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

by Stephen C. Meyer

Darwin, however, proposed a possible solution. He suggested that the fossil record may be significantly incomplete: either the ancestral forms of the Cambrian animals were not fossilized or they hadn’t been found yet. “I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect,” Darwin wrote. “Of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. . .

Agassiz, for his part, would have none of it.

However broken the geological record may be, there is a complete sequence in many parts of it, from which the character of the succession may be ascertained.” On what basis did he make this claim? “Since the most exquisitely delicate structures, as well as embryonic phases of growth of the most perishable nature, have been preserved from very early deposits, we have no right to infer the disappearance of types because their absence disproves some favorite [i.e., Darwinian] theory.”25

many leading naturalists—Joseph Hooker, Thomas Huxley, Ernst Haeckel, and Asa Gray—all younger than Agassiz, quickly aligned themselves with his evolutionary line of thinking. True, some scientists, notably the Scottish engineering professor Fleeming Jenkin and (later) the English geneticist William Bateson, expressed persistent doubts about the efficacy of natural selection. But despite the views of some weighty scientific critics, Darwin’s revolutionary theory won increasingly wide support and soon defined the terms of the debate about the history of life. Those who rejected it wholesale, as Agassiz did, consigned themselves to increasing irrelevance.

As science advanced in the late nineteenth century, it increasingly excluded appeals to divine action or divine ideas as a way of explaining phenomena in the natural world. This practice came to be codified in a principle known as methodological naturalism. According to this principle, scientists should accept as a working assumption that all features of the natural world can be explained by material causes without recourse to purposive intelligence, mind, or conscious agency.

“What is the great difference,” he wrote, “between supposing that God makes variable species or that he makes laws by which species vary?”

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.

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.

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 been known even in Darwin’s time and detailing a greater diversity of biological form and architecture than had hitherto been imagined.

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

eventually the slab was traced to a layer of shale—later called the Burgess Shale—3000 feet above the town of Field.1

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.

unique body plans.

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.

novel body plans exemplified by the Burgess animals

But in the great debate between Agassiz and Darwin, Walcott sided with the Englishman. Thus, the Burgess Shale struck Walcott as not only fascinating, but puzzling.

Over the years, as paleontologists have reflected on the overall pattern of the Precambrian–Cambrian fossil record in light of Walcott’s discoveries, they too have noted several features of the Cambrian explosion that are unexpected from a Darwinian point of view11 in particular: (1) the sudden appearance of Cambrian animal forms; (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; and (4) a pattern in which radical differences in form in the fossil record arise before more minor, small-scale diversification and variations. This pattern turns on its head the Darwinian expectation of small incremental change only gradually resulting in larger and larger differences in form.

the problem posed by the Burgess Shale is not the increase in complexity, but the sudden quantum leap in complexity.

The jump from the simpler Precambrian organisms (further explored in the next chapters) to the radically different Cambrian forms appears to occur far too suddenly to be readily explained by the gradual activity of natural selection and random variations. Neither the Burgess Shale nor any other series of sedimentary strata known in Walcott’s day recorded a pattern of novel body plans arising gradually from a sequence of intermediates. Instead, completely unique organisms such as the bizarre arthropod Opabinia (see Fig. 2.9)—with its fifteen articulated body segments, twenty-eight gills, thirty flipper-like swimming lobes, long trunk-like proboscis, intricate nervous system, and five separate eyes12—appear fully formed in the Cambrian strata along with representatives of other fundamentally different body plans and designs of equal complexity.

William Paley. Paley had argued that just as complex structures such as watches necessarily issue from intelligent watchmakers, the complex structures in living organisms must likewise owe their origin to a designing intelligence. 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. If evolution progressed by “whole watches”—that is, by entire anatomical systems like the trilobite’s eye—then biology would have fallen back to the old absurdity of imagining that a watch could fall together purely at random and all at once. Thus, unless Darwin’s evolutionary mechanism worked gradually by preserving the tiniest of random changes over many millions of years, it didn’t work at all.

