Tiers of Time and Trials of Extrapolationism 1307
then held. Luis reasoned that Walter's hypothesis could therefore be tested by
measuring iridium concentrations in latest Cretaceous sediments, arguing that a small
positive excursion would validate Walter's supposition, as the constant cosmic influx
became diluted by less than the usual amount of iridium-free terrestrial sediment.
But when the Alvarez team measured iridium in boundary-layer sediments, they
found a value so high that they had to invert their initial assumption in the most
radical manner. Terrestrial sedimentation would have to cease for longer than the
earth's entire history to produce such a high spike from a low and constant cosmic
influx. Rather, they now reasoned, a true and sudden influx of iridium must have
occurred right at the K-T boundary itself—with the obvious "culprit" as a large
extraterrestrial body striking the earth and im-placing an enormous and momentary
dose of cosmic iridium. Thus, the revival of catastrophic theories for mass extinction
began with an empirical surprise generated during a test for a conventional
gradualistic hypothesis— the exact opposite (in both form and utility) of previous
exercises in evidence-free and catastrophically driven speculation.
This thoroughly different character of the Alvarez hypothesis—as an evidence-
driven claim bursting with seeds of testability, rather than a sterile speculation—
should have caught the attention and intrigue of all scientists from the start. But the
anti-catastrophic biases of Lyellian and Darwinian traditions ran so deep, and the
knee-jerk fear and disdain of paleontologists therefore stood so high, that even this
welcome novelty of operationality did not allay rejection and outright disdain from
nearly all established professional students of the fossil record (whereas other
relevant subdisciplines with other traditions, planetary scientists and students of the
physics and engineering of impacts, for example, reacted in markedly more mixed or
positive ways—see Glen, 1994). I will never forget a 1979 phone conversation (as the
preprints of Alvarez et al., 1980, circulated) with David Raup, perhaps the only other
invertebrate paleontologist of my generation who reacted with initial warmth to the
impact hypothesis: * It was one of those laconic affairs,
- Since I have far more often been wrong than right in my intuitions about
iconoclastic ideas, perhaps I may be excused for taking some pride, in retrospect, in calling
this affair, even for the right reasons. I can take no credit whatsoever for the theory's
success, since I work in other areas of paleontology and never (unlike Raup and others) did
any primary research on the subject. At most, I did write many private letters to participants,
and several popular essays, trying to explain both to my colleagues and to general readers,
why they should take impact seriously, and why the Alvarez hypothesis differed so
dramatically from previous speculations. In a climate of such negativity, these efforts may
have done some good—and Glen (1994, p. 49) was kind enough to write: "By contrast, the
paleontologist Stephen Jay Gould, perhaps earth's science's most widely read spokesman,
was intellectually predisposed to welcome the impact hypothesis straightaway, in part
because his unorthodox theory of punctuated equilibrium... articulated well with impact
theory. Gould thus provided welcome and vital encouragement through sustained
communication with the Alvarez group early on, when only the iridium evidence was at
hand and paleontologic backlash against the theory was strong." Moreover, I cannot assert
any general claim for anything inellectually admirable in myself or the very few initial
paleontological supporters of impact—as we all had purely personal reasons for favorable
predispositions