Wonderful Life: The Burgess Shale and the Nature of History (43 page)

In this key passage, the shoehorn becomes an instrument of God. If the history of life shows God’s direct benevolence in its ordered march to human consciousness, then decimation by lottery, with a hundred thousand possible outcomes (and so very few leading to any species with self-conscious intelligence), cannot be an option for the fossil record. The creatures of the Burgess Shale must be primitive ancestors to an improved set of descendants. The Burgess shoehorn was more than a buttress to a comfortable and convenient view of life; it was also a moral weapon, and virtually a decree of God.

If Walcott had never encountered a Cambrian rock before discovering the Burgess Shale, his persona and general attitude toward evolution would by themselves have generated the shoehorn. But Walcott also had highly specific reasons for his view, based upon his lifelong commitment to Cambrian studies, particularly his obsession with the problem of the Cambrian explosion.

I devoted the first chapter of this book to documenting the influence of iconography upon concepts. I showed how two basic pictures—the ladder of progress and the cone of increasing diversity—buttressed a general view of life based on human hopes, and forced a specific interpretation of Burgess animals as primitive precursors. In the present chapter, my two previous sections, on Walcott’s persona and attitude toward evolution, invoke the ladder; his more specific argument about the Cambrian rests upon the cone.

Evolutionary trees as the standard iconography for phylogeny had been introduced in the 1860s by the German morphologist Ernst Haeckel. (Others, including Darwin in his single drawing for the
Origin of Species
, had used botanical metaphors and drawn abstract, branching diagrams as general guides to relationships among organisms. But Haeckel developed this iconography as the preferred representation of evolution. He drew numerous trees with real bark and gnarled branches. And he placed an actual organism on each twig of his copious arborescences.) To native speakers of English, Haeckel’s name may not be so well known as Thomas Henry Huxley’s, but he was surely the most dogged and influential publicist that ever spoke for evolution. Those trees, the mainstay of instruction when Walcott studied and taught paleontology, embody the themes of ladder and cone in both flamboyantly overt and deceptively subtle ways.

To begin, all of Haeckel’s trees branch continually upward and outward, forming a cone (Haeckel sometimes allowed the two peripheral branches in each subcone to grow inward at the top, in order to provide enough room on the page for all groups—but note how he carefully preserved the general impression of up and out whenever he used this device). Haeckel’s placement of groups reinforces the great conflation of low with primitive, thus uniting the central themes of cone and ladder.

Consider, for example, Haeckel’s treatment of vertebrate phylogeny (figure 4.4; all figures from Haeckel appear in his
Generelle Morphologie
of 1866). The entire tree branches upward and outward, forming two levels, with greater diversity at the top. The lower tier, for fishes and amphibia, clearly denotes limited spread and primitivity; the upper, for reptiles, birds, and mammals, implies both more and better. Yet fishes and amphibians live still, whatever their time of origin—and fishes are by far the most diverse of vertebrates both in range of morphology and number of species. Haeckel’s tree of mammals (figure 4.5) dramatically illustrates the conflation of high with advanced, and the misrepresentation of relative diversity that may arise when a small twig is equated with an entire upper level of progress. On this tree, the highly diverse and morphologically specialized artiodactyls (cattle, sheep, deer, giraffes, and their relatives) are squeezed together in the lower tier. By contrast, the primates, forming a comparatively small group, occupy nearly half the upper level on the culturally favored right-hand side. The most diverse of all mammals, the rodents, must squash into a little bubble of space, caught in limbo between the two main layers—for there is no room for them to spread out at the top, where Haeckel’s two favored groups—carnivores (for general valor) and primates (for smarts)—hog all the space.

4.4. Haeckel’s evolutionary tree of the vertebrates (1866). Fishes (Pisces) actually encompass more disparity than all the rest of the vertebrates combined, but this false iconography, based on the cone of increasing diversity, confines them to a lower branch that gains in breadth as it expands upward.

