Wonderful Life: The Burgess Shale and the Nature of History (7 page)

I write this book to suggest a third alternative, off the line. I believe that the reconstructed Burgess fauna, interpreted by the theme of replaying life’s tape, offers powerful support for this different view of life: any replay of the tape would lead evolution down a pathway radically different from the road actually taken. But the consequent differences in outcome do not imply that evolution is senseless, and without meaningful pattern; the divergent route of the replay would be just as interpretable, just as explainable
after
the fact, as the actual road. But the diversity of possible itineraries does demonstrate that eventual results cannot be predicted at the outset. Each step proceeds for cause, but no finale can be specified at the start, and none would ever occur a second time in the same way, because any pathway proceeds through thousands of improbable stages. Alter any early event, ever so slightly and without apparent importance at the time, and evolution cascades into a radically different channel.

This third alternative represents no more nor less than the essence of history. Its name is contingency—and contingency is a thing unto itself, not the titration of determinism by randomness. Science has been slow to admit the different explanatory world of history into its domain—and our interpretations have been impoverished by this omission. Science has also tended to denigrate history, when forced to a confrontation, by regarding any invocation of contingency as less elegant or less meaningful than explanations based directly on timeless “laws of nature.”

This book is about the nature of history and the overwhelming improbability of human evolution under themes of contingency and the metaphor of replaying life’s tape. It focuses upon the new interpretation of the Burgess Shale as our finest illustration of what contingency implies in our quest to understand the evolution of life.

I concentrate upon details of the Burgess Shale because I don’t believe that important concepts should be discussed tendentiously in the abstract (much as I have disobeyed the rule in this opening chapter!). People, as curious primates, dote on concrete objects that can be seen and fondled. God dwells among the details, not in the realm of pure generality. We must tackle and grasp the larger, encompassing themes of our universe, but we make our best approach through small curiosities that rivet our attention—all those pretty pebbles on the shoreline of knowledge. For the ocean of truth washes over the pebbles with every wave, and they rattle and clink with the most wondrous din.

We can argue about abstract ideas forever. We can posture and feint. We can “prove” to the satisfaction of one generation, only to become the laughingstock of a later century (or, worse still, to be utterly forgotten). We may even validate an idea by grafting it permanently upon an object of nature—thus participating in the legitimate sense of a great human adventure called “progress in scientific thought.”

But the animals of the Burgess Shale are somehow even more satisfying in their adamantine factuality. We will argue forever about the meaning of life, but
Opabinia
either did or did not have five eyes—and we can know for certain one way or the other. The animals of the Burgess Shale are also the world’s most important fossils, in part because they have revised our view of life, but also because they are objects of such exquisite beauty. Their loveliness lies as much in the breadth of ideas that they embody, and in the magnitude of our struggle to interpret their anatomy, as in their elegance of form and preservation.

The animals of the Burgess Shale are holy objects—in the unconventional sense that this word conveys in some cultures. We do not place them on pedestals and worship from afar. We climb mountains and dynamite hillsides to find them. We quarry them, split them, carve them, draw them, and dissect them, struggling to wrest their secrets. We vilify and curse them for their damnable intransigence. They are grubby little creatures of a sea floor 530 million years old, but we greet them with awe because they are the Old Ones, and they are trying to tell us something.

Soured, perhaps, by memories of the multiplication tables, college students hate the annual ritual of memorizing the geological time scale in introductory courses on the history of life. We professors insist, claiming this venerable sequence as our alphabet. The entries are cumbersome—Cambrian, Ordovician, Silurian—and refer to such arcana as Roman names for Wales and threefold divisions of strata in Germany. We use little tricks and enticements to encourage compliance. For years, I held a mnemonics contest for the best entry to replace the traditional and insipid “Campbell’s ordinary soup does make Peter pale …” or the underground salacious versions that I would blush to record, even here. During political upheavals of the early seventies, my winner (for epochs of the Tertiary, see figure 2.1) read: “Proletarian efforts off many pig police. Right on!” The all-time champion reviewed a porno movie called
Cheap Meat
—with perfect rhyme and scansion and only one necessary neologism, easily interpreted, at the end of the third line. This entry proceeds in unconventional order, from latest to earliest, and lists all the eras first, then all the periods:

The winner also provided an epilogue, for the epochs of the Cenozoic era:

When such blandishments fail, I always say, try an honest intellectual argument: if these names were arbitrary divisions in a smooth continuum of events unfolding through time, I would have some sympathy for the opposition—for then we might take the history of modern multicellular life, about 600 million years, and divide this time into even and arbitrary units easily remembered as 1–12 or A–L, at 50 million years per unit.

