A third problem with the official portrait of Darwin’s chief rival concerns Lurie’s suggestion that Agassiz was a master of particulars, but not of generalizing from those particulars. The historical record suggests otherwise. For example, Agassiz was the man who ably generalized from a wide array of particular clues in his work on the ice age, winning over the geological establishment by demonstrating how a range of facts were best explained by the action of retreating glaciers.
Here a direct comparison between Darwin and Agassiz is possible. Each searched for an explanation of a curious geological phenomenon in the Scottish Highlands, the parallel roads of Glen Roy. Glen Roy is the valley of the River Roy and, although it’s a place of breathtaking beauty, what visitors found most intriguing about it over the years were its three parallel roads that wind along the canyon wall on either side of the river (see Fig. 1.7).48 Scottish legend held that they were hunting paths built for use by early Scottish kings or perhaps even for the mythical warrior Fingal. Scientists later argued that the roads were natural rather than artificial. Darwin and Agassiz were both convinced that natural processes were the cause, but they nevertheless arrived at different explanations. What was the end of the matter? In his autobiography, Darwin explained, “Having been deeply impressed with what I had seen of the elevation of the land in S. America, I attributed the parallel lines to the action of the sea; but I had to give up this view when Agassiz propounded his glacier-lake theory.”49 Subsequent investigations in the late nineteenth and early twentieth centuries confirmed that Agassiz’s interpretation was the correct one.50
Agassiz, then, was far more than just a walking encyclopedia or an insatiable gatherer of fossils who couldn’t see the proverbial forest for the trees. Those who insist otherwise can point to but one example to support their position, namely, his rejection of Darwin’s theory; but they cannot use that example to establish his general inability to interpret evidence and then turn around and use that supposed inability to explain his failure to accept Darwin’s theory. That is to argue in a circle.
There is a far more obvious solution to the historical puzzle posed by the great Agassiz’s objection to Darwin’s theory: the fossils of the Cambrian strata do, in fact, arise abruptly in the geological record, in clear defiance of what Darwin’s theory would lead us to expect. In short, a genuine mystery is at hand.
FIGURE 1.7
Parallel roads of Glen Roy.
Two final considerations lend support to this view. First, as already noted, Darwin himself accepted the validity of Agassiz’s objection.51 As he acknowledged elsewhere in the Origin, “To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer… . The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained.”52
Second, Darwin’s attempt to account for the absence of the expected fossil ancestors of the Cambrian forms failed to address the full strength and subtlety of Agassiz’s objection. As Agassiz explained, the problem with Darwin’s theory was not just the general incompleteness of the fossil record or even a pervasive absence of ancestral forms of life in the fossil record. Rather, the problem, according to Agassiz, was the selective incompleteness of the fossil record.
Why, he asked, does the fossil record always happen to be incomplete at the nodes connecting major branches of Darwin’s tree of life, but rarely—in the parlance of modern paleontology—at the “terminal branches” representing the major already known groups of organisms? These terminal branches were well represented (see Fig. 1.8), often stretching over many generations and millions of years, while the “internal branches” at the connecting nodes on Darwin’s tree of life were nearly always—and selectively—absent. As Agassiz explained, Darwin’s theory “rests partly upon the assumption that, in the succession of ages, just those transition types have dropped out from the geological record which would have proved the Darwinian conclusions had these types been preserved.”53 To Agassiz, it sounded like a just-so story, one that explains away the absence of evidence rather than genuinely explaining the evidence we have.
Was there any easy answer to Agassiz’s argument? If so, beyond his stated willingness to wait for future fossil discoveries, Darwin didn’t offer one.
FIGURE 1.8
The vertical lines in these diagrams represent known animal phyla. The dots within the vertical lines represent animals from those phyla that have been found fossilized in different strata. The diagram on the left shows the animal tree of life as expected based upon Darwinian theory. The diagram on the right shows a simplified representation of the actual pattern of the Precambrian–Cambrian fossil record. Notice that fossils representing the internal branches and nodes, but not the terminal branches, are missing.
An Enduring Mystery
In the years immediately following the publication of On the Origin of Species, many of Agassiz’s concerns were temporarily swept aside as public and scientific fascination with Darwin’s ideas grew. Even so, a persistent mystery lay at the feet of biologists, one that subsequent generations of scientists would revisit and repeatedly seek to resolve. As Darwin noted, in his time, the fossils of the Cambrian were relatively few, and the period of the explosion only vaguely understood. But perhaps future scientists would come to his rescue with fresh discoveries.
The story of the successive attempts to solve the Cambrian mystery stretches from Darwin’s time to the present, and from the Swansea Valley in southern Wales to remote fossil sites in southern China. In the next chapter, the work of detection moves from the late nineteenth century to the early twentieth, from the British Isles to British Columbia, and to a fossil site above the Kicking Horse River so astonishing that, even today, paleontologists and some of the most skeptical and hardened of scientific rationalists speak its name with childlike reverence.