4.5. The evolutionary tree of mammals according to Haeckel (1866).

Echinoderms provide the test case for the iconography of the tree, for in well-preserved hard parts already well-documented in Haeckel’s time, they tell the same tale as the Burgess Shale—maximal early disparity followed by decimation. Note how Haeckel acknowledges this maximal early disparity with a forest of primary stems at the geological beginning (figure 4.6). But the cone decrees that trees must spread outward as they grow, so all these early groups are shrunk into the insignificant space available at the outset. The radically decimated modern tree concentrates nearly all its diversity in two groups of strictly limited range in design—the starfish (Haeckel’s “Asterida”) and the sea urchins (his “Echinida”). Yet Haeckel’s iconography conveys the impression of a continuous increase in range.

Finally, consider Haeckel’s tree of annelids and arthropods (figure 4.7), the framework upon which Walcott would hang all the Burgess organisms that have fueled our new interpretation. Upon this ultimate expression of up and out, Walcott put all the Burgess arthropods on two adjacent branches of the lower tier—
Sidneyia
and its relatives in Haeckel’s “Poecilopoda” with horseshoe crabs and eurypterids, and nearly all other forms on the branchiopod-trilobite branch.

4.6. The evolutionary tree of echinoderms as depicted by Haeckel (1866), in accordance with the cone of increasing diversity. This group actually displays the Burgess pattern of maximal early disparity followed by decimation, but Haeckel’s iconography conveys the impression of continuously increasing diversity and range.

4.7. The evolutionary tree of arthropods and their relatives as depicted by Haeckel (1866), once again in accordance with the cone of increasing diversity.

Walcott followed all these iconographical conventions in the three sketchy trees that represent his only published attempts to draw a phylogeny for Burgess organisms. All appear in his major paper on Burgess arthropods (Walcott, 1912). Considered in their original order, they beautifully illustrate the restriction of ideology by iconography. His first chart (figure 4.8) claims to be a simple description of “stratigraphic distribution” in a phylogenetic context. Yet even here, both conventions of cone and ladder conspire to confine Burgess disparity within the limits of a few recognized major groups. The
ladder
acts to compress one group of five “merostomoid” genera into a single line: by treating
Habelia–Molaria–Emeraldella–Amiella–Sidneyia
as a structural sequence of ancestors for eurypterids and horseshoe crabs, Walcott conveyed an impression of temporal succession for these contemporaneous (and, we now know, quite unrelated) genera.

4.8. Walcott’s first chart showing the phylogeny of Burgess arthropods (1912). Walcott forcibly shaped his data in accordance with the cone and ladder by drawing speculative lines of convergence toward a common ancestry in his hypothetical Lipalian interval. He also minimized the explosion of disparity in the Burgess itself by lining up, in an apparently temporal sequence, five forms that were actually contemporaneous (right) and by drawing a hypothetical line at the left boundary to suggest continuing diversity after the Burgess where no evidence exists.

The
cone
then forces all other genera into two major groups—the branchiopod and the trilobite-to-merostome lineages. All these genera were contemporaneous, but Walcott framed the entire picture with two vertical lines, implying that later ranges continued to match recorded Burgess disparity—although no direct evidence supports this assumption. Note, especially, that the left-hand boundary line corresponds to no organism at all—the line is an iconographical device added to guide the eye into seeing a cone. Without this line, disparity would be maximal in the Burgess, and markedly decreased thereafter. Never doubt the power of such tiny and apparently insignificant moves. In a way, everything that I am trying to say in this book achieves an elegant epitome in this one vertical stroke—added to represent a philosophy of life, not the empirical record of organisms.

As a second device, buttressed by no data and added to support a traditional interpretation, Walcott drew the origin of Burgess genera at different levels within a Precambrian interval that he called Lipalian. He connected these levels with two slanted lines that point downward toward a distant Precambrian ancestor for the entire tree. This device provides the tree with a root, in an early period of restricted disparity. But Walcott had no evidence at all—and we have none today—for such evolutionary order among the Burgess arthropods.