Eight Little Piggies (8 page)

The second major approach—historical contingency in my favored terminology (see my recent book,
Wonderful Life
)—argues that five was not meant to be, but just happens to be. Other configurations would have worked and might have evolved, but they didn’t—and five works well enough. The obvious supports for this alternative view lie scattered throughout this essay. If five is so good, why do so many species devise such curious and devious means to produce six (prepollux or converted wrist bone)? If five is so predictable, why does one of two lineages grow but four? (I should say right up front that neither of these two positions—adaptation or contingency—really addresses the greatest puzzle of all: the recalcitrant stability of five once it evolves. I suspect that this is a question for embryologists and geneticists; phylogenetic history may offer little in the way of clues. Why should five, once attained by whatever route and for whatever reason, be so stubbornly intractable as an upper limit thereafter, so that any lineage, again evolving six or more, must do so by a different path? The inquiry could not be more important, for this issue of digits is a microcosm for the grandest question of all about the history of animal life: Why, following a burst of anatomical exploration in the Cambrian explosion some 550 million years ago, have anatomies so stabilized that not a single new phylum [major body plan] has ever evolved since?)

But the greatest boost to contingency lies in the discovery that prompted this essay in the first place—seven digits in
Ichthyostega
and eight in
Acanthostega
. If tetrapods had five at the beginning, and always retained five thereafter, then some predictability or inevitability could legitimately be maintained. (At the very least, no fuel would exist for an alternative proposal.) But if the first members of the lineage had six, seven, or eight toes, then alternative possibilities are legion, and an eventual five may be a happenstance, not a necessity.

Embryologist Jonathan Cooke, in a commentary written to accompany Coates and Clack’s paper, agrees with me that possible contingency of pentadactyly is the most interesting implication of the new discovery. But he makes a curious statement in his advocacy. Cooke writes:

I love the idea, but I decry the apology and abnegation implied by the designation of “philistine” for contingency. This unnecessary humility follows an unfortunate tradition of self-hate among scientists who deal with the complex, unrepeatable, and unpredictable events of history. We are trained to think that the “hard science” models of quantification, experimentation, and replication are inherently superior and exclusively canonical, so that any other set of techniques can only pale by comparison. But historical science proceeds by reconstructing a set of contingent events, explaining in retrospect what could not have been predicted beforehand. If the evidence be sufficient, the explanation can be as rigorous and confident as anything done in the realm of experimental science. In any case, this is the way the world works: No apologies needed.

Contingency is rich and fascinating; it embodies an exquisite tension between the power of individuals to modify history and the intelligible limits set by laws of nature. The details of individual and species’s lives are not mere frills, without power to shape the large-scale course of events, but particulars that can alter entire futures, profoundly and forever.

Consider the primary example from American history. Northern victory was not inevitable in the Civil War, for the South was not fighting a war of conquest (unwinnable given their inferiority in manpower and economic wealth), but a struggle to induce war weariness and to compel the North to recognize their boundaries. The Confederacy had almost succeeded in 1863. Their armies were deep into Pennsylvania; draft riots were about to break out in New York City; Massachusetts was arming the first regiment of free black volunteers—not from an abstract sense of racial justice but from an urgent need for more bodies. In this context, the crucial Battle of Gettysburg occurred in early July. Robert E. Lee made a fateful error in thinking that his guns had knocked out the Union battery, and he sent his men into the nightmare of Pickett’s Charge. Suppose we could rerun history and give Lee another chance. This time, armed with better intelligence perhaps, he does not blunder and prevails. On this replay, the South might win the war, and all subsequent American history becomes radically different. The actual outcome at Gettysburg is no minor frill in an inevitable unrolling of events, but a potential setting point of all later patterns.

