Wonderful Life: The Burgess Shale and the Nature of History (6 page)

If we elect the second strategy, our maneuvers are severely restricted by our geological history. When we infested all but the first five days of time, the history of life could easily be rendered in our terms. But if we wish to assert human centrality in a world that functioned without us until the last moment, we must somehow grasp all that came before as a grand preparation, a foreshadowing of our eventual origin.

The old chain of being would provide the greatest comfort, but we now know that the vast majority of “simpler” creatures are not human ancestors or even prototypes, but only collateral branches on life’s tree. The cone of increasing progress and diversity therefore becomes our iconography of choice. The cone implies predictable development from simple to complex, from less to more.
Homo sapiens
may form only a twig, but if life moves, even fitfully, toward greater complexity and higher mental powers, then the eventual origin of self-conscious intelligence may be implicit in all that came before. In short, I cannot understand our continued allegiance to the manifestly false iconographies of ladder and cone except as a desperate finger in the dike of cosmically justified hope and arrogance.

I leave the last word on this subject to Mark Twain, who grasped so graphically, when the Eiffel Tower was the world’s tallest building, the implications of geology’s most frightening fact:

The iconography of the cone made Walcott’s original interpretation of the Burgess fauna inevitable. Animals so close in time to the origin of multicellular life would have to lie in the narrow neck of the funnel. Burgess animals therefore could not stray beyond a strictly limited diversity and a basic anatomical simplicity. In short, they had to be classified either as primitive forms within modern groups, or as ancestral animals that might, with increased complexity, progress to some familiar form of the modern seas. Small wonder, then, that Walcott interpreted every organism in the Burgess Shale as a primitive member of a prominent branch on life’s later tree.

1.17. The false but still conventional iconography of the cone of increasing diversity, and the revised model of diversification and decimation, suggested by the proper reconstruction of the Burgess Shale.

I know no greater challenge to the iconography of the cone—and hence no more important case for a fundamentally revised view of life—than the radical reconstructions of Burgess anatomy presented by Whittington and his colleagues. They have literally followed our most venerable metaphor for revolution: they have turned the traditional interpretation on its head. By recognizing so many unique anatomies in the Burgess, and by showing that familiar groups were then experimenting with designs so far beyond the modern range, they have inverted the cone. The sweep of anatomical variety reached a maximum right after the initial diversification of multicellular animals. The later history of life proceeded by elimination, not expansion. The current earth may hold more species than ever before, but most are iterations upon a few basic anatomical designs. (Taxonomists have described more than a half million species of beetles, but nearly all are minimally altered Xeroxes of a single ground plan.) In fact, the probable increase in number of species through time merely underscores the puzzle and paradox. Compared with the Burgess seas, today’s oceans contain many more species based upon many fewer anatomical plans.

Figure 1.17 presents a revised iconography reflecting the lessons of the Burgess Shale. The maximum range of anatomical possibilities arises with the first rush of diversification. Later history is a tale of restriction, as most of these early experiments succumb and life settles down to generating endless variants upon a few surviving models.
*

This inverted iconography, however interesting and radical in itself, need not imply a revised view of evolutionary predictability and direction. We can abandon the cone, and accept the inverted iconography, yet still maintain full allegiance to tradition if we adopt the following interpretation: all but a small percentage of Burgess possibilities succumbed, but the losers were chaff, and predictably doomed. Survivors won for cause—and cause includes a crucial edge in anatomical complexity and competitive ability.

But the Burgess pattern of elimination also suggests a truly radical alternative, precluded by the iconography of the cone. Suppose that winners have not prevailed for cause in the usual sense. Perhaps the grim reaper of anatomical designs is only Lady Luck in disguise. Or perhaps the actual reasons for survival do not support conventional ideas of cause as complexity, improvement, or anything moving at all humanward. Perhaps the grim reaper works during brief episodes of mass extinction, provoked by unpredictable environmental catastrophes (often triggered by impacts of extraterrestrial bodies). Groups may prevail or die for reasons that bear no relationship to the Darwinian basis of success in normal times. Even if fishes hone their adaptations to peaks of aquatic perfection, they will all die if the ponds dry up. But grubby old Buster the Lungfish, former laughingstock of the piscine priesthood, may pull through—and not because a bunion on his great-grandfather’s fin warned his ancestors about an impending comet. Buster and his kin may prevail because a feature evolved long ago for a different use has fortuitously permitted survival during a sudden and unpredictable change in rules. And if we are Buster’s legacy, and the result of a thousand other similarly happy accidents, how can we possibly view our mentality as inevitable, or even probable?

We live, as our humorists proclaim, in a world of good news and bad news. The good news is that we can specify an experiment to decide between the conventional and the radical interpretations of extinction, thereby settling the most important question we can ask about the history of life. The bad news is that we can’t possibly perform the experiment.

