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