tinctions may have had a similar cause.
New forms of supporting evidence are reported on a monthly basis in
almost every issue of major journals. In the past few weeks we have learned
about minute diamonds in sediments from the impact boundary. Diamonds
are a form of pure carbon produced under immense pressures that impacts,
and no other known process active at the earth's surface, can generate.
This discovery may represent the literal fulfillment of that Beatles'
classic about psychic hallucinations, "Lucy in the Sky with Diamonds"
(and its obvious acronym). Lucy is, literally, light--and the impact
was quite a blast. Moreover, the smoking gun may now have been located
as a massive crater in the Gulf of Mexico, off the Yucatán Peninsula.
Paleontologists, with very few exceptions, reacted negatively, to say
the least, and Luis Alvarez, a virtual model for the stereotype of the
self-assured physicist, was fit to be tied. Luis, in retrospect, was also
mostly right, so I forgive his fulminations against my profession. I,
if I may toot my horn, was among his few initial supporters, but not for
the right reason of better insight into the evidence. Catastrophic extinction
simply matched my idiosyncratic preference for rapidity, born of the debate
over punctuated equilibrium (see my essay of August 1991). After all,
my colleagues had been supporting Darwinian gradualism for a century,
and the fossil record, read literally, seemed to indicate a petering-out
of most groups before the boundary. How could an impact cause the extinction
if most species were already dead? But the extraterrestrial impact theory
soon proved its mettle in the most sublime way of all: by Darwin's criterion
of provoking new observations that no one had thought of making under
old views. The theory, in short, engendered its own test and broke the
straitjacket of previous certainty.
My colleagues may have disliked the Alvarez hypothesis with unconcealed
vigor, but we are an honorable lot, and as debate intensified and favorable
evidence accumulated, paleontologists had to take another look at their
previous convictions. Many new kinds of observations can be made, but
let us focus on the simplest, most obvious, and most literal example.
In the light of new prestige for impact and sudden termination, the Signor-Lipps
argument began to sink in, and paleontologists realized that catastrophic
wipeouts might be recorded as gradual declines in the fossil record.
How then to break the impasse of this indecisive broad appearance of
peteringout? Many procedures, some rather subtle and mathematical, have
been proposed and pursued, but why not start with the most direct approach.
If rare species actually lived right to the impact boundary but have not
yet been recorded from the uppermost strata, why not look a whole lot
harder. The obvious analogy to the usual cliché suggests itself.
If I search for a single needle in the haystack by sampling ten handfuls
of hay, I have very little chance of locating it. But if I take apart
the stack, straw by straw, I will recover the needle. Similarly, if I
really search every inch of sediment in every known locality, I might
eventually find even the rarest species right near the boundary--if
it truly survived.
This all seems rather obvious. I cannot possibly argue that such an approach
was unthinkable before the Alvarez hypothesis. I cannot claim that conceptual
blinders of gradualism made it impossible even to imagine pulling apart
the haystack rather than sampling it. But this example becomes so appealing
precisely through its entirely pedestrian character. I could cite many
fancy cases of original theories that open entirely new worlds of observation:
think of Galileo's telescope and all the impossible phenomena thus revealed. |