2
The Burgess Bestiary
Only in fiction can we expect such fine orchestration of setting and dramatic action. Gothic tales haunted by demons of the past have their thunderstorms and crumbling mansions, the existentialist novel its disorienting cityscapes, the romance its unattainable balconies laced with jasmine. In ordinary life, the staging is usually less precise. Intricate family tragedies unravel in tidy suburban ranch-style homes, while enchanted romances blossom over cubicle walls. But the twentieth century’s most revolutionary fossil discovery was more like fiction: the setting was commensurate with the moment.
Photographs taken during the summer’s expedition show a lean and balding man with pleasant crinkles at the corners of his eyes and a deep thought line slashing down between his brows; he stands precariously on rocky ascents, pick and hoe at hand, gazing far into the distance from a stony peak, at ease among the forbidding slopes and treacherous ridges. Working his way over one ridge and then above the tree line of the next, Charles Doolittle Walcott reached a place where he could see for miles. To the northwest, the crude arrowhead of Mount Wapta jutted skyward. Below lay Emerald Lake, its waters green from the mineral-rich glacial till. To the east and west, snowy peaks stretched to the horizon (see Figs. 2.1 and 2.2). Only the view to the northeast lacked a vista. Here was the homely shale of a barren ridge. Of course, as in any fairy tale, there lay the real prize, a hidden vista measured not in miles, but in ages.
FIGURE 2.1
The scenery of the Burgess Shale and surrounding area. Courtesy Corbis.
FIGURE 2.2
Charles Doolittle Walcott in the field (c. 1911). Courtesy Smithsonian Institution Archives.
Walcott, already the director of the Smithsonian Institution, was about to enter the most significant phase of his professional life. More than this, he was about to make perhaps the most dramatic discovery in the history of paleontology, a rich trove of middle Cambrian–era fossils, including many previously unknown animal forms, preserved in exquisite detail, suggesting an event of greater suddenness than had b
een known even in Darwin’s time and detailing a greater diversity of biological form and architecture than had hitherto been imagined.
Where did this wealth of biological form come from, and why, again, did it seem to arise so suddenly during the Cambrian period? Walcott was the first to explore the Burgess Shale, and he would be the first to suggest an answer to the questions it raised.
The Bestiary
Among paleontologists, the fateful clue that led to the Burgess Shale’s discovery is the stuff of legend. Paleontologist Stephen Jay Gould considered it to have been rendered best in an obituary of Charles Walcott written by Walcott’s former research assistant, Charles Schuchert:
One of the most striking of Walcott’s faunal discoveries came at the end of the field season of 1909, when Mrs. Walcott’s horse slid on going down the trail and turned up a slab that at once attracted her husband’s attention. Here was a great treasure—wholly strange Crustacea of Middle Cambrian time—but where in the mountain was the mother rock from which the slab had come? Snow was even then falling, and the solving of the riddle had to be left to another season, but next year the Walcotts were back again on Mount Wapta, and eventually the slab was traced to a layer of shale—later called the Burgess Shale—3000 feet above the town of Field.1
Gould quotes the legend to celebrate its archetypal appeal even as he debunks it: “Consider the primal character of this tale—the lucky break provided by the slipping horse, … the greatest discovery at the very last minute of a field season (with falling snow and darkness heightening the drama of finality), the anxious wait through a winter of discontent, the triumphant return and careful, methodical tracing of errant block to mother lode.”2 A compelling story, Gould concludes, but pure fiction. Walcott’s own diaries reveal that his team had plenty of time to begin excavating the site that very summer amid cooperative weather and even warm nights. As for their return the following summer, locating the mother lode was apparently the work of a single day rather than a full week, a conclusion Gould drew from both Walcott’s diaries and his knowledge of Walcott’s expertise as a geologist.3
The motifs of the lucky break, the frustrating delay, and the final and fortuitous triumph will resurface later (see Chapter 7) as a tall tale of Gould’s own, but for now consider only the scientific community’s weakness for staging the Burgess discovery with various fictional props, as if the stunning scenery around it were not setting enough. This weakness for theater is understandable, considering what Walcott and later investigators found there. Over the next several years, Walcott’s team alone collected more than 65,000 specimens, many of them astonishingly well preserved, some so bizarre that paleontologists would cast about for more than half a century for the proper categories in which to contain them.
Consider just one odd couple from Walcott’s quarry, Marrella and Hallucigenia. Marrella, also called a lace crab, is an unusual form. Walcott described it as a type of trilobite, but later studies by Cambridge paleontologist Harry Whittington classified it not as a trilobite, nor a chelicerate (the subgroup of arthropods that includes spiders), and not even as a crustacean, but rather as a fundamentally distinct form of arthropod.4 The creature is divided into twenty-six segments, each with a jointed leg for walking and a feathery gill branch for swimming. Its head shield has two long pairs of spikes directed backwards, and the underside of the head features two pairs of antennae. One is short and stout, the other long and sweeping (see Fig. 2.3).
