There may be an additional way to shed light on the role of impact: by focusing not on the mass extinctions themselves, but on the normal intervals of background extinction in between. If impact drives mass extinctions, then in the times between impact events, few extinctions would be expected. Can we tell whether the tempo of extinction and evolution in between the large extinctions is consistent with a history of impact? After Darwin, evolutionists came to have the view that natural selection operates steadily, all the time. Organisms continually undergo small changes that, when summed together, produce large effects. As long as environments are stable, natural selection operates to adapt organisms ever more perfectly. When environments change, natural selection allows them to adapt just enough to keep pace. This model conforms exactly to the uni-formitarian view of earth behavior: Change is gradual, but cumulative, and in time can be prodigious.
If this gradualistic view is correct, the fossil record should reflect it. As natural selection works its way, plants and animals should evolve steadily, little by little, leaving much of the work of evolution to be done in between mass extinctions. But, as Darwin's contemporaries knew, this is not really what the fossil record reveals. Instead, most evolutionary change occurs in a burst right after a new species diverges from its ancestor. After the initial spurt, species change little, sometimes remaining static for millions of years. For example, the lampshell brachiopod genus Lingula found today appears just like its 450-million-year-old fossil ancestor. Niles Eldredge of the American Museum of Natural History and Stephen Jay Gould called this model "punctuated equilibrium," known to aficionados as "punc eek," and when they proposed it in 1972, most evolutionists scoffed. 2 3,2 4 Today, however, many believe that not only species, but whole ecosystems, remain stable for long periods of time, until something disturbs them enough to cause a multitude of extinctions. The motor of evolution then revs up and gives rise to new species that are adapted to the postextinction conditions. Eldredge believes that "Nothing much happens in evolution without extinction first disrupting ecosystems and driving many preexisting, stable species extinct. And extinction is almost always the result of the physical environment's changing beyond the point where species can relocate by finding familiar habitat elsewhere."25
If punc eek is the rule, then something punctuates evolution. That something produces extinctions that in turn open up ecological niches into which the pressure of natural selection propels a new set of organisms. According to this notion, the driving force behind evolution is the punctuator itself. What could it be? If we follow where Raup, Gould, and Eldredge would lead us, we see that the punctuator must be unfamiliar to species over long periods of geologic time and must disrupt the environment beyond the ability of species to adapt or migrate. Though mass extinction may have more than one cause, however we come at the question, we find hints, if not corrob-oration, that impact may have played a more important role in the history of the earth than almost anyone has appreciated.
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