Darwin's Doubt

by Stephen C. Meyer

  This book addresses Darwin’s most significant doubt and what has become of it. It examines an event during a remote period of geological history in which numerous animal forms appear to have arisen suddenly and without evolutionary precursors in the fossil record, a mysterious event commonly referred to as the “Cambrian explosion.” As he acknowledged in the Origin, Darwin viewed this event as a troubling anomaly—one that he hoped future fossil discoveries would eventually eliminate.

  The book is divided into three main parts. Part One, “The Mystery of the Missing Fossils,” describes the problem that first generated Darwin’s doubt—the missing ancestors of the Cambrian animals in the earlier Precambrian fossil record—and then tells the story of the successive, but unsuccessful, attempts that biologists and paleontologists have made to resolve that mystery.

  Part Two, “How to Build an Animal,” explains why the discovery of the importance of information to living systems has made the mystery of the Cambrian explosion more acute. Biologists now know that the Cambrian explosion not only represents an explosion of new animal form and structure but also an explosion of information—that it was, indeed, one of the most significant “information revolutions” in the history of life. Part Two examines the problem of explaining how the unguided mechanism of natural selection and random mutations could have produced the biological information necessary to build the Cambrian animal forms. This group of chapters explains why so many leading biologists now doubt the creative power of the neo-Darwinian mechanism and it presents four rigorous critiques of the mechanism based on recent biological research.

  Part Three, “After Darwin, What?” evaluates more current evolutionary theories to see if any of them explain the origin of form and information more satisfactorily than standard neo-Darwinism does. Part Three also presents and assesses the theory of intelligent design as a possible solution to the Cambrian mystery. A concluding chapter discusses the implications of the debate about design in biology for the larger philosophical questions that animate human existence. As the story of the book unfolds, it will become apparent that a seemingly isolated anomaly that Darwin acknowledged almost in passing has grown to become illustrative of a fundamental problem for all of evolutionary biology: the problem of the origin of biological form and information.

  To see where that problem came from and why it has generated a crisis in evolutionary biology, we need to begin at the beginning: with Darwin’s own doubt, with the fossil evidence that elicited it, and with a clash between a pair of celebrated Victorian naturalists—the famed Harvard paleontologist Louis Agassiz and Charles Darwin himself.

  Part One

  The Mystery of the Missing Fossils

  1

  Darwin’s Nemesis

  When Charles Darwin finished his famous book, he thought that he had explained every clue but one.

  By anyone’s measure, On the Origin of Species was a singular achievement. Like a great Gothic cathedral, the ambitious work integrated many disparate elements into a grand synthesis, explaining phenomena in fields as diverse as comparative anatomy, paleontology, embryology, and biogeography. At the same time, it was impressive for its simplicity. Darwin’s Origin explained many classes of biological evidence with just two central organizing ideas. The twin pillars of his theory were the ideas of universal common ancestry and natural selection.

  The first of these pillars, universal common ancestry, represented Darwin’s theory of the history of life. It asserted that all forms of life have ultimately descended from a single common ancestor somewhere in the distant past. In a famous passage at the end of the Origin, Darwin argued that “all the organic beings which have ever lived on this earth have descended from some one primordial form.”1 Darwin thought that this primordial form gradually developed into new forms of life, which in turn gradually developed into other forms of life, eventually producing, after many millions of generations, all the complex life we see in the present.

  Biology textbooks today usually depict this idea just as Darwin did, with a great branching tree. The trunk of Darwin’s tree of life represents the first primordial organism. The limbs and branches of the tree represent the many new forms of life that developed from it (see Fig. 1.1). The vertical axis on which the tree is plotted represents the arrow of time. The horizontal axis represents changes in biological form, or what biologists call “morphological distance.”

  FIGURE 1.1

  Darwin’s evolutionary tree of life, as depicted by the nineteenth-century German evolutionary biologist Ernst Haeckel.

  Biologists often call Darwin’s theory of the history of life “universal common descent” to indicate that every organism on earth arose from a single common ancestor by a process of “descent with modification.” Darwin argued that this idea best explained a variety of biological evidences: the succession of fossil forms, the geographical distribution of various species (such as Galápagos finches), and the anatomical and embryological similarities among otherwise highly distinct organisms.

  The second pillar of Darwin’s theory affirmed the creative power of a process he called natural selection, a process that acted on random variations in the traits or features of organisms and their offspring.2 Whereas the theory of universal common descent postulated a pattern (the branching tree) to represent the history of life, Darwin’s idea of natural selection referred to a process that he said could generate the change implied by his branching tree of life.