But the earth scorns our simplifications, and becomes much more interesting in its derision. The history of life is not a continuum of development, but a record punctuated by brief, sometimes geologically instantaneous, episodes of mass extinction and subsequent diversification. The geological time scale maps this history, for fossils provide our chief criterion in fixing the temporal order of rocks. The divisions of the time scale are set at these major punctuations because extinctions and rapid diversifications leave such clear signatures in the fossil record. Hence, the time scale is not a devil’s ploy for torturing students, but a chronicle of key moments in life’s history. By memorizing those infernal names, you learn the major episodes of earthly time. I make no apologies for the central importance of such knowledge.

The geological time scale (figure 2.1) is divided hierarchically into eras, periods, and epochs. The boundaries of the largest divisions—the eras—mark the greatest events. Of the three era boundaries, two designate the most celebrated of mass extinctions. The late Cretaceous mass extinction, some 65 million years ago, sets the boundary between Mesozoic and Cenozoic eras. Although not the largest of “great dyings,” this event surpasses all others in fame, for dinosaurs perished in its wake, and the evolution of large mammals (including, much later, ourselves) became possible as a result. The second boundary, between the Paleozoic and Mesozoic eras (225 million years ago), records the granddaddy of all extinctions—the late Permian event that irrevocably set the pattern of all later history by extirpating up to 96 percent of marine species.

The third and oldest boundary, between Precambrian times and the Paleozoic era (about 570 million years ago), marks a different and more puzzling kind of event. A mass extinction may have occurred at or near this boundary, but the inception of the Paleozoic era denotes a concentrated episode of diversification—the “Cambrian explosion,” or first appearance of multicellular animals with hard parts in the fossil record. The importance of the Burgess Shale rests upon its relationship to this pivotal moment in the history of life. The Burgess fauna does not lie within the explosion itself, but marks a time soon afterward, about 530 million years ago, before the relentless motor of extinction had done much work, and when the full panoply of results therefore stood on display. As the only major soft-bodied fauna from this primordial time, the Burgess Shale provides our sole vista upon the inception of modern life in all its fullness.

2.1. The geological time scale.

The Cambrian explosion is a tolerably ancient event, but the earth is 4.5 billion years old, so multicellular life of modern design occupies little more than 10 percent of earthly time. This chronology poses the two classic puzzles of the Cambrian explosion—enigmas that obsessed Darwin (1859, pp. 306–10) and remain central riddles of life’s history: (1) Why did multicellular life appear so late? (2) And why do these anatomically complex creatures have no direct, simpler precursors in the fossil record of Precambrian times?

These questions are difficult enough now, in the context of a rich record of Precambrian life, all discovered since the 1950s. But when Charles Doolittle Walcott found the Burgess Shale in 1909, they seemed well-nigh intractable. In Walcott’s time, the slate of Precambrian life was absolutely blank. Not a single well-documented fossil had been found from any time before the Cambrian explosion, and the earliest evidence of multicellular animals coincided with the earliest evidence of any life at all! From time to time, claims had been advanced—more than once by Walcott himself—for Precambrian animals, but none had withstood later scrutiny. These creatures of imagination had been founded upon hope, and were later exposed as ripple marks, inorganic precipitates, or genuine fossils of later epochs misdiagnosed as primordial.

This apparent absence of life during most of the earth’s history, and its subsequent appearance at full complexity, posed no problem for anti-evolutionists. Roderick Impey Murchison, the great geologist who first worked out the record of early life, simply viewed the Cambrian explosion as God’s moment of creation, and read the complexity of the first animals as a sign that God had invested appropriate care in his initial models. Murchison, writing five years before Darwin’s
Origin of Species
, explicitly identified the Cambrian explosion as a disproof of evolution (“transmutation” in his terms), while he extolled the compound eye of the first trilobites as a marvel of exquisite design:

Darwin, honest as always in exposing the difficulties of his theory, placed the Cambrian explosion at the pinnacle of his distress, and devoted an entire section to this subject in the
Origin of Species
. Darwin acknowledged the anti-evolutionary interpretation of many important geologists: “Several of the most eminent geologists, with Sir. R. Murchison at their head, are convinced that we see in the organic remains of the lowest Silurian
*
stratum the dawn of life on this planet” (1859, p. 307). Darwin recognized that his theory required a rich Precambrian record of precursors for the first complex animals:

Darwin invoked his standard argument to resolve this uncomfortable problem: the fossil record is so imperfect that we do not have evidence for most events of life’s history. But even Darwin acknowledged that his favorite ploy was wearing a bit thin in this case. His argument could easily account for a missing stage in a single lineage, but could the agencies of imperfection really obliterate absolutely all evidence for positively every creature during most of life’s history? Darwin admitted: “The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained” (1859, p. 308).

Darwin has been vindicated by a rich Precambrian record, all discovered in the past thirty years. Yet the peculiar character of this evidence has not matched Darwin’s prediction of a continuous rise in complexity toward Cambrian life, and the problem of the Cambrian explosion has remained as stubborn as ever—if not more so, since our confusion now rests on knowledge, rather than ignorance, about the nature of Precambrian life.