Wonderful Life: The Burgess Shale and the Nature of History (44 page)

Walcott’s second chart (figure 4.9) illustrates the tyranny of the cone in an even more striking manner. Walcott claimed that five distinct lineages could be recognized among Burgess arthropods—the extinct trilobites, and four prominent groups of organisms inhabiting modern waters. Again, he used two devices to compress Burgess disparity into the narrow end of a cone. First, he showed all five lineages as converging toward the bottom (subtly for four, perhaps because he felt sheepish about making such an assertion with no supporting data at all; more boldly, with a distinct angular bend, for the merostome lineage, where he adduced some evidence—see below). Second, he placed all these contemporaneous fossils at different positions on his vertical branches, implying that they represented evolutionary diversification through time. On the merostome branch, he lined up eight genera (five of which are known only as contemporaries in the Burgess Shale) to forge a hypothetical link between merostomes and crustaceans: “Such forms as
Habelia, Molaria
and
Emeraldella
serve to fill in the gap between the Branchiopoda and the Merostomata as represented by
Sidneyia
and later the eurypterids” (1912, p. 163). Finally, figure 4.10 shows Walcott’s last and most abstract phylogeny for the Burgess arthropods. Even larger groups are lined up on vertical branches, and the entire tree converges to a branchiopod root.

These phylogenies embody the crucial link between Walcott’s interpretation of Burgess arthropods and the previous focus of a career that had spanned more than thirty intense years—the study of Cambrian rocks and the problem of the Cambrian explosion. The linkage between the Burgess and Walcott’s view of the Cambrian explosion provides a final, and more specific, explanation for his inevitable embrace of the shoehorn as an interpretation for Burgess fossils.

4.9. Walcott’s second chart showing the phylogeny of Burgess arthropods (1912). Again, the lineages converge toward a hypothetical common ancestry, and several contemporaneous forms are placed in ladder-like order, on the left-hand and middle lines.

In short, Walcott viewed the Burgess arthropods as members of five major lineages, already stable and well established at this early Cambrian date. But if life had already become so well differentiated along essentially modern lines, the five lineages must have existed at the inception of the Cambrian explosion as recorded by fossil evidence—for evolution is stately and gradual, not a domain of sudden jumps and mad eruptions of diversity. And if the five lineages existed as well-differentiated groups right at the beginning of the Cambrian, then their common ancestor must be sought
far back
in the Precambrian. The Cambrian explosion must therefore be an artifact of an imperfect fossil record; the late Precambrian seas, in Darwin’s words, must have “swarmed with living creatures” (1859, p. 307).

Walcott thought that he had discovered why we have no evidence for this necessary Precambrian richness. In other words, he thought that he had solved the riddle of the Cambrian explosion in orthodox Darwinian terms. The ordering of Burgess arthropods into five well-known and stable groups cemented his solution:

4.10. Walcott’s third and last attempt at depicting arthropod evolution (1912). The lineages now converge to a common point, and major groups are lined up, one above the other, on one of the three diverging branches.

We must remember that the Cambrian explosion was no ordinary riddle, and its potential solution therefore no minor plum, but something more akin to the Holy Grail. Darwin, as already noted, had publicly fretted that “the case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained” (1859, p. 308).

Two different kinds of explanations for the absence of Precambrian ancestors have been debated for more than a century: the artifact theory (they did exist, but the fossil record hasn’t preserved them), and the fast-transition theory (they really didn’t exist, at least as complex invertebrates easily linked to their descendants, and the evolution of modern anatomical plans occurred with a rapidity that threatens our usual ideas about the stately pace of evolutionary change).

Darwin, making his characteristic (and invalid) conflation of leisurely, gradual evolution and change by natural selection, rejected the fast-transition theory out of hand. He insisted that any complex Cambrian creature must have arisen from a lengthy series of Precambrian ancestors
with the same basic anatomy:
“I cannot doubt that all the Silurian [Cambrian, in modern terminology] trilobites have descended from some one crustacean, which must have lived long before the Silurian [Cambrian] age” (1859, p. 306).

Accordingly, Darwin searched for a believable version of the artifact theory, finally proposing that, in Precambrian times, “clear and open oceans may have existed where our continents now stand.” Such tracts of uninterrupted water would have received little or no sediment. Hence our current continents, containing all rocks available to our view, rose from an area that accumulated no strata during the crucial span of late Precambrian faunas, while regions of shallow water that did receive Precambrian sediments now lie in inaccessible oceanic depths.

Walcott had long maintained a firm commitment to the artifact theory. It provided the keystone for his entire approach to Cambrian geology and life. He never doubted that Cambrian complexity and diversity required a long series of abundant Precambrian ancestors of similar anatomy. In an early article he wrote: “That the life in the pre-
Olenellus
sea was large and varied there can be little, if any, doubt.… It is only a question of search and favorable conditions to discover it” (1891).
Olenellus
, as then defined, was the oldest Cambrian trilobite, so pre
-Olenellus
meant Precambrian. And in one of his late papers: “When the advanced stage of development of some of the earliest-known forms is considered it seems almost certain that such existed far back in Precambrian time” (1916, p. 249).