Hallucigenia belongs to a genus and family of one. It has a rounded mass at one end (possibly the head) connected to a cylinder-shaped trunk sporting seven pairs of spines projecting upward and to either side, each of them almost as long as the trunk itself (see Fig. 2.4). On the underside of the creature are seven pairs of limbs, each corresponding in position to one of the pairs of spines on the back, though with the tentacle farthest back offset. The underbelly also features three pairs of shorter tentacles before the trunk tapers and curves upward in what was probably a flexible extension from the body. Each of the larger tentacles appears to have a hollow tube connected to the gut and a pincer at the tip. This ancient creature was so peculiar that paleontologists feigned disbelief at what they saw, giving it its memorable name.
FIGURE 2.3
Figure 2.3a (left): Artist rendering of Marrella splendens. Figure 2.3b (right): Photograph of Marrella splendens fossil. Courtesy Wikimedia Commons, user Smith609.
FIGURE 2.4
Figure 2.4a (top): Artist rendering of Hallucigenia sparsa. Figure 2.4b (bottom): Photograph of Hallucigenia sparsa fossil. Courtesy Smithsonian Institution.
The term “Cambrian explosion” was to become common coin, because Walcott’s site suggested the geologically abrupt appearance of a menagerie of animals as various as any found in the gaudiest science fiction. During this explosion of fauna, representatives of about twenty of the roughly twenty-six total phyla present in the known fossil record made their first appearance on earth (see Fig. 2.5).5
The term “phyla” (singular: “phylum”) refers to divisions in the biological classification system. The phyla constitute the highest (or widest) categories of biological classification in the animal kingdom, with each exhibiting a unique architecture, organizational blueprint, or structural body plan. Familiar examples of phyla are cnidarians (corals and jellyfish), mollusks (squid and clams), echinoderms (sea stars and sea urchins), arthropods (trilobites and insects), and the chordates, to which all vertebrates including humans belong.
The animals within each phylum exhibit distinguishing features that enable taxonomists to divide and group them further into other, progressively smaller divisions, beginning with classes and orders, and eventually coming to families, genera, and individual species. The broadest and highest categories within the animal kingdom—such as phyla and classes—designate the major categories of animal life, typically designating unique body plans. Lower taxonomic categories—like genus and species—designate smaller degrees of difference among organisms that typically exemplify similar overall ways of organizing their body parts and structures.
Throughout the book I will use these conventional categories of classification, as do most Cambrian paleontologists. Nevertheless, I am aware that some paleontologists and systematists (experts in classification) today prefer “phylogenetic classification,” a method that often uses a “rank-free” classification scheme.6 Advocates of modern phylogenetic classification argue that the traditional classification system lacks objective criteria by which to decide whether a certain group of organisms should be assigned a particular rank of, for example, phylum or class or order.7 Proponents of rank-free classification attempt to eliminate subjectivity in classification (and ranking) by grouping together animals that are thought, based upon studies of similar molecules in different groups, to share a common ancestor. This method of classification treats groups that emerge at roughly the same time on the tree of life as equivalent. Nevertheless, even proponents of phylogenetic classification often use the conventional taxonomic categories in their technical discussions of specific organisms because of their common scientific usage. So despite my own sympathy with some of the concerns of rank-free advocates (see below), I have chosen to do the same.
FIGURE 2.5
Figure 2.5a (top): Chart showing when representatives of the different animal phyla first appeared in the fossil record. According to Darwinian theory, differences in biological form should increase gradually, steadily increasing the number of distinct body plans and phyla, over time. References for first appearances are found in note 5 of this chapter. Figure 2.5b (bottom, left) expresses that expectation graphically, showing the number of new phyla increasing steadily as members of one phylum diversify and give rise to new phyla. Figure 2.5c (bottom, right) shows the actual pattern of first appearance showing a spike in the number of phyla that first appear in the Cambrian, followed by either few or no new phyla arising in subsequent periods of geological history.
In any case, it’s worth noting th
at using a rank-free classification system does not minimize the mystery of the Cambrian explosion. The Cambrian explosion presents a puzzle for evolutionary biologists, not just because of the number of phyla that arise, but rather because of the number of unique animal forms and structures that arise (as measured, perhaps, by the number of phyla)—however biologists decide to classify them. Thus, whether scientists decide to use newer rank-free classification schemes or older, more conventional, Linnaean categories, the “evolutionary novelties”—that is, the new anatomical structures and modes of organization—that arise suddenly with the Cambrian animals remain as facts of the fossil record, requiring explanation. (For an expanded technical discussion of these issues, go to this endnote.)8
One especially dramatic fact of the Cambrian explosion is the first appearance of many novel marine invertebrate animals (representatives of separate invertebrate9 phyla, subphyla, and classes in the traditional classification scheme). Some of these animals have mineralized exoskeletons, including those representing phyla, such as echinoderms, brachiopods, and arthropods, and each represent clearly distinct and novel body plans. Further, these are just three of dozens of novel body plans exemplified by the Burgess animals—animals in which both soft and hard parts are well preserved (see Fig. 2.6).
FIGURE 2.6
Representatives of some of the major animal groups that first appeared in the sedimentary rock record during the Cambrian period.
Darwin's Doubt Page 4
