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

by Stephen C. Meyer

There are also several reasons to question the second version of this hypothesis—the idea that the presumed Cambrian ancestors were too soft to be preserved. First, some paleontologists have questioned whether soft-bodied ancestral forms of the hard-bodied Cambrian animals would have even been anatomically viable.16 They argue that many animals representing phyla such as brachiopods and arthropods could not have evolved their soft parts first and then added shells later, since their survival depends upon their ability to protect their soft parts from hostile environmental forces. Instead, they argue that soft and hard parts had to arise together.

“Animals such as brachiopods . . . cannot exist without a mineralized skeleton. Arthropods bear jointed appendages and likewise require a hard, organic or mineralized outer covering.”

Thus, the absence of hard-bodied ancestors of these Cambrian animals in the Precambrian strata shows that these animals first arose in the Cambrian period.

it seems unlikely on a Darwinian view of the history of life that all Cambrian arthropod or brachiopod ancestors, especially the relatively recent ancestors of these animals, would have lacked hard parts entirely.

As we saw earlier, Precambrian sedimentary rocks in several places around the world have preserved fossilized colonial blue-green algae, single-celled algae, and cells with a nucleus (eukaryotes).25 These microorganisms were not only small, but they also entirely lacked hard parts.

Another class of late Precambrian organisms called the Vendian or Ediacaran biota included the fossilized remains of many soft-bodied organisms, including many that may well have been lichens, algae, or protists (microorganisms with cells containing nuclei). Cambrian-era strata themselves preserve many soft-bodied creatures and structures. The Burgess Shale in particular preserved the soft parts of several types of hard-bodied Cambrian animals, such as Marrella splendens,26 Wiwaxia,27 and Anomalocaris. The Burgess Shale also documents entirely soft-bodied representatives28 of several phyla,

“The existing [Burgess] collections represent approximately 70,000 specimens. Of these, about 95 percent are either soft-bodied or have thin skeletons.”37

Any doubts about the ability of sedimentary rocks to preserve soft and small body parts were permanently laid to rest by a series of dramatic fossil finds in southern China beginning in the 1980s.

The discoveries near Chengjiang demonstrated beyond any reasonable doubt that sedimentary rocks can preserve soft-bodied fossils of great antiquity and in exquisite detail, thereby challenging the idea that the absence of Precambrian ancestors is a consequence of the fossil record’s inability to preserve soft-bodied animals from that period.

Sure enough, under magnification he found little round balls that he and Paul Chien identified as sponge embryos.

Chen found these sponge embryos beneath the Cambrian–Precambrian boundary in late Precambrian rock. Yet these Precambrian layers did not preserve remains of any clearly ancestral or intermediate forms leading to the other main groups of Cambrian animals. This raised an obvious question. If the Precambrian sedimentary strata beneath the Maotianshan Shale preserved the soft tissues of tiny, microscopic sponge embryos, why didn’t they also preserve the near ancestors of the adult animals that arose in the Cambrian, especially since some of those animals must have had at least some hard parts as a condition of their viability? If these strata could preserve embryos, then they should have preserved fully developed animals—at least, if such animals were present at the time. That well-developed, clearly ancestral animal forms were not preserved, when tiny sponge embryos51 were, strongly indicates that such forms were simply not present in the Precambrian layers.

Precambrian strata include many types of sediments that can preserve—and in the case of the Doushantuo formation in China, have preserved—animal remains in fine detail, including small and vulnerable sponge embryos.

Since the discovery of the Burgess Shale, Precambrian- and Cambrian-era discoveries have repeatedly uncovered fossil forms that either establish radically disparate new forms of life or, increasingly, forms that fall into existing higher taxonomic groups (such as class, subphylum, or phylum).

Recall that Louis Agassiz thought that this pattern could not be explained by appealing to an incomplete fossil record, because the fossil record was strangely selective in its incompleteness, preserving abundant evidence of the terminal branches but consistently neglecting to preserve the representatives of the internal branches or nodes.

as more and more fossil discoveries fall within existing higher taxonomic groups (e.g., phyla, subphyla, and classes), and as they fail to document the rainbow of intermediate forms expected in the Darwinian view of the history of life, it grows ever more improbable that the absence of intermediate forms reflects a sampling bias—that is, an “artifact” of either incomplete sampling or preservation.

