Darwin's Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design

by Stephen C. Meyer

(corals and jellyfish),

echin...

arthr...

(trilobites and i...

which all vertebrates including h...

orders,

families,

ge...

bro...

p...

cla...

Lower taxonomic ca...

spe...

Throughout the book I will use these conventional categories of classification, as do most Cambrian paleontologists.

Advocates of modern phylogenetic classification argue

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.

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 tradit...

mineralized exosk...

arthr...

(2) an absence of transitional intermediate fossils connecting the Cambrian animals to simpler Precambrian forms;

(3) a startling array of completely novel animal forms with novel body plans;

Figures 2.7 and 2.8 illustrate the difficulty posed by the first two of these features:

sudden appearance

missing interm...

The first shows the Darwinian expectation that changes in morphology should arise only a...

This Darwinian commitment to gradual change through microevolutionary variations produces the classic representation of evolu...

Instead, the Precambrian–Cambrian fossil record, especially in light of the Burgess Shale after Walcott, points to the geologically sudden appearance of complex and novel body plans.

According to Darwinian theory, the strata beneath the Cambrian rocks should evidence many ancestral and intermediate forms. Such forms have not been found for the vast majority of animal phyla. These anticipated but missing forms are represented by the gray circles. Lines and dark circles depict fossilized representatives of phyla that have been found.

Darwin, as we know, regarded the sudden appearance of the Cambrian animals as a significant challenge to his theory.13

Darwin’s recognition15 of this constraint was prescient. Geologists in his day employed relative dating methods. They did not have modern radiometric methods for determining the “absolute” ages of rocks. For this reason, they did not yet fully understand how long it would have taken to accumulate the great columns of sedimentary rock and, thus, the great expanses of time that were available to the evolutionary process.

With natural selection, Darwin proposed a purely natural mechanism for constructing the complex organs and structures (such as eyes) present in many forms of life. His mechanism of natural selection worked by constructing such systems one tiny step at a time, discarding the harmful variations and seizing upon the rare improvement.

MORE MISSING LINKS

Two other features of the Cambrian explosion revealed in the Burgess Shale, features (3) and (4) described earlier, not only confirmed the reality of the Cambrian mystery, but broadened and deepened it—and at just the time when paleontologists were looking to resolve the mystery with new fossil discoveries.

F...

Each new and exotic Cambrian creature—the anomalocarids (see Fig. 2.10), Marrella, Opabinia, and the bizarre and appropriately named Hallucigenia—for which there were again no obvious ancestral forms in the lower strata, required its own series of transitional ancestors. But where were they?

Darwin had hoped that later fossil discoveries would eventually eliminate what he regarded as the one outstanding anomaly associated with his theory. Walcott’s discovery was not that discovery.

Not only did the Burgess Shale fail to reveal the expected ancestral precursors of the known Cambrian animal forms, but it revealed a motley crew of previously unknown animal forms and body plans that now demanded their own lengthy chain of evolutionary precursors, only compl...

The Burgess Shale raised an additional difficulty (feature 4, discussed earlier), though not one that Walcott recognized during his lifetime. Instead, its exposition would await a later generation of Cambrian experts, particularly Stephen Jay Gould.

Darwin’s theory implied that as new animal forms first began to emerge from a common ancestor, they would at first be quite similar to each other, and that large differences in the forms of life—what paleontologists call disparity—would only emerge much later as the result of the accumulation of many incremental changes. In

The site of the Burgess Shale and its setting nicely illustrates the difference between

and disparity.

Reaching it involves a six-mile hike through the picturesque scenery of Yoho National Park—Takakkaw Falls, Emerald Lake, and glaciers and glacier-cut mountain peaks thrusting into view at almost every turn. In this ecologically diverse setting, hikers have a chance of spotting squirrels, marmots, deer, moose, elk, wolves, and mountain goats. Rare sightings might include a grizzly bear or Canadian lynx, while alert birdwatchers might glimpse a horned lark, a white-tailed ptarmigan, the rare water pipit, or a gray-crowned rosy finch; an eagle, hawk, or grassland falcon; dippers, jays, migrating warblers, or harlequin ducks.16

As richly various as this array of animals is, all of them come from a single phylum, Chordata—and even from a single subphylum, Vertebrata.

According to Darwin’s theory, the differences in form, or “morphological distance,” between evolving organisms should increase gradually over time as small-scale variations accumulate by natural selection to produce increasingly complex forms and structures (including, eventually, new body plans).

“The fossil record suggests that the major pulse of diversification of phyla occurs before that of classes, classes before that of orders, orders before that of families. . . . The higher taxa do not seem to have diverged through an accumulation of lower taxa.”

If bottom up development is the rule the theory works if not another explanation must ensue that explains the anomaly.

classification dispenses with taxonomic ranks and hierarchies. In their system, there is no “top” and no “down.” Nevertheless, advocates of phylogenetic classification do acknowledge that different combinations of “character” states (characteristics or features of organisms) can mark either bigger or smaller morphological differences between clades (closely related groups of organisms that presumably share a common ancestor). And some leading advocates of phylogenetic classification have noted that the fossil record exhibits a pattern in which a few character traits marking large morphological differences between clades arise first, followed later in each clade by the addition of other combinations of characters that mark smaller differences within those clades. Larger differences between clades arise first, smaller differences within them arose later—the themes precede the variations.

The top-down pattern of appearance in the fossil record: disparity precedes diversity.

In his classic work Phylogenetic Systematics, Hennig quoted another paleontologist approvingly who observed: