Geoscience Reference
In-Depth Information
of John Maddox, the editor of
Nature.
Hoffman proffered three
objections. First, he criticized Raup and Sepkoski for removing from
their analysis species that were still living and those whose range is
poorly known. This criticism is questionable, however, because al-
though it is easy to imagine how the removal of some families could
degrade an existing cyclic pattern, it is hard to see how the removal
could create a strong periodicity where none existed. Surely it would
merely produce more "noise." Second, Hoffman noted that because
the periodicity is degraded, or disappears altogether, when a different
time scale than the one used by Raup and Sepkoski is employed,
their conclusion must be wrong. But this criticism is tantamount to
claiming that, using an incorrect time scale, one can generate a false
pattern that is periodic at a high confidence level, which seems con-
trary to logic. It is more likely that the degrading of the periodicity
when a different scale is used means that (1) the fossil record is peri-
odic, and (2) the time scale used by Raup and Sepkoski is closer to
the true scale.
Hoffman's third argument was in a different class and purported
to be the knockout punch to the proposal of periodic extinctions, and
by extension, to the general notion of extraterrestrial impacts. Raup
and Sepkoski had to distinguish mass extinction from the normal
background extinction rate. They defined a mass extinction as having
occurred whenever their data showed a rise in extinction rate from
one geologic stage to the next, followed by a decline in rate in the
third stage. Thus a mass extinction has occurred only when the rate
of extinction is greater in a given geologic stage than in the stages
above and below it. Hoffman pointed out that there is a 25 percent
probability of this happening by chance. To understand his argument,
label the three successive stages 1, 2, and 3. At random, stage 2 has a
50 percent probability of having a higher extinction rate than stage 1
and a 50 percent probability of having a lower rate. Stage 3 likewise
has a 50 percent probability of having a higher rate than stage 2
and a 50 percent probability of having a lower one. Since probabil-
ities multiply, 0.50 x 0.50 = 0.25 and the chance of stage 2 hav-
ing a higher rate than either stages 1 or 3 is one in four, or 25 per-
cent. Hoffman then went on to his clincher: 39 stages in 250 million
years works out to an average stage length of 6.4 million years. But
four times 6.4, rounded up a little, equals 26 million years—the
periodicity found by Raup and Sepkoski! In other words, in a random
set of extinction events, the 26-million-year frequency would show
up 25 percent of the time,
on the average.
This seemingly irresistible argument tempted John Maddox fur-
ther out on a limb than journal editors ought to go. Not content to
