Less drastic though even more interesting, was the 

 period of rapid change that occurred more recently at the 

 boundary between the Cretaceous and Tertiary Periods, 

 about 65 million years ago, the time at which the 

 dinosaurs at last went extinct. This "mass extinction" is 

 fascinating because we find dinosaurs fascinating, but as 

 far as numbers are concerned, dinosaurs played only a 

 minor role. Incidentally, the "mass extinctions" always 

 seem to come at the boundaries between geological 

 periods, but that is simply because this is the way that 

 these periods were distinguished in the first place. 



Thus, when a high-energy physicist and Nobel 

 laureate, Luis Alvarez, his son Walter, a geologist, and 

 two chemists, Frank Asaro and Helen Michel, published 

 a paper in Science in the spring of 1980 suggesting that 

 the extinction of the dinosaurs and other species was 

 actually caused by the impact of some sort of an 

 extraterrestrial body such as a large meteor, those 

 scientists most directly concerned with explaining ex- 

 tinctions were aghast. 



Collisions with meteors are certainly not mi- 

 raculous events, but any collision that could kill off so 

 many kinds of organisms over a very short period of time 

 certainly counts as a catastrophe. If the issue had been 

 simply whether or not a large meteor had hit the earth 65 

 million years ago, it would have roused little attention. 

 Meteors hit the earth all the time. But the implications 

 were startling. If Alvarez and company were right, then 

 everyone else was wrong. The most important feature of 

 this paper and the only one that justified its publication 

 in such a prestigious journal as Science was that the 

 authors presented independent evidence that a huge 

 meteor had actually hit the earth at the time necessary to 

 produce this mass extinction — an unusually high 

 concentration of iridium, an element that is normally 

 absent from the earth's crust but relatively common in 

 some types of meteorites. But if meteor impacts caused 

 one mass extinction, why not others? In the next few 

 years, numerous papers were published arguing the 

 existence and relevance of iridium deposits at important 

 boundaries in the fossil record. 



What was Raup doing during all of this time? For 

 one thing, he had served as one of the referees for the 

 paper by Alvarez and company. He found the paper 

 poorly written and somewhat pretentious in style. 

 Besides, several years before, he himself had investigated 

 the possibility of collisions with extraterrestrial bodies 

 causing mass extinctions and, by means of computer 

 simulations, had found the cause not up to the effects. 

 He concluded that the manuscript was potentially 

 excellent and exciting, meriting rapid publication, but 

 actually mediocre and should not be published. "If a 



David M. Raup is Sewell L. Avery Distinguished Service Professor 

 of Geophysical Sciences at the University of Chicago and a 

 Research Associate, Department of Geology, Field Museum. He 

 formerly served as Field Museum's Dean of Science and, prior to 

 that, as Chairman of the Museum's Department of Geology, kwm 



