Wonderful Life: The Burgess Shale and the Nature of History (5 page)

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 phylogeny.

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.
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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:

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

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:

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.