First, the great profusion of completely novel forms of life in the Burgess assemblage (feature 3) demanded even more transitional forms than had previously been thought missing.

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 complicating the task of explaining the Cambrian explosion in Darwinian terms.

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.

the animal forms preserved in the Burgess Shale display considerable disparity.

on a Darwinian view, small-scale variations and differences should arise first, gradually giving rise to larger-scale differences in form—just the opposite of the pattern evident in the fossil record.

On shale of a very fine grain, the Burgess fossils look like lithographic pictures, dark on light (see color insert plates 15 and 16). Even the soft parts like the gills and guts are sometimes preserved. This is not the norm in the world of paleontology. Usually soft tissues decay before they can be fossilized, leaving behind only harder parts, such as bone, teeth, and shells, to be preserved. The Burgess event that captured the Cambrian fauna for future discovery was different. Although it took the lives of untold Cambrian animals, it did so with an exquisite delicacy that preserved soft tissue.

All of the fossilized animals of the Burgess Shale were sea creatures living near an enormous carbonate reef that later was thrust upward by plate tectonic activity to form what is now called the Cathedral Escarpment. Long after the sea creatures of the Burgess Shale were entombed, these tectonic forces drove the fossils upward from the seafloor carrying them many miles eastward along faults, at the same time building the mountains that Walcott would climb millions of years later.

Thanks to this tectonic movement of earth’s major plates, the continents are now located in very different places than they were millions of years ago. At the time these Cambrian creatures were alive, the land masses that would later form North America lay on the equator. Plate tectonic activity explains why a trove of sea creatures were found fossilized in the mountains of Yoho National Park rather than along a seafloor somewhere.

this disparity arose at a most unexpected time, assuming Darwinian theory, namely, right at the dawn of animal life.

And so, Walcott, ever the geologist, proposed an ingenious geological solution to the biological problem of the origin of animal life. He noted that the Precambrian period was a period of dramatic continental uplift. He then suggested that the ancestors of the trilobites first evolved at a time when the Precambrian seas had receded from the landmasses.

Thus, according to Walcott, ancestral precursors to the trilobites and other distinctive Cambrian forms had existed, but they were not fossilized in sediments that would later be elevated above sea level until early in the Cambrian; instead, before the Cambrian, during a period when sea levels were lower, trilobites and their ancestral forms were being deposited offshore in what are now only deep-sea sediments.24 Walcott named this cryptic period of time in which trilobites and other animals were rapidly evolving offshore as the “Lipalian interval.” (The term “Lipalian” is derived from the Greek word for lost.)

In this view, the abrupt appearance of the Cambrian body plans in the geological column was merely an “artifact” of incomplete sampling of the fossil record and, indeed, the inability to access the undersea sedimentary layers where the ancestors of the Cambrian fauna presumably lay encased.

His artifact hypothesis (also known as the “Lipalian interval” hypothesis) was a distinct advance over Darwin’s unadorned claim that the fossil ancestors of the Cambrian animals had not yet been discovered.

It also could be tested, at least once offshore drilling technology advanced to allow for the sampling of the buried offshore sedimentary rocks.

“I fully realize that the conclusions above outlined are based primarily on the absence of a marine fauna in Algonkian [Precambrian] rocks,” he wrote, “but until such is discovered I know of no more probable explanation of the abrupt appearance of the Cambrian fauna than that I have presented.”25

Gould also noted that Walcott used this strategy to minimize the challenge posed by the morphological disparity of the Burgess forms.27 Some paleontologists today reject Gould’s criticism of Walcott’s inclusion of so many Burgess animal forms into existing taxonomic categories.Nevertheless,

few paleontologists think Walcott’s use of lumping explained away the Cambrian explosion.

They had only to wait for the technologies of seafloor drilling to emerge and hope that nature had seen fit, unmolested under the ocean deeps, to leave concrete evidence of the gradual emergence of the major Cambrian body plans.