Imagine that you reach into an enormous barrel full of marbles and randomly pull out a yellow, a red, and a blue marble. At this point your brief sampling should leave you undecided as to whether you have a representative sample of the barrel’s contents. You might at first imagine that the barrel also contains marbles representing a rainbow of intermediate colors. But as you continue to sample from every place in the barrel and find that the barrel disgorges only those same three colors you begin to suspect that it may offer a much more limited selection of colors than, say, the rack of color samples at your local paint store. Over the past 150 years or so, paleontologists have found many representatives of the phyla that were well known in Darwin’s time (by analogy, the equivalent of the three primary colors) and a few completely new forms altogether (by analogy, some other distinct colors such as green and orange, perhaps). And, of course, within these phyla there is a great deal of variety. Nevertheless, the analogy holds at least insofar as the differences in form between any member of one phylum and any member of another phylum are vast, and paleontologists have utterly failed to find forms that would fill these yawning chasms in what biologists call “morphological space.” In other words, they have failed to find the paleontological equivalent of the numerous finely graded intermediate colors (Pendleton blue, dusty rose, gun barrel gray, magenta, etc.) that interior d...

Foote’s analysis suggests that since paleontologists have reached repeatedly into the proverbial barrel, sampled it from one end to the other, and found only representatives of various radically distinct phyla but no rainbow of intermediates, we shouldn’t hold our breath expecting such intermediates to eventually emerge. He asks “whether we have a representative sample of morphological diversity and therefore can rely on patterns documented in the fossil record.” The answer, he says, is yes.55

“Although we have much to learn about the evolution of form,” he writes, the statistical pattern created by our existing fossil data demonstrates that “in many respects our view of the history of biological diversity is mature.”

As the name implies, the fossils documenting the Cambrian explosion appear within a relatively narrow slice of geologic time. Until the early 1990s, most paleontologists thought the Cambrian period began 570 million and ended 510 million years ago, with the Cambrian explosion of novel animal forms occurring within a 20- to 40-million-year window during the lower Cambrian period. Two developments have led paleontologists and geochronologists to revise those estimates downward.

Radiometric analyses of these crystals fixed the start of the Cambrian period at 544 million years ago,58 and the beginning of the Cambrian explosion itself to about 530 million years ago (see Fig. 3.8).

These studies also suggested that the explosion of the novel Cambrian animal forms occurred within a window of geologic time much shorter than previously believed, lasting no more than 10 million years, and the main “period of exponential increase of diversification” lasting only 5 to 6 million years.59

Geologically speaking, 5 million years represents a mere 1/10 of 1 percent (0.11 percent, to be precise) of earth’s history. J. Y. Chen explains that “compared with the 3-plus-billion-year history of life on earth, the period [of the explosion] can be likened to one minute in 24 hours of one day.”60

An analysis by MIT geochronologist Samuel Bowring has shown that the main pulse of Cambrian morphological innovation occurred in a sedimentary sequence spanning no more than 6 million years.

62 Yet during this time representatives of at least sixteen completely novel phyla and about thirty classes first appeared in the rock record. In a more recent paper using a slightly different dating scheme, Douglas Erwin and colleagues similarly show that thirteen new phyla appear in a roughly 6-million-year window.63 As we’ve seen, among these animal forms were the first trilobites, with their lens-focusing compound eyes among other complex anatomical features. The problem of explaining how so many new forms and structures arose so rapidly in the first explosive period of the Cambrian remains, whether or not one decides to include within the designation “Cambrian explosion” other distinct events (see Part Two).

Discoveries at Chengjiang contradict the bottom-up pattern that neo-Darwinism expects. The site does not show the gradual emergence of unique species followed slowly but surely by the emergence of representatives of ever higher and more disparate taxa, leading to novel phyla. Instead, like the Burgess Shale, it shows body plan–level disparity arising first and suddenly, with no evidence of a gradual unfolding and ranging through the lower taxonomic groups.

For weeks before our event in September of 2009, outspoken evolutionary biology students and an atheist student group, both egged on by militant off-campus bloggers, had threatened to disrupt the screening. Members of the biology faculty pledged to come, so that well before the official start time a large crowd had gathered. The museum and the geology department, not wanting to complicate matters in their own minds by watching the film first, decided to launch a preemptive first strike by issuing a disclaimer and scheduling an official lecture designed to rebut the film. In the disclaimer the museum stated that, given its public funding, it had no choice but to rent the auditorium to groups irrespective of their “religious beliefs” or “scientific literacy.” The disclaimer further noted that the Sam Noble Science Museum did “not support unscientific views masquerading as science, such as those of the Discovery Institute.” The museum flyer also announced the lecture of one of their curators, a paleontologist from the university, and mocked the topic of the film—the Cambrian “explosion”—with carefully placed scare quotes. The lecture’s start time of 5:00 P.M. also ensured a confluence afterward of the audience made upset by the lecture and the more friendly audience coming to see the film.

many prominent paleontologists have sought to explain the Cambrian explosion as an artifact of our incomplete sampling of an incomplete fossil record. The lecture that my colleague heard that night in Oklahoma took a very different approach, giving the strong impression that the Precambrian fossil record actually does preserve the ancestral forms of the Cambrian animals and that the Ediacaran fauna, in particular, provide several striking examples of such forms. In public presentations about the Cambrian explosion, I’ve often encountered this claim, though usually in the form of an unfocused question: “What about the Ediacaran?”