  Darwin formulated the idea of natural selection by analogy to a well-known process, that of “artificial selection” or “selective breeding.” Anyone in the nineteenth century familiar with the breeding of domestic animals—dogs, horses, sheep, or pigeons, for example—knew that human breeders could alter the features of domestic stock by allowing only animals with certain traits to breed. A sheepherder from the north of Scotland might breed for a woollier sheep to enhance its chances of survival in a cold northern climate (or to harvest more wool). To do so, he would choose only the woolliest males and woolliest ewes to breed. If generation after generation he continued to select and breed only the woolliest sheep among the resulting offspring, he would eventually produce a woollier breed of sheep. In such cases, “the key is man’s power of accumulative selection,” wrote Darwin. “Nature gives successive variations; man adds them up in certain directions useful to him.”3

  Darwin noted that pigeons have been coaxed into a dizzying variety of breeds: the carrier, with its elongated eyelids and a “wide gape of mouth”; the “short-faced tumbler,” with its “beak in outline almost like that of a finch”; the common tumbler, with its penchant for flying in close formation and “tumbling in the air head over heels”; and, perhaps strangest of all, the pouter, with its elongated legs, wings, and body overshadowed by its “enormously developed crop, which it glories in inflating” for its astonished patrons.4

  Of course, pigeon breeders achieved these startling metamorphoses by carefully sifting and selecting. But, as Darwin pointed out, nature also has a means of sifting: defective creatures are less likely to survive and reproduce, while those offspring with beneficial variations are more likely to survive, reproduce, and pass on their advantages to future generations. In the Origin, Darwin argued that this process, natural selection acting on random variations, could alter the features of organisms just as intelligent selection by human breeders can. Nature itself could play the role of the breeder.

  Consider once more our flock of sheep. Imagine that instead of a human selecting the woolliest males and ewes to breed, a series of very cold winters ensures that all but the very woolliest sheep in a population die. Now again only very woolly sheep will remain to breed. If the cold winters continue over several generations, will the result not be the same as before? Won’t the population of sheep eventually become discernibly woollier?

  This was Darwin’s great insight. Nature—in the form of environmental changes or other factors—could have the same effect on a population of organisms as the intentional
decisions of an intelligent agent. Nature would favor the preservation of certain features over others—specifically, those that conferred a functional or survival advantage upon the organisms possessing them—causing the features of the population to change. And the resulting change will have been produced not by an intelligent breeder choosing a desirable trait or variation—not by “artificial selection”—but by a wholly natural process. What’s more, Darwin concluded that this process of natural selection acting on randomly arising variations had been “the chief agent of change” in generating the great branching tree of life in all its variety.

  On the Origin of Species seized the attention of the scientific community like a thunderclap. Darwin’s analogy to artificial selection was powerful, his proposed mechanism of natural selection and random variation easily grasped, and his skill in dispensing with potential objections unrivalled. Moreover, the explanatory scope of his argument for universal common descent constituted something of a tour de force. By the close of the Origin, it seemed to many that Darwin had dispensed with every conceivable objection to his theory but one.

  The Anomaly: Darwin’s Doubt

  Despite the scope of his synthesis, there was one set of facts that troubled Darwin—something he conceded his theory couldn’t adequately explain, at least at present. Darwin was puzzled by a pattern in the fossil record that seemed to document the geologically sudden appearance of animal life in a remote period of geologic history, a period that at first was commonly called the Silurian, but later came to be known as the Cambrian.

  During this geological period, many new and anatomically sophisticated creatures appeared suddenly in the sedimentary layers of the geologic column without any evidence of simpler ancestral forms in the earlier layers below, in an event that paleontologists today call the Cambrian explosion. Darwin frankly described his concerns about this conundrum in the Origin: “The difficulty of understanding the absence of vast piles of fossiliferous strata, which on my theory were no doubt somewhere accumulated before the Silurian [i.e., Cambrian] epoch, is very great,” he wrote. “I allude to the manner in which numbers of species of the same group suddenly appear in the lowest known fossiliferous rocks.”5 The sudden appearance of animals so early in the fossil record did not easily accord with Darwin’s new theory of gradual evolutionary change, and there was one scientist who would not let him forget it.

  The Antagonist

  Swiss-born paleontologist Louis Agassiz, of Harvard University, was one of the best-trained scientists of his age, and he knew the fossil record better than any man alive. Hoping to enlist Agassiz as an ally, Darwin sent him a copy of On the Origin of Species and asked him to consider the argument with an open mind (see Fig. 1.2). One can almost see the great, aging naturalist receiving the unremarkable package from the postman, unwrapping the small green volume that had stirred such a tempest on both sides of the Atlantic. Perhaps he retired to his study the better to concentrate, scrutinizing the book’s prepossessing title, recalling what he had already heard about the work. He read the book with deep interest, making notes in the margin as he moved through it, but in the end his verdict would disappoint its author. Agassiz concluded that the fossil record, particularly the record of the explosion of Cambrian animal life, posed an insuperable difficulty for Darwin’s theory.

  FIGURE 1.2

  Figure 1.2a (left): Louis Agassiz. Figure 1.2b (right): Charles Darwin.

