refinements in exploiting one kind of environment might be nothing compared to a fast reproductive rate and an ability to eat anything and everything. If every niche were opened by the extinctions, the world would be welcoming to all sorts of mutations.
    Nature had suddenly become a kind of Wild West, far less picky about whom it welcomed. It was a new frontier of sorts, something like western North America when it was being taken over by European-Americans. In the West manners were much more varied than in the East or in England. Behavior that would not be tolerated in a stratified society in which all the niches were filled was tolerated, or accepted, in a land that offered all sorts of opportunities, once the original inhabitants were gotten rid of. In the American West social mobility was great. After a mass extinction evolutionary mobility was greatly enhanced.
    About 35 percent of the existing species were wiped out in the K/T extinction, a mere interruption compared to the catastrophe at the end of the Permian. The consensus in science seems to be that the asteroid impact was the primary cause of the extinction. And the meteor crash is so astonishing in its destructive power that it tends to obscure the time before and after it landed off the Yucatán.
    But the period before the crash is fascinating. If we go back ten million years before the meteor hit, dinosaur diversity was at a peak. This is a time when the Judith River Formation in Montana was laid down. And this formation is rich in the numbers of different species. But then, when we turn to the Hell Creek Formation, ten million years later, we find many fossils, but far fewer different species. And the more we study the fossils, the fewer species we find.
    Recently, some of the species of the Hell Creek Formation have gone extinct. In this case, the cause is paleontological. As we understand more about the growth of dinosaurs, we find that some specimens that we thought were different species are just different ages of the same species. We used to think that Dracorex, Pachycephalosaurus, and Stygimoloch were three different species of dome-headed dinosaurs that were found in the Hell Creek Formation. Now we have found that Dracorex and Stygimoloch are juvenile stages of Pachycephalosaurus .
    Many other species actually disappeared, rather than being reidentified. In fact, the biggest drop in diversity in the 140-million-year history of the dinosaurs occurred in the 10 million years before that meteor crashed. Something took out a lot of dinosaur species before the meteor finished the job. That is what interests me more than the mass extinction of the meteor crash, perhaps because the mechanism is so unknown and hard to understand. A meteor crash is, at heart, simple. Not that it isn’t a challenge to figure out exactly what kind of havoc the meteor wreaked, but it’s pretty clear that it caused a world of trouble. We don’t know anything about why dinosaur species disappeared at a rapid rate in 10 million years before the extinction.
    The unanswered questions may only increase the sense of awe that comes from standing in the Hell Creek