Most paleontologists doubt that well-known Ediacaran forms represent ancestors of the Cambrian animals and few think the late Precambrian fossil record as a whole makes the Cambrian explosion appreciably less explosive. The claim is important to address, however, since it persists as a kind of paleontological urban legend, one that even occasionally finds its way into the mouths of paleontologists.

So do either the remains of the specific organisms from the Ediacaran Hills or the Ediacaran or Vendian biota as a whole solve the problem of the Cambrian explosion? Do these exotic forms represent a kind of fuse to the Cambrian explosion that eliminates the need to explain the rapid emergence of novel body plans and forms of animal life? There are many good reasons to doubt this idea.

of sponges and the possible exception of Kimberella, the body plans of visibly fossilized organisms (as opposed to trace fossils) bear no clear relationship to any of the organisms that appear in the Cambrian explosion (or thereafter).8 The most noted Ediacaran organisms such as Dickinsonia, Spriggina, and Charnia do not have an obvious head, a mouth, bilateral symmetry (see below), a gut, or sense organs such as eyes. Some paleontologists question whether these organisms even belong in the animal kingdom. Dickinsonia, for example, has been interpreted by University of Oregon paleontologist Gregory Retallack as having “fungal-lichen” affinities, since its mode of fossil preservation “is comparable not with that of soft-bodied jellyfish, worms, and cnidarians, but with the fossil record of fungi and lichens.”

“Although the soft-bodied fossils that appear about 565 million years ago are animal-like, their classifications are hotly debated. In just the past few years these fossils have been viewed as protozoans; as lichens; as close relatives of the cnidarians; as a sister group to cnidarians plus all other animals; as representatives of more advanced, extinct phyla; and as representatives of a new kingdom entirely separate from the animals.”

those paleontologists who do regard the Ediacaran fauna as animals rarely classify them the same way, underscoring their lack of clear affinities to any known animal groups.

As Nature recently noted, if the Ediacaran fauna “were animals, they bore little or no resemblance to any other creatures, either fossil or extant.”27 This absence of clear affinities has led an increasing number of paleontologists to reject ancestor-descendant relationships between all but (at most) a few of the Ediacaran and Cambrian fauna.

many alleged trace fossils actually show evidence of inorganic origin: “There are numerous reports of older trace fossils, but most can be immediately shown to represent either inorganic sedimentary structures or metaphytes [land plants], or alternatively they have been misdated.”31

“Some such traces date back to 1.5 billion to 1.8 billion years ago, which outdates even the boldest claims of the time of origin of animal multi-cellularity

Even the most favorable interpretations of these trace fossils suggest that they indicate the presence of no more than two animal body plans (of largely unknown characteristics). Thus, the Ediacaran record falls far short of establishing the existence of the wide variety of transitional intermediates that a Darwinian view of life’s history requires. The Cambrian explosion attests to the first appearance of organisms representing at least twenty phyla and many more subphyla and classes, each manifesting distinctive body plans. In a best case, the Ediacaran forms represent possible ancestors for, at most, four distinct Cambrian body plans, even counting those documented only by trace fossils. This leaves the vast majority of the Cambrian phyla with no apparent ancestors in the Precambrian rocks (i.e., at least nineteen of the twenty-three phyla present in the Cambrian have no representative in Precambrian strata).33

Third, even if representatives of four animal phyla were present in the Ediacaran period, it does not follow that these forms were necessarily transitional or intermediate to the Cambrian animals. The Precambrian sponges (phylum Porifera), for example, were quite similar to their Cambrian brethren, thus demonstrating, not a gradual transformation from a simpler precursor or the presence of an ancestor common to many forms, but quite possibly only an earlier first appearance of a known Cambrian form. The same may be true of whatever kind of worm may be attested by Precambrian tracks and burrows. Moreover, even assuming, as some evolutionary biologists do,34 that later Cambrian animals had a sponge-like Precambrian ancestor, the gap in complexity as measured by the number of cell types alone, to say nothing of the specific anatomical structures and distinct modes of body plan organization that are present in later animals but not in sponges, leaves a massive discontinuity in the fossil record that requires explanation (much like the morphological gap between Spriggina and actual arthropods).

The Ediacaran fossils themselves provide evidence of a puzzling leap in biological complexity, though not one nearly great enough (or of the right kind) to account for the Cambrian explosion. Before organisms like Kimberella, Dickinsonia, and sponges appeared, the only living forms documented in the fossil record for over 3 billion years were single-celled organisms and colonial algae. Producing sponges, worms, and mollusks from single-celled organisms is a little like transforming a spinning top into a bicycle.

Thus, the Ediacaran biota attest to a separate sudden increase in biological complexity within a short window of geological time (about 15 million years), following roughly 3 billion years in which only single-celled organisms inhabited the earth.

