The Top-Down Pattern in Stark Relief
The Chengjiang fauna makes the Cambrian explosion more difficult to reconcile with the Darwinian view for yet another reason. The Chengjiang discoveries intensify the top-down pattern of appearance in which individual representatives of the higher taxonomic categories (phyla, subphyla, and classes) appear and only later diversify into the lower taxonomic categories (families, genera, and species).
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.
Consider the case of the early chordates, a phylum consisting of creatures possessing a flexible, rod-shaped structure called a notochord. Mammals, fish, and birds are familiar members of this phylum. Prior to the discovery of the Chengjiang biota, chordates were unknown in the Cambrian period and were thought to have first appeared only much later, during the Ordovician period.64 Now, following the discoveries at Chengjiang, the first appearance of chordates in the Cambrian period has been amply documented.
For example, J. Y. Chen and several other Chinese paleontologists have found a spindle-shaped eel-like animal called Yunnanozoon lividum, which many paleontologists have interpreted as an early chordate because it possesses, among other features, a digestive tract, branchial arches, and a large notochord much like a spinal cord.65 In addition, J. Y. Chen and colleagues have reported the discovery of a sophisticated craniate-like chordate called Haikouella lanceolata from the lower Cambrian Maotianshan Shale. According to Chen and others, Haikouella has many of the same features of Yunnanozoon lividum as well as several additional anatomical features including a “heart, ventral and dorsal aorta, an anterior branchial arterial, gill filaments, a caudal (posterior) projection, a neural cord with a relatively large brain, a head with possible lateral eyes, and a ventrally situated buccal cavity with short tentacles.”66
FIGURE 3.9
Figure 3.9a (top): Drawing of the Cambrian fish, Myllokunmingia fengjiaoa. Figure 3.9b (bottom): Photograph of Myllokunmingia fengjiaoa fossil. Reprinted by permission from Macmillan Publishers Ltd.: Nature, Shu et al., “Lower Cambrian Vertebrates from South China,” Nature, 402 (November 4, 1999): 42–46. Copyright 1999.
Simon Conway Morris, with D. G. Shu and several Chinese colleagues, has reported an even more dramatic find. They have discovered the fossilized remains of two small Cambrian fish, Myllokunmingia fengjiaoa and Haikouichthys ercaicunensis (see Fig. 3.9), suggesting a much earlier appearance for both fishes and vertebrates (a class of chordates), both of which were first thought to have originated in the Ordovician period, about 475 million years ago. Both of these taxa are jawless fish (agnathans) and are considered by Shu and others to be closely allied to modern lampreys.67 Finally, a paper by Shu and others reports the first convincing specimen of another type of chordate from the Cambrian, a urochordate (tunicate).68 This specimen, Cheungkongella ancestralis, is likewise found in the early Cambrian shales (Qiongzhusi Formation) near Chengjiang. These recent finds demonstrate that not only did the phylum Chordata first appear in the Cambrian, but also each one of the chordate subphyla (Cephalochordata, Craniata, and Urochordata) first emerged then as well. In any case, the discovery in China of chordates, and other previously undiscovered phyla in the Cambrian, only accentuates the puzzling top-down pattern of appearance that other Cambrian discoveries had previously established.69
More Questions Than Answers
Thus, despite the efforts to explain the Cambrian explosion using various versions of the artifact hypotheses, the mystery of the Cambrian explosion has only grown more acute as a result of the dramatic discoveries in southern China that have turned Darwin’s tree of life upside down.
When I first heard J. Y. Chen describe these discoveries in 2000, I had been investigating another unsolved question about the history of life: What caused the first living cell, and the information it contains, to arise? As I heard Dr. Chen speak that day in Seattle, my interest in another puzzling question about the history of life began to germinate. Could it be that the origin of animal life was in its own way just as difficult a problem as that of the origin of life itself? Though I eventually concluded that the Cambrian explosion does, indeed, present a profound challenge to contemporary Darwinian theory, it didn’t take me long to discover that some scientists believed that the mystery of the Cambrian explosion had already been resolved by the discovery of some rather enigmatic Precambrian fossils. We turn to those next.
4
The Not Missing Fossils?
The atmosphere in the auditorium of the Sam Noble Science Museum at the University of Oklahoma was uncomfortably tense, with a security detail of local Norman, Oklahoma, police officers on hand to keep the peace—a conspicuous change from the campus security guard who might be present at a typical university event. The occasion? A new documentary, Darwin’s Dilemma, that Jonathan Wells, a colleague of mine from the Discovery Institute, and I were scheduled to show. The film would explore the challenge to Darwin’s theory posed by the Cambrian fossil record.