The discovery, near the town of Chengjiang in the Yunnan Province, revealed a trove of early Cambrian animal forms.

widely acknowledged to surpass even the legendary Burgess Shale as the most extensive and significant Cambrian-era locality.

fossils from the Maotianshan Shale near Chengjiang (see Fig. 3.2) had established an even greater variety of Cambrian body plans from an even older layer of Cambrian rock than those of the Burgess, and they did so with an almost photographic fidelity.

So there was little doubt about the significance of the discoveries that Chen came to report that day. What was soon in doubt, however, was Chen’s scientific orthodoxy. In his presentation, he highlighted the apparent contradiction between the Chinese fossil evidence and Darwinian orthodoxy. As a result, one professor in the audience asked Chen, almost as if in warning, if he wasn’t nervous about expressing his doubts about Darwinism so freely—especially given China’s reputation for suppressing dissenting opinion. I remember Chen’s wry smile as he answered. “In China,” he said, “we can criticize Darwin, but not the government. In America, you can criticize the government, but not Darwin.”

Walcott argued that the ancestral precursors of the Cambrian animals were missing from the Precambrian fossil record because of the transgression and regression of seas.

When Walcott proposed his ingenious geological scenario, it could not yet be tested. But with the development of offshore drilling technology in the 1940s, 1950s, and 1960s, oil companies began to drill through thousands of feet of marine sedimentary rock.6 As geologists evaluated the contents of these drill cores, they did not find Walcott’s predicted Precambrian fossils.

modern plate tectonic theory now affirms that oceanic crustal material eventually plunges back into the earth and melts in a process known as subduction. After surface rocks melt during subduction, they form a new supply of molten magma. Eventually, magma from other locations deep in the earth wells up at mid-oceanic ridges to form new igneous rocks, in a process known as seafloor spreading. It follows that any oceanic sediments deposited atop the oceanic igneous crust will have a limited “life span” on the surface of the earth. Eventually, these sedimentary rocks collide with the continental margin, plunge deep into the upper mantle, and melt to form magma.

As a consequence of this cycle, the maximum age of any marine sediment is strictly limited. And according to modern estimates, the oldest section of oceanic crust has existed only since the Jurassic (or about 180 million years ago8)—far too young to contain fossil ancestors of the trilobites. As the evidence for plate tectonics mounted, scientists discarded Walcott’s artifact hypothesis and Lipalian interval as nonstarters. Paleontologists today do not expect to find any Precambrian ancestors of the trilobites in oceanic sediments, since they realize that there are no Precambrian sediments in the ocean basins. If Precambrian strata are to be found anywhere, continents are the place.

Walcott rejected the idea that paleontologists simply had not looked in, or sampled, enough places. He noted that geologists already had extensively investigated “the great series of Cambrian and Precambrian strata in eastern North America.” Though they had looked “from Alabama to Labrador; in western North America [and] from Nevada and California far into Alberta and British Columbia, and also in China” their investigations had turned up nothing of significant interest.9 In Walcott’s view the continents simply had not preserved the fossilized remains of the Cambrian ancestors. Before Walcott, some geologists had gone a step farther and suggested that all Precambrian sedimentary rocks had been destroyed via extreme heat and pressure, a process called “universal metamorphism.” Walcott rejected this hypothesis, since he himself had encountered a “great series of pre-Cambrian sedimentary rocks on the North American continent” among other places. Other geologists suggested that major bursts of evolutionary innovation occurred only during periods when sedimentary deposition had ceased, thus again resulting in a lack of fossil preservation. But, as Gould remarked of Walcott’s artifact hypothesis, this explanation also appeared to many scientists “forced and ad hoc . . . born of frustration, rather than the pleasure of discovery.”10

As for the idea that the ancestors of the Cambrian animals were too small to be preserved, paleontologists have known for some time that the cells of filament-shaped microorganisms (probably cyanobacteria) have been preserved in ancient Precambrian rocks.

If paleontologists can find tiny fossilized cells in these far older and rarer formations, shouldn’t they also be able to find some ancestral forms of the Cambrian animals in younger and more abundant sedimentary rocks? Yet few such precursors have been found.