Wonderful Life: The Burgess Shale and the Nature of History

by Stephen Jay Gould

  1.8. World terrorism parachutes into its appropriate place in the march of progress. By Szep, in the Boston Globe.

  1.9. A “scientific creationist” takes his appropriate place in the march of progress. By Bill Day, in the Detroit Free Press.

  1.10. More mileage from the iconography of the ladder. By Mike Peters, in the Dayton Daily News. (Reprinted by permission of UFS, Inc.)

  1.11. The highest stage of human advance as photographed from an English billboard.

  Life is a copiously branching bush, continually pruned by the grim reaper of extinction, not a ladder of predictable progress. Most people may know this as a phrase to be uttered, but not as a concept brought into the deep interior of understanding. Hence we continually make errors inspired by unconscious allegiance to the ladder of progress, even when we explicitly deny such a superannuated view of life. For example, consider two errors, the second providing a key to our conventional misunderstanding of the Burgess Shale.

  First, in an error that I call “life’s little joke” (Gould, 1987a), we are virtually compelled to the stunning mistake of citing unsuccessful lineages as classic “textbook cases” of “evolution.” We do this because we try to extract a single line of advance from the true topology of copious branching. In this misguided effort, we are inevitably drawn to bushes so near the brink of total annihilation that they retain only one surviving twig. We then view this twig as the acme of upward achievement, rather than the probable last gasp of a richer ancestry.

  1.12. The march of progress as portrayed in another advertisement.

  1.13. The vernacular equation of evolution with progress. Andy’s quadrupedal posture is interpreted as evolution in reverse. (By permission of © M.G.N. 1989, Syndication International/North America Syndicate, Inc.)

  Consider the great warhorse of tradition—the evolutionary ladder of horses themselves (figure 1.14). To be sure, an unbroken evolutionary connection does link Hyracotherium (formerly called Eohippus) to modern Equus. And, yes again, modern horses are bigger, with fewer toes and higher crowned teeth. But Hyracotherium-Equus is not a ladder, or even a central lineage. This sequence is but one labyrinthine pathway among thousands on a complex bush. This particular route has achieved prominence for just one ironic reason—because all other twigs are extinct. Equus is the only twig left, and hence the tip of a ladder in our false iconography. Horses have become the classic example of progressive evolution because their bush has been so unsuccessful. We never grant proper acclaim to the real triumphs of mammalian evolution. Who ever hears a story about the evolution of bats, antelopes, or rodents—the current champions of mammalian life? We tell no such tales because we cannot linearize the bounteous success of these creatures into our favored ladder. They present us with thousands of twigs on a vigorous bush.

  Need I remind everyone that at least one other lineage of mammals, especially dear to our hearts for parochial reasons, shares with horses both the topology of a bush with one surviving twig, and the false iconography of a march to progress?

  In a second great error, we may abandon the ladder and acknowledge the branching character of evolutionary lineages, yet still portray the tree of life in a conventional manner chosen to validate our hopes for predictable progress.

  The tree of life grows with a few crucial constraints upon its form. First, since any well-defined taxonomic group can trace its origin to a single common ancestor, an evolutionary tree must have a unique basal trunk.* Second, all branches of the tree either die or ramify further. Separation is irrevocable; distinct branches do not join.†

  1.14. The original version of the ladder of progress for horses, drawn by the American paleontologist O. C. Marsh for Thomas Henry Huxley after Marsh had shown his recently collected Western fossils to Huxley on his only visit to the United States. Marsh convinced his English visitor about this sequence, thus compelling Huxley to revamp his lecture on the evolution of horses given in New York in 1876. Note the steady decrease in number of toes and increase in height of teeth. Since Marsh drew all his specimens the same size, we do not see the other classical trend of increase in stature.

  Yet, within these constraints of monophyly and divergence, the geometric possibilities for evolutionary trees are nearly endless. A bush may quickly expand to maximal width and then taper continuously, like a Christmas tree. Or it may diversify rapidly, but then maintain its full width by a continuing balance of innovation and death. Or it may, like a tumbleweed, branch helter-skelter in a confusing jumble of shapes and sizes.

  Ignoring these multifarious possibilities, conventional iconography has fastened upon a primary model, the “cone of increasing diversity,” an upside-down Christmas tree. Life begins with the restricted and simple, and progresses ever upward to more and more and, by implication, better and better. Figure 1.15 on the evolution of coelomates (animals with a body cavity, the subjects of this book), shows the orderly origin of everything from a simple flatworm. The stem splits to a few basic stocks; none becomes extinct; and each diversifies further, into a continually increasing number of subgroups.

  1.15. A recent iconography for the evolution of coelomate animals, drawn according to the convention of the cone of increasing diversity (Valentine, 1977).

  Figure 1.16 presents a panoply of cones drawn from popular modern textbooks—three abstract and three actual examples for groups crucial to the argument of this book. (In chapter IV, I discuss the origin of this model in Haeckel’s original trees and their influence upon Walcott’s great error in reconstructing the Burgess fauna.) All these trees show the same pattern: branches grow ever upward and outward, splitting from time to time. If some early lineages die, later gains soon overbalance these losses. Early deaths can eliminate only small branches near the central trunk. Evolution unfolds as though the tree were growing up a funnel, always filling the continually expanding cone of possibilities..

  In its conventional interpretation, the cone of diversity propagates an interesting conflation of meanings. The horizontal dimension shows diversity—fishes plus insects plus snails plus starfishes at the top take up much more lateral room than just flatworms at the bottom. But what does the vertical dimension represent? In a literal reading, up and down should record only younger and older in geological time: organisms at the neck of the funnel are ancient; those at the lip, recent. But we also read upward movement as simple to complex, or primitive to advanced. Placement in time is conflated with judgment of worth.

