We live, as our humorists proclaim, in a world of good news and bad news. The good news is that we can specify an experiment to decide between the conventional and the radical interpretations of extinction, thereby settling the most important question we can ask about the history of life. The bad news is that we can’t possibly perform the experiment.
I call this experiment “replaying life’s tape.” You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past—say, to the seas of the Burgess Shale. Then let the tape run again and see if the repetition looks at all like the original. If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur. But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life? What could we then say about the predictability of self-conscious intelligence? or of mammals? or of vertebrates? or of life on land? or simply of multicellular persistence for 600 million difficult years?
* * *
THE MEANINGS OF DIVERSITY AND DISPARITY
I must introduce at this point an important distinction that should allay a classic source of confusion. Biologists use the vernacular term diversity in several different technical senses. They may talk about “diversity” as number of distinct species in a group: among mammals, rodent diversity is high, more than 1,500 separate species; horse diversity is low, since zebras, donkeys, and true horses come in fewer than ten species. But biologists also speak of “diversity” as difference in body plans. Three blind mice of differing species do not make a diverse fauna, but an elephant, a tree, and an ant do—even though each assemblage contains just three species.
The revision of the Burgess Shale rests upon its diversity in this second sense of disparity in anatomical plans. Measured as number of species, Burgess diversity is not high. This fact embodies a central paradox of early life: How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species?—for the two are correlated, more or less in lockstep, by the iconography of the cone (see figure 1.16).
When I speak of decimation, I refer to reduction in the number of anatomical designs for life, not numbers of species. Most paleontologists agree that the simple count of species has augmented through time (Sepkoski et al., 1981)—and this increase of species must therefore have occurred within a reduced number of body plans.
Most people do not fully appreciate the stereotyped character of current life. We learn lists of odd phyla in high school, until kinorhynch, priapulid, gnathostomulid, and pogonophoran roll off the tongue (at least until the examination ends). Focusing on a few oddballs, we forget how unbalanced life can be. Nearly 80 percent of all described animal species are arthropods (mostly insects). On the sea floor, once you enumerate polychaete worms, sea urchins, crabs, and snails, there aren’t that many coelomate invertebrates left. Stereotypy, or the cramming of most species into a few anatomical plans, is a cardinal feature of modern life—and its greatest difference from the world of Burgess times.
Several of my colleagues (Jaanusson, 1981; Runnegar, 1987) have suggested that we eliminate the confusion about diversity by restricting this vernacular term to the first sense—number of species. The second sense—difference in body plans—should then be called disparity. Using this terminology, we may acknowledge a central and surprising fact of life’s history—marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs.
* * *
We can now appreciate the central importance of the Burgess revision and its iconography of decimation. With the ladder or the cone, the issue of life’s tape does not arise. The ladder has but one bottom rung, and one direction. Replay the tape forever, and Eohippus will always gallop into the sunrise, bearing its ever larger body on fewer toes. Similarly, the cone has a narrow neck and a restricted range of upward movement. Rewind the tape back into the neck of time, and you will always obtain the same prototypes, constrained to rise in the same general direction.
But if a radical decimation of a much greater range of initial possibilities determined the pattern of later life, including the chance of our own origin, then consider the alternatives. Suppose that ten of a hundred designs will survive and diversify. If the ten survivors are predictable by superiority of anatomy (interpretation 1), then they will win each time—and Burgess eliminations do not challenge our comforting view of life. But if the ten survivors are protégés of Lady Luck or fortunate beneficiaries of odd historical contingencies (interpretation 2), then each replay of the tape will yield a different set of survivors and a radically different history. And if you recall from high-school algebra how to calculate permutations and combinations, you will realize that the total number of combinations for 10 items from a pool of 100 yields more than 17 trillion potential outcomes. I am willing to grant that some groups may have enjoyed an edge (though we have no idea how to identify or define them), but I suspect that the second interpretation grasps a central truth about evolution. The Burgess Shale, in making this second interpretation intelligible by the hypothetical experiment of the tape, promotes a radical view of evolutionary pathways and predictability.
Rejection of ladder and cone does not throw us into the arms of a supposed opposite—pure chance in the sense of coin tossing or of God playing dice with the universe. Just as the ladder and the cone are limiting iconographies for life’s history, so too does the very idea of dichotomy grievously restrict our thinking. Dichotomy has its own unfortunate iconography—a single line embracing all possible opinions, with the two ends representing polar opposites—in this case, determinism and randomness.
An old tradition, dating at least to Aristotle, advises the prudent person to stake out a position comfortably toward the middle of the line—the aurea mediocritas (“golden mean”). But in this case the middle of the line has not been so happy a place, and the game of dichotomy has seriously hampered our thinking about the history of life. We may understand that the older determinism of predictable progress cannot strictly apply, but we think that our only alternative lies with the despair of pure randomness. So we are driven back toward the old view, and finish, with discomfort, at some ill-defined confusion in between.
I strongly reject any conceptual scheme that places our options on a line, and holds that the only alternative to a pair of extreme positions lies somewhere between them. More fruitful perspectives often require that we step off the line to a site outside the dichotomy.
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.
CHAPTER II
A Background for the Burgess Shale
LIFE BEFORE THE BURGESS: THE CAMBRIAN EXPLOSION AND THE ORIGIN OF ANIMALS
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:
Cheap Meat performs passably,
Quenching the celibate’s jejune thirst,
Portraiture, presented massably,
Drowning sorrow, oneness cursed.
The winner also provided an epilogue, for the epochs of the Cenozoic era:
Rare pornography, purchased meekly
O Erogeny, Paleobscene.*
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 scrut
iny. 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:
The earliest signs of living things, announcing as they do a high complexity of organization, entirely exclude the hypothesis of a transmutation from lower to higher grades of being. The first fiat of Creation which went forth, doubtlessly ensured the perfect adaptation of animals to the surrounding media; and thus, whilst the geologist recognizes a beginning, he can see in the innumerable facets of the eye of the earliest crustacean, the same evidences of Omniscience as in the completion of the vertebrate form (1854, p. 459).
Wonderful Life: The Burgess Shale and the Nature of History Page 4