  The Two-Pronged Challenge

  To see why, consider brachiopods and trilobites, two of the best-documented creatures in the Cambrian fossil record by 1859. The brachiopod (see Fig. 1.3), with its two shells, looks like a clam or an oyster, but is very different inside. As shown in the accompanying figure, it possesses a gonad, mantle, mantle cavity, anterior body wall, body cavity, gut, and lophophore, the last of which is a feeding organ like a ring of tentacles, usually in the shape of a coil or horseshoe, with a mouth inside the ring of tentacles, and an anus outside. The brachiopod exhibits a highly complex overall body plan, with many individually complex and functionally integrated anatomical systems and parts. Its tentacles, for instance, are covered by cilia precisely arranged to generate and direct a current of water toward the mouth.6

  FIGURE 1.3

  Figure 1.3a (top): Brachiopod internal anatomy. Figure 1.3b (bottom, left): Brachiopod fossil showing remains of internal structure. Courtesy Paul Chien. Figure 1.3c (bottom, right): Fossil showing exterior structure of brachiopod shell. Courtesy Corbis.

  FIGURE 1.4

  Figure 1.4a (top): Trilobite anatomy. Figure 1.4b (bottom): Trilobite fossil of the species Kuanyangia pustulosa. Courtesy Illustra Media.

  Even more sophisticated was the trilobite (see Fig. 1.4), with its three longitudinal lobes across its head (a raised middle lobe and a flatter pleural lobe to either side) and a body divided into three parts—head, chest, and tail, the former two consisting of as many as thirty segments. It had a pair of legs for every pleural groove and another three pairs for the head. Most dramatic of all were the compound eyes found on even some of the very early trilobites—eyes that afforded these not so primitive animals a 360-degree field of vision.7

  The abrupt appearance of such complex anatomical designs presented a challenge to each of the two main parts of Darwin’s theory of evolution.

  The Cambrian Explosion and the Action of Natural Selection

  The Cambrian fossil evidence represented a significant challenge to Darwin’s claim that natural selection had the capacity to produce novel forms of life. As Darwin described it, the ability of natural selection to produce significant biological change depends upon the presence of three distinct elements: (1) randomly arising variations, (2) the heritability of those variations, and (3) a competition for survival, resulting in differences in reproductive success among competing organisms.

  According to Darwin, variations in traits arise randomly. Some variations (such as thicker fleece) might confer advantages in the competition for survival in particular environmental conditions. Those variations that are heritable and that impart functional or survival advantage will be preserved in the next generation. As nature “selects” these successful variations, the features of a population change.

  Darwin conceded that the beneficial variations responsible for permanent change in species are both rare and necessarily modest. Major variations in forms, what later evolutionary biologists would term “macromutations,” inevitably produce deformity and death. Only minor variations meet the test of viability and heritability.

  It followed that, over human timescales, the benefits of this evolutionary mechanism would be difficult or impossible to spot. But given enough time, favorable variations would gradually accumulate and give rise to new species and, given more time, even fundamentally new groups of organisms and body designs. If artificial selection could conjure so many strange breeds from a wild strain in a few centuries, Darwin argued, imagine what natural selection could achieve over many millions of years. Even the origin of complex structures such as the mammalian eye—which seemed at first to present a significant challenge to his theory—could be explained if one postulated the existence of an initially simpler structure (such as a light sensitive spot) that could be gradually modified over long periods of time.

  And that was the rub. Darwin’s mechanism of natural selection and random variation necessarily required a lot of time to generate wholly novel organisms, creating a dilemma that Agassiz was keen to expose.

  In an 1874 Atlantic Monthly essay titled “Evolution and the Permanence of Type,” Agassiz explained his reasons for doubting the creative power of natural selection. Small-scale variations, he argued, had never produced a “specific difference” (i.e., a difference in species). Meanwhile, large-scale variations, whether achieved gradually or suddenly, inevitably resulted in sterility or death. As he put it, “It is a matter of fact that extreme variations finally degenerate or become sterile; like monstrosities they die out.”8

  Darwin himself insisted
that the process of evolutionary change he envisioned must occur very gradually for the same reason. Thus, Darwin realized that building, for instance, a trilobite from single-celled organisms by natural selection operating on small, step-by-step variations would require countless transitional forms and failed biological experiments over vast stretches of geologic time. As University of Washington paleontologist Peter Ward would later explain, Darwin had very specific expectations for what paleontologists would find below the lowest known strata of animal fossils—in particular, “intervening strata showing fossils of increasing complexity until finally trilobites appeared.”9 As Darwin noted, “If my theory be true, it is indisputable that before the lowest Silurian [Cambrian] stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.”10

  The mechanism of natural selection necessarily had to work gradually on small incremental variations. And, indeed, the kinds of variations that Darwin actually observed and described in developing his analogy between natural and artificial selection were in every case minor. Only by selecting and accumulating minor variations over many generations were breeders able to produce the striking changes in the features of a breed, changes that were, nevertheless, extraordinarily modest compared to the radical differences in form between, say, Precambrian and Cambrian forms of life. At the end of the day, as Agassiz hastened to note, the pigeons Darwin cited in support of the creative power of artificial and, by analogy, natural selection were still pigeons. More significant changes to the form and anatomical structure of organisms would, by the logic of Darwin’s mechanism, require untold millions of years, precisely what seemed unavailable in the case of the Cambrian explosion.