I call this experiment “replaying life’s tape.” You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past—say, to the seas of the Burgess Shale. Then let the tape run again and see if the repetition looks at all like the original. If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur. But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life? What could we then say about the predictability of self-conscious intelligence? or of mammals? or of vertebrates? or of life on land? or simply of multicellular persistence for 600 million difficult years?

THE MEANINGS OF DIVERSITY AND DISPARITY

I must introduce at this point an important distinction that should allay a classic source of confusion. Biologists use the vernacular term
diversity
in several different technical senses. They may talk about “diversity” as number of distinct species in a group: among mammals, rodent diversity is high, more than 1,500 separate species; horse diversity is low, since zebras, donkeys, and true horses come in fewer than ten species. But biologists also speak of “diversity” as difference in body plans. Three blind mice of differing species do not make a diverse fauna, but an elephant, a tree, and an ant do
—
even though each assemblage contains just three species
.

The revision of the Burgess Shale rests upon its diversity in this second sense of
disparity
in anatomical plans. Measured as number of species, Burgess diversity is not high. This fact embodies a central paradox of early life: How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species?
—
for the two are correlated, more or less in lockstep, by the iconography of the cone (see figure 1.16
).

When I speak of decimation, I refer to reduction in the number of anatomical designs for life, not numbers of species. Most paleontologists agree that the simple count of species has augmented through time (Sepkoski
et al.,
1981
)—
and this increase of species must therefore have occurred
within
a reduced number of body plans
.

Most people do not fully appreciate the stereotyped character of current life. We learn lists of odd phyla in high school, until kinorhynch, priapulid, gnathostomulid, and pogonophoran roll off the tongue (at least until the examination ends). Focusing on a few oddballs, we forget how unbalanced life can be. Nearly 80 percent of all described animal species are arthropods (mostly insects). On the sea floor, once you enumerate polychaete worms, sea urchins, crabs, and snails, there aren’t that many coelomate invertebrates left. Stereotypy, or the cramming of most species into a few anatomical plans, is a cardinal feature of modern life
—
and its greatest difference from the world of Burgess times
.

Several of my colleagues (Jaanusson, 1981; Runnegar, 1987) have suggested that we eliminate the confusion about diversity by restricting this vernacular term to the first sense
—
number of species. The second sense
—
difference in body plans
—
should then be called
disparity
. Using this terminology, we may acknowledge a central and surprising fact of life’s history
—
marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs
.

We can now appreciate the central importance of the Burgess revision and its iconography of decimation. With the ladder or the cone, the issue of life’s tape does not arise. The ladder has but one bottom rung, and one direction. Replay the tape forever, and
Eohippus
will always gallop into the sunrise, bearing its ever larger body on fewer toes. Similarly, the cone has a narrow neck and a restricted range of upward movement. Rewind the tape back into the neck of time, and you will always obtain the same prototypes, constrained to rise in the same general direction.

But if a radical decimation of a much greater range of initial possibilities determined the pattern of later life, including the chance of our own origin, then consider the alternatives. Suppose that ten of a hundred designs will survive and diversify. If the ten survivors are predictable by superiority of anatomy (interpretation 1), then they will win each time—and Burgess eliminations do not challenge our comforting view of life. But if the ten survivors are protégés of Lady Luck or fortunate beneficiaries of odd historical contingencies (interpretation 2), then each replay of the tape will yield a different set of survivors and a radically different history. And if you recall from high-school algebra how to calculate permutations and combinations, you will realize that the total number of combinations for 10 items from a pool of 100 yields more than 17 trillion potential outcomes. I am willing to grant that some groups may have enjoyed an edge (though we have no idea how to identify or define them), but I suspect that the second interpretation grasps a central truth about evolution. The Burgess Shale, in making this second interpretation intelligible by the hypothetical experiment of the tape, promotes a radical view of evolutionary pathways and predictability.

Rejection of ladder and cone does not throw us into the arms of a supposed opposite—pure chance in the sense of coin tossing or of God playing dice with the universe. Just as the ladder and the cone are limiting iconographies for life’s history, so too does the very idea of dichotomy grievously restrict our thinking. Dichotomy has its own unfortunate iconography—a single line embracing all possible opinions, with the two ends representing polar opposites—in this case, determinism and randomness.

An old tradition, dating at least to Aristotle, advises the prudent person to stake out a position comfortably toward the middle of the line—the
aurea mediocritas
(“golden mean”). But in this case the middle of the line has not been so happy a place, and the game of dichotomy has seriously hampered our thinking about the history of life. We may understand that the older determinism of predictable progress cannot strictly apply, but we think that our only alternative lies with the despair of pure randomness. So we are driven back toward the old view, and finish, with discomfort, at some ill-defined confusion in between.

I strongly reject any conceptual scheme that places our options on a line, and holds that the only alternative to a pair of extreme positions lies somewhere between them. More fruitful perspectives often require that we step off the line to a site outside the dichotomy.