  Our ordinary discourse about animals follows this iconography. Nature’s theme is diversity. We live surrounded by coeval twigs of life’s tree. In Darwin’s world, all (as survivors in a tough game) have some claim to equal status. Why, then, do we usually choose to construct a ranking of implied worth (by assumed complexity, or relative nearness to humans, for example)? In a review of a book on courtship in the animal kingdom, Jonathan Weiner (New York Times Book Review, March 27, 1988) describes the author’s scheme of organization: “Working in loosely evolutionary order, Mr. Walters begins with horseshoe crabs, which have been meeting and mating on dark beaches in synchrony with tide and moon for 200 million years.” Later chapters make the “long evolutionary leap to the antics of the pygmy chimpanzee.” Why is this sequence called “evolutionary order”? Anatomically complex horseshoe crabs are not ancestral to vertebrates; the two phyla, Arthropoda and Chordata, have been separate from the very first records of multicellular life.

  1.16. The iconography of the cone of increasing diversity, as seen in six examples from textbooks. All these diagrams are presented as simple objective portrayals of evolution; none are explicit representations of diversification as opposed to some other evolutionary process. Three abstract examples (A–C) are followed by conventional views of three specific phylogenies–vertebrate (D), arthropod (E), and mammalian (F, on p. 42). The data of the Burgess Shale falsify this central view of arthropod evolution as a continuous process of increasing diversification.

  1.16 A conventional view of mammalian phyl
ogeny.

  In another recent example, showing that this error infests technical as well as lay discourse, an editorial in Science, the leading scientific journal in America, constructs an order every bit as motley and senseless as White’s “regular gradation” (see figure 1.3). Commenting on species commonly used for laboratory work, the editors discuss the “middle range” between unicellular creatures and guess who at the apex: “Higher on the evolutionary ladder,” we learn, “the nematode, the fly and the frog have the advantage of complexity beyond the single cell, but represent far simpler species than mammals” (June 10, 1988).

  The fatuous idea of a single order amidst the multifarious diversity of modern life flows from our conventional iconographies and the prejudices that nurture them—the ladder of life and the cone of increasing diversity. By the ladder, horseshoe crabs are judged as simple; by the cone, they are deemed old.* And one implies the other under the grand conflation discussed above—down on the ladder also means old, while low on the cone denotes simple.

  I don’t think that any particular secret, mystery, or inordinate subtlety underlies the reasons for our allegiance to these false iconographies of ladder and cone. They are adopted because they nurture our hopes for a universe of intrinsic meaning defined in our terms. We simply cannot bear the implications of Omar Khayyám’s honesty:

  Into this Universe, and Why not knowing,

  Nor whence, like Water willy-nilly flowing:

  And out of it, as Wind along the Waste

  I know not Whither, willy-nilly blowing.

  A later quatrain of the Rubáiyát proposes a counteracting strategy, but acknowledges its status as a vain hope:

  Ah Love! could you and I with Fate conspire

  To grasp this sorry Scheme of Things entire,

  Would we not shatter it to bits—and then

  Re-mold it nearer to the Heart’s Desire!

  Most myths and early scientific explanations of Western culture pay homage to this “heart’s desire.” Consider the primal tale of Genesis, presenting a world but a few thousand years old, inhabited by humans for all but the first five days, and populated by creatures made for our benefit and subordinate to our needs. Such a geological background could inspire Alexander Pope’s confidence, in the Essay on Man, about the deeper meaning of immediate appearances:

  All Nature is but art, unknown to thee;

  All chance, direction, which thou canst not see;

  All discord, harmony not understood;

  All partial evil, universal good.

  But, as Freud observed, our relationship with science must be paradoxical because we are forced to pay an almost intolerable price for each major gain in knowledge and power—the psychological cost of progressive dethronement from the center of things, and increasing marginality in an uncaring universe. Thus, physics and astronomy relegated our world to a corner of the cosmos, and biology shifted our status from a simulacrum of God to a naked, upright ape.

  To this cosmic redefinition, my profession contributed its own special shock—geology’s most frightening fact, we might say. By the turn of the last century, we knew that the earth had endured for millions of years, and that human existence occupied but the last geological millimicrosecond of this history—the last inch of the cosmic mile, or the last second of the geological year, in our standard pedagogical metaphors.

  We cannot bear the central implication of this brave new world. If humanity arose just yesterday as a small twig on one branch of a flourishing tree, then life may not, in any genuine sense, exist for us or because of us. Perhaps we are only an afterthought, a kind of cosmic accident, just one bauble on the Christmas tree of evolution.

  What options are left in the face of geology’s most frightening fact? Only two, really. We may, as this book advocates, accept the implications and learn to seek the meaning of human life, including the source of morality, in other, more appropriate, domains—either stoically with a sense of loss, or with joy in the challenge if our temperament be optimistic. Or we may continue to seek cosmic comfort in nature by reading life’s history in a distorted light.

  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:

  Man has been here 32,000 years. That it took a hundred million years to prepare the world for him* is proof that that is what it was done for. I suppose it is. I dunno. If the Eiffel Tower were now representing the world’s age, the skin of paint on the pinnacle knob at its summit would represent man’s share of that age; and anybody would perceive that the skin was what the tower was built for. I reckon they would, I dunno.

  REPLAYING LIFE’S TAPE: THE CRUCIAL EXPERIMENT

  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?