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.
Jonathan Wells, a biologist well known for his skepticism about contemporary Darwinian theory, attended the pre-film lecture. Ignoring a few hostile glares, he listened as the paleontologist from the university argued that the Cambrian explosion presented no actual dilemma for Darwinian evolution, and as this same paleontologist speculated that if Darwin had only known what paleontologists today know of the Cambrian fossil record, he (Darwin) would have celebrated it as confirmation of his theory. This particular paleontologist also denied that novel animal forms emerged suddenly in the Cambrian. Instead, he argued that they arose in rudimentary form much earlier in the late Precambrian. He noted that paleontologists had discovered in late Precambrian sediments fossilized sponges, a type of primitive mollusk, and the burrows of worms.
He also laid particular stress on the significance of a group of enigmatic organisms first discovered in the Ediacaran Hills in southern Australia dating from about 565 million years ago, in a Precambrian period known as the Vendian or Ediacaran. Jonathan and I were well aware that most paleontologists do not regard these fossilized organisms as plausible ancestors to the Cambrian fauna. But on that evening the expert from the university claimed the opposite. He also claimed that some obscure Ediacaran organisms (with exotic names such as Vernanimalcula, Parvancorina, and Arkarua) represented early bilaterians (bilaterally symmetrical animals), arthropods, and echinoderms. He insisted that these organisms pushed back the explosi
on of animal life by some 40 million years, establishing a “fuse” for the Cambrian explosion in the form of primitive and presumably ancestral animal forms for several of the most significant Cambrian phyla and body designs.
An hour before I was to walk over to lead a discussion and answer questions about the Cambrian film from what proved to be an intensely hostile audience, Jonathan Wells called me with a report on the museum’s attempt to refute the film preemptively. The presentation claimed to have resolved the Cambrian mystery—Darwin’s dilemma—by showing that the ancestral precursors to the major groups of Cambrian animals had been found after all. But is this true?
The Ediacaran Fauna and Vendian Radiation
In the previous chapter, we saw that 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?” Nevertheless, in writing about the Cambrian, I take care not to attribute the idea that the Ediacaran fauna represent Cambrian ancestors to leading Ediacaran or Cambrian experts, lest I critique a straw man. 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.
The Ediacaran fauna derive their name from their most notable discovery site, the Ediacaran Hills in the outback of southeastern Australia. These fauna date from late Precambrian time, a period that the International Union of Geological Sciences has recently renamed the “Ediacaran period.”1 Since geologists used to call the last period of Precambrian time the “Vendian period,” paleontologists also refer to the Ediacaran fauna as the Vendian fauna or biota (see Fig. 1.6). Paleontologists have made additional discoveries of Edicaran-or Vendian-era creatures in England, Newfoundland, the White Sea in northwestern Russia, and the Namibian desert in southern Africa, suggesting a near worldwide distribution. Although these fossils were originally dated to between 700 and 640 million years old, volcanic ash beds both below and above the Namibian site have recently provided more accurate radiometric dates. These studies fix the date for the first appearance of the Ediacaran fauna at about 570–565 million years ago, and the last appearance at the Cambrian boundary about 543 million years ago, or about 13 million years before the onset of the Cambrian explosion itself.2
The late Precambrian-era sediments around the world have yielded four main types of fossils, all of which are dated between about 570 and 543 million years ago. The first group consists of the Precambrian sponges mentioned in the previous chapter. These animals first arose about 570 to 565 million years ago.
FIGURE 4.1
Examples of enigmatic Ediacaran fossils: Dickinsonia, Spriggina, and Charnia. Fossil photos in Figures 4.1a and 4.1b courtesy Peterson, K. J., Cotton, J. A., Gehling, J. G., and Pisani, D., “The Ediacaran Emergence of Bilaterians: Congruence Between the Genetic and the Geological Fossil Records,” Philosophical Transactions of the Royal Society B, 2008, 363 (1496): 1435–43, Figure 2, by permission of the Royal Society.
The second is the distinctive group of fossils from the Ediacaran Hills. The creatures fossilized there include such well-known forms as the flat, air mattress-like body of Dickinsonia; the enigmatic Spriggina, with its elongated and segmented body and possible head shield; and the frond-like Charnia (see Fig. 4.1). These organisms were at least mostly soft-bodied and are large enough to identify with the naked eye.