Walcott had long defended a particular approach to the artifact theory that a profusion of new Burgess phyla would have undermined. The artifact theory demanded long Precambrian histories for many modern groups, yet no fossils had been found. Therefore, the existence of Precambrian life would have to be inferred from some aspect of later, recorded history. Accordingly, Walcott sought support for the artifact theory in the concept of stability. If the number of basic anatomical designs had not changed throughout life’s recorded history, then such stability must surely guide our concept of what came before. Could a system so constant for hundreds of millions of years arise in a geological flash just a moment before? Protracted stability surely implied a very long and stately approach from a common ancestry deep in the distant Precambrian mists, not a gigantic burp of creativity from a starting point just below the Cambrian borderline.

We can now understand why Walcott was virtually compelled to propose the Burgess shoehorn. He interpreted his new fauna in the light of thirty previous years spent (largely in frustration) trying to prove the artifact theory, as an ultimate tribute to Darwin from a Cambrian geologist. He could not grant Burgess organisms the uniqueness that seems so evident to us today because a raft of new phyla would have threatened his most cherished belief. If evolution could produce ten new Cambrian phyla and then wipe them out just as quickly, then what about the surviving Cambrian groups? Why should they have had a long and honorable Precambrian pedigree? Why should they not have originated just before the Cambrian, as the fossil record, read literally, seems to indicate, and as the fast-transition theory proposes? This argument, of course, is a death knell for the artifact theory.

If, instead, he could shoehorn all Burgess creatures into modern groups, he would be giving the strongest possible boost to the artifact theory. For such a condensation of disparity increased the proportion of modern groups already represented right at the start of life’s recorded history—and greatly enhanced the apparent stability of major designs through time. Obviously, and with both vigor and delight, Walcott chose this alternative. What does any man do when faced with destruction or affirmation?

Walcott approached the artifact theory from both geological directions—down from the Cambrian, as illustrated by the Burgess shoehorn, and up from the Precambrian. His argument about the Precambrian has become, in the typically perverse manner of textbook histories, his most enduring legacy. Most textbooks contain a traditional, almost mandatory, two-or three-page introductory section on the history of their discipline. These travesties of scholarship dismiss the fine thinkers of our past with two-liners about some error, usually misinterpreted, that shows how stupid they were and how enlightened we have become. Charles Doolittle Walcott was one of the most powerful men in the history of American science. Yet ask any student of geology about him, and if you get any response at all, you will probably hear: “Oh yeah, that doofus who invented the nonexistent Lipalian interval to explain the Cambrian explosion.” I first heard of Walcott in this context, long before I knew about the Burgess Shale. History can be either enlightening or cruel. However, with the preceding discussion of the Burgess and the artifact theory in mind, I think we can finally understand the story of the Lipalian interval properly—and recognize Walcott’s proposal as a reasonable, if outstandingly wrong, inference within his general commitments.

The artifact theory was central to Walcott’s scientific perspective. His conclusions about the Burgess fauna supported this theory, but he needed a more direct argument from the Precambrian. Where had all the Precambrian animals gone? Other ideas included a universal metamorphism (alteration of rocks by heat and pressure) that had destroyed all Precambrian fossils, and an absence of fossilizable hard parts in Precambrian creatures. Walcott rejected the metamorphism theory because he had found many unaltered Precambrian rocks, and he argued that the hard-part theory, while probably true, could not explain the entire phenomenon.

Walcott was primarily a field geologist, specializing in Cambrian rocks. Following the proclivities of any field man, he approached his growing interest in the problem of the Cambrian explosion in the obvious way—he decided to search the latest Precambrian rocks for the elusive ancestors of Cambrian fossils. He worked for many years in the western United States, the Canadian Rockies (where he discovered the Burgess), and in China, but he found no Precambrian fossils. So he tried to reconstruct the geological and topographic history of the late Precambrian earth in a way that would explain this frustrating absence.

Walcott eventually reached a conclusion opposite to Darwin’s speculation but in the same tradition—the rocks that might house abundant Precambrian fossils just aren’t accessible to us. Darwin had suggested vast Precambrian oceans with no continents nearby to serve as a source of sediments. Walcott argued that the late Precambrian was a time of uplift and mountain building, with continents far more extensive than today’s. Since life, according to Walcott and others, had evolved in the oceans and had not yet colonized land or fresh waters, these vast Precambrian continents permitted no marine sedimentation in areas now accessible to us. (Walcott wrote long before the era of continental drift and never doubted the permanent position of continents. Thus, he argued that places available for geological observation today were the centers of more extensive Precambrian continents, and were therefore devoid of late Precambrian marine sediments. Late Precambrian sediments might lie under miles of deep ocean, but no technology then existed to recover or even to sample such potential treasures.)

The infamous “Lipalian interval” was Walcott’s name for this time of Precambrian nondeposition. Walcott proposed a world-wide break in accessible marine sedimentation, just during the critical interval of extensive Precambrian ancestry for modern groups. In a famous address to the Eleventh International Geological Congress, meeting in Stockholm on August 18,1910, he stated: