Tires to Sandals
As we strive to understand nature, do we seek truth or solace?
by Stephen Jay Gould
Did you ever wonder what happens to old automobile tires? Since the
United States boasts almost as many vehicles as people, our nation may
feature nearly twice as many wheels as feet, and therefore (depending
upon the Imelda Marcos factor of pairs per person) more tires than shoes.
Well, I don't know what happens to our discarded wheel wear, but I do
know the fate of many worn tires in third-world nations. They are cut
apart and made into the soles and straps of sandals (I own pairs purchased
in the open-air markets of three continents--Quito, Nairobi, and Delhi).
In our world of material wealth, where so many broken items are thrown
away, rather than mended, because the cost of repair now exceeds the price
of replacement, we forget that most of the world fixes everything and
discards nothing. (Streets in crowded Indian cities, contrary to our usual
assumptions, are often spotlessly clean because any item has value in
reuse or resale. Scraps of paper are immediately scavenged; even cow pies
remain but a few seconds on the street before they are collected for fuel
and slapped against a wall to dry.) I have never visited a place more
fascinating than the recycling market of Nairobi--a true testimony
to human ingenuity. Here sandals are made from tires, bracelets from telephone
wire, kerosene lamps from bisected tin cans, containers from scraps of
metal, and cooking pots from the tops of oil drums.
This little prologue may strike you as interesting enough, but dubiously
related to the stated subject of this column--evolution and the history
of life. Yet this theme of recycling for purposes almost comically different
from original intent not only occupies a central place in the history
of life but also completes an argument that I left half-finished in the
last essay. The previous column examined Darwin's justification for predictable
progress in the history of life and argued that recent discoveries about
the frequency, rapidity, and extent of mass extinction suggest that life
follows a much more quirky and uncertain path. I pointed out that Darwin
did not locate the source of progress in the basic mechanics of natural
selection itself--for he recognized natural selection as a theory
of local adaptation only, not a statement about general advance. He justified
progress with another argument about nature embodied in his favorite metaphor
of the wedge. Nature is chock-full of species (like a surface covered
with wedges) all struggling for a bit of limited space. New species usually
win an address by driving out others in overt competition (a process that
Darwin often described in his notebooks as "wedging"). This
constant battle and conquest provides a rationale for progress, since
victors, on average, may secure their success by general superiority in
design.
I argued last month that mass extinction prevents wedging from establishing
long-term patterns in the history of life. Progress by competition may
occur in normal times, but episodes of mass extinction undo, disrupt,
and redirect this process so frequently that wedging cannot put a dominant
stamp upon the overall course of life. I do not believe that mass extinctions
work with absolute randomness, treating each species as a coin to be flipped
or a die to be rolled. Survivors probably prevail for reasons, but here's
the rub (and the role for the wheel of fortune): the rules for survival
change in these extraordinary episodes, and the features that help species
to prevail through catastrophes are not the same as the sources of success
in normal times. Getting through a mass extinction may require a stroke
of fortune in the following special sense: a feature evolved for one function
in the wedging of normal times must, by good luck and for different reasons,
provide a crucial benefit under the different rules of episodic catastrophe.
(The diatoms of last month's column, the only group of planktonic microorganisms
to sail through the Cretaceous debacle, had evolved a life cycle with
a dormant stage in order to weather predictable seasonal fluctuations
of light and nutrients in normal times. If an impacting comet entrained
a dust cloud that darkened the earth for several months, the survival
of diatoms might be linked to a capacity for dormancy evolved for quite
different reasons. Mammals may have prevailed, in large part, by virtue
of small body size. But we can hardly label limited size as a preparation
for long-term success or even as an active adaptation at all. Mammals
may have remained small for primarily negative reasons--because dinosaurs
dominated the ecospace of large creatures, and mammals could not displace
them by wedging in normal times.) Thus, mass extinction sets a quirky
and interesting course for life by opening opportunities to new groups
and by basing success upon fortuitous side consequences of features evolved
for other reasons. It doesn't matter how well a tire works when the rules
change and we run out of gas; existence now depends upon a fortuitous
capacity for conversion to some other, utterly unanticipated role--retreading
for sandals, for example.
But this argument--as far as I went last month--grants the wheel
of fortune only half a loaf. For any champion of the wedge will swiftly
reply: mass extinction is a negative force. It makes nothing and can only
pick and choose among creatures fashioned by natural selection. Sure,
mass extinction can disrupt a trend, wipe out a complex group, or send
life down an unpredicted channel--but evolution is about making, not
about differential removal. The creative face in evolution, the
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