The third group includes what are called trace fossils, the possible remains of animal activity such as tracks, burrows, and fecal pellets. Some paleontologists have attributed these trace fossils to ancient worms.
The fourth group is the fossils of what may be primitive mollusks, a possibility that received support from a recent discovery in the cliffs along the White Sea in northwestern Russia. There, Russian scientists have discovered thirty-five distinctive specimens of a possible mollusk called Kimberella, probably a simple animal form. These new White Sea specimens, dated to 550 million years ago, suggest that Kimberella “had a strong [though not hard], limpet-like shell, crept along the sea floor, and resembled a mollusk.”3 Paleontologist Douglas Erwin, of the Smithsonian Institution, has commented: “It’s the first animal that you can convincingly demonstrate is more complicated than a flatworm.”4 Additionally, seafloor tracks from Precambrian sediments in both Canada and Australia have been attributed to mollusks, since the tracks resemble what might have been left by a row of small teeth on the tongue-like ribbon of some mollusks as they scraped food particles off the seafloor. In this case, Kimberella may well have been the track maker.5 The authors of the original descriptive paper in Nature, Mikhail Fedonkin, from the Russian Academy of Sciences, and Benjamin Waggoner, then at the University of California at Berkeley, conclude as much and suggest that such creatures “began to diversify before the beginning of the Cambrian.”6 Paleontologists, however, are still weighing the evidence.7
The Significance of the Ediacaran
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.
First, with the exception 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.” Dickinsonia’s taxonomic position, Retallack notes, has long been an unsolved puzzle. “Biological affinities of Dickinsonia remain problematic,” he writes, since it has been “variously considered a polychaete, turbellarian or annelid worm, jellyfish, polyp, xenophyophoran protist, lichen or mushroom.”9
Similar disputes have characterized attempts to classify Spriggina. In 1976, Martin Glaessner, the first paleontologist to study the Ediacaran in detail, described Spriggina as a possible annelid polychaete worm based largely upon its segmented body. Nevertheless, Simon Conway Morris later rejected that hypothesis because Spriggina shows no evidence of the distinguishing “chaetes,” leg-like bristled protrusions that polychaete worms possess. Glaessner himself later repudiated his original hypothesis that Spriggina was ancestral to polychaetes, noting that Spriggina “cannot be considered as a primitive polychaete, having none of the possible ancestral characters indicated … by specialists on the systematics and evolution of this group.”10
In 1981, paleontologist Sven Jorgen Birket-Smith produced a reconstruction of a Spriggina fossil showing that it possessed a head and legs similar to those of trilobites, though examinations of subsequent Spriggina specimens have shown no evidence of it possessing limbs of any kind.11 In 1984, Glaessner weigh
ed in on this discussion as well. He argued that “Spriggina shows no specific characters of the arthropods, particularly of the trilobites.”12 He also noted that the body segmentation of Spriggina, and “its known appendages are at the level of polychaete annelids”13 (although, as noted, by this time he had rejected Spriggina as a possible polychaete ancestor). Instead, he proposed that Spriggina represented a side branch on the animal tree of life—one that resulted, “metaphorically” perhaps, in “an unsuccessful attempt to make an arthropod.”
In a presentation to the Geological Society of America in 2003, geologist Mark McMenamin revived the idea that Spriggina might represent a trilobite ancestor. He argued that several features present in Spriggina fossils are comparable to those in trilobites such as “the presence of genal spines” and an effaced head or “cephalic region.”14 Nevertheless, many Ediacaran experts, including McMenamin, have also noted that Spriggina specimens show no evidence of eyes, limbs, mouths, or anuses, most of which are known from fossil trilobites.15 Other paleontologists remain skeptical about whether Spriggina does in fact exhibit genal spines, noting that good specimens seem to show relatively smooth edges with no protruding spines.16 In addition, analysis of the best specimens of Spriggina shows that it does not exhibit bilateral symmetry, undermining earlier attempts to classify it as a bilaterian animal, and by implication an arthropod.17 Instead, Spriggina exhibits something called “glide symmetry” in which the body segments on either side of its midline are offset rather than aligned.18 As geologist Loren Babcock of Ohio State University notes, “The zipper-like body plans of some Ediacaran (Proterozoic) animals such as Dickinsonia and Spriggina involve right and left halves that are not perfect mirror images of each other.”19 The lack of such symmetry, a distinctive feature of all bilaterian animals, and the absence in Spriggina specimens of many other distinguishing features of trilobites, has left the classification of this enigmatic organism uncertain.
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