35 This leap in complexity, in a relatively short span of geological time, may well exceed the explanatory resources of natural selection working on random mutations. We will return to that question in Part Two. The Ediacaran fossils therefore do not solve the problem of the sudden increase in biological form and complexity during the Cambrian. Instead, they represent an earlier, if less dramatic, manifestation of the same kind of problem. To biology’s “big bang,”36 the Ediacaran biota add a significant “pow.” As paleobiologist Kevin Peterson, of Dartmouth College, and his colleagues note, these fauna represent “an apparent quantum leap in ecological complexity as compared with the ‘boring billions’ [of years] that characterize Earth before the Ediacaran,” even if these organisms are “still relatively simple when compared with the Cambrian,” which they characterize as another “quantum leap in organismal and ecological complexity.”37 Many paleontologists now refer to the Ediacaran radiation as an explosion in its own right.38 This Precambrian “pow” makes the problem of fossil discontinuity only more acute, since credible intermediates leading to the Ediacaran layers are completely nonexistent in the even more sparsely populated strata beneath them.

even if one regards the appearance of the Ediacaran fossils as evidence of a “fuse” leading to the Cambrian explosion as some have proposed,39 the total time encompassed by the Ediacaran and Cambrian radiations still remains exceedingly brief relative to the expectations...

To anyone unfamiliar with the equations of population genetics by which neo-Darwinian evolutionary biologists estimate how much morphological change is likely to occur in a given period of time, 40 to 50 million years may seem like an eternity. But empirically derived estimates of the rate at which mutations accumulate imply that 40 to 50 million years does not constitute anything like enough time to build the necessary anatomical novelties that arise in the Cambrian and Ediacaran periods.

Recall that bilaterians are animals whose parts found on one side of the body midline are also found in mirror image on the other

the bilaterian animals that later first appeared in the Cambrian period.

“There is no evidential basis for interpreting Vernanimalcula as an animal, let alone a bilaterian.”

Their article upbraided David J. Bottjer, the main paleontologist who has promoted the interpretation of Vernanimalcula as a bilaterian ancestor, for seeing what he wanted to see and disregarding the clear evidence of nonbiological mineralization.

In a 2005 Scientific American article, Bottjer interpreted Vernanimalcula as the “oldest fossil animal with a bilaterian body plan yet discovered.” In that article, Bottjer claimed that Vernanimalcula confirmed the “suspicion that complex animals have a much deeper root in time” and “that the Cambrian was less of an explosion and more of a flowering of animal life.”51 After unequivocally rejecting Bottjer’s interpretation on the basis of their geochemical analysis, Bengtson and his coauthors rebuked Bottjer in rather personal terms: It is likely that the fossils referred to [as] Vernanimalcula were interpreted as bilaterians because this was . . . the explicit quarry of its authors. If you know from the beginning not only what you are looking for, but what you are going to find, you will find it, whether or not it exists. As Richard Feynman (1974) famously remarked: “The first principle is that you must not fool yourself—and you are the easiest person to fool.” . . . Once you have fooled yourself you will fool other scientists.

Bengtson and his colleagues insist that however much a few paleontologists such as Bottjer might have wanted to “heap” “evolutionary significance” on Vernanimalcula in order to relieve their cognitive dissonance about the Cambrian explosion, the evidence does not bear the weight of interpretation that had been placed upon it.

Jonathan Wells explained why these and other obscure and enigmatic Precambrian fossils (or imprints) failed to qualify as convincing precursors to any of the Cambrian animals, citing the work of leading authorities in paleontology. In each case, he noted that similarities between the Ediacaran forms and later Cambrian animals had proven superficial, because the Ediacaran forms lacked many key diagnostic features of specific Cambrian phyla.

Many defenders of the Darwinian picture of the history of life seemed to assume that the discovery of any alleged Precambrian animal forms, however implausible as ancestors of specific Cambrian animals or however sparsely distributed in the vast sequences of Precambrian strata, would solve the mystery of the Cambrian explosion, especially if these forms exemplified some abstractly perceived commonality such as bilateral symmetry. To see what’s wrong with this way of thinking, imagine an ambitious distance swimmer claiming that it would be possible to swim between California and Hawaii over a period of many months or years because of the small islands that provide way stations where he could eat, rest, and overnight at each stage along his marathon journey. But instead of showing that an archipelago dotting the route between California and Hawaii at reasonably accessible intervals actually exists, he points to a couple of barren atolls in the South Pacific far from the most likely course to Hawaii. Clearly, in that case, the claims of our intrepid hypothetical swimmer would not be credible. In a similar way, the claims of those who assert that a few isolated and anatomically enigmatic forms of life in the Precambrian solve the problem of the Cambrian explosion also lack credibility. To