Never apologize for an explanation that is “only” contingent and not ordained by invariant laws of nature, for contingent events have made our world and our lives. If you ever feel the slightest pull in that dubious direction, think of poor Heathcliff who would have been spared so much agony if only he had stayed a few more minutes to eavesdrop upon the conversation of Catherine and Nelly (yes, the book wouldn’t have been as good, but consider the poor man’s soul). Think of Bill Buckner who would never again let Mookie Wilson’s easy grounder go through his legs—if only he could have another chance. Think of the alternative descendants of
Ichthyostega
, with only four fingers on each hand. Think of arithmetic with base eight, the difficulty of playing triple fugues on the piano, and the conversion of this essay into an illegible Roman tombstone, for how could I separate words withoutathumbtopressthespacebaronthistypewriter.

WE ALL DREAM
about retirement projects that might recapture the lost pleasures of youth, or perfect what we had, perforce, abandoned when the practicalities of making a living and supporting a family intervened. Some day, in a rosy future after the millennium, I will take out my old stamp album or sit down at the piano and finally progress beyond the first of Bach’s two-part inventions and the Prelude in C Major from Book 1 of the Well-Tempered Clavier.

Charles Darwin, my hero and role model, achieved this exquisite pleasure, so I may yet have hope for emulation. His last book, published a year before his death, treated the apparently arcane, but vitally important subject of earthworms and their role in forming the topsoil of England. This wonderful and disarming book unites Darwin’s end, in the calmness of old age, secure in the knowledge of accomplishment, with his more tumultuous youth, sparked by the fires of unrealized ambition. For Darwin wrote the précis of his worm book in 1838, just two years after the
Beagle
docked—a brilliant five-page article, presenting the entire argument that would fill a book more than forty years later. Darwin concluded:

Odd juxtapositions always intrigue me. I do not grant them deep meaning, and firmly believe that they represent nothing more than coincidence. Nonetheless, we do take notice, if only because we must find patterns to tell stories. Darwin published his paper in the fifth volume of the second series of the
Transactions of the Geological Society of London
in 1838. I was reading this paper a few months ago, and couldn’t help turning the last page to note the subsequent article, a four-page “Note on the dislocation of the tail at a certain point observed in the skeleton of many Ichthyosauri,” written by Richard Owen.

Richard Owen, then a young man, became England’s greatest comparative anatomist and first director for the Natural History division of the British Museum when the collections finally escaped the shadows of the Elgin Marbles at Bloomsbury and won their own magnificent home in South Kensington (one of the world’s great Victorian buildings and an essential stop on any visit to London).

Owen and Darwin had a long and problematical relationship. Darwin originally courted Owen’s friendship and support. (Owen, at Darwin’s request, formally described for publication the fossil mammals that Darwin had collected on the
Beagle
. Darwin’s famous
Toxodon
, for example, was named, described, and illustrated by Owen.) But the relationship inevitably soured, in part because Owen’s vanity could not bear Darwin’s successes. Legend holds that Owen’s rejection of evolution prompted their final break, but such a falsehood only records our propensity for simplifying stories told in the heroic mode, thus making “bad guys” both nasty and stupid. Owen did reject natural selection, and with vigor, as an excessively materialistic theory depending too much on external environments and too little on laws of organic structure, but he embraced evolution as a guiding principle in natural history.

In any case, the juxtaposition of worm and ichthyosaur dates from 1838, an early period of their friendship. I couldn’t help noticing another link more interesting than mere spatial proximity. Darwin wrote, as quoted above, that his subject seemed trivial but really unleashed a cascade of implications leading to substantial importance. Owen then made the very same point, arguing that an apparently broken tail in an ichthyosaur might seem entirely devoid of interest, but that close study yielded generalities of more than passing concern. Since the conversion of detail to wide message, through links of tangential connection, forms the stock-in-trade of these essays, I could hardly avoid such a double invitation to discourse at greater length on the tail bend of ichthyosaurs.

Ichthyosaurs are a group of marine reptiles with bodies so fishlike in external form that they have become the standard textbook example of “convergence”—evolved similarity from two very different starting points as independent adaptive responses to a common environment and mode of life (wings of birds and bats, eyes of squids and fishes). Ichthyosaurs are not closely related to dinosaurs, though they arose at about the same time and became extinct before the great wipeout that ended the dinosaurs’ reign some 65 million years ago. (The god-awful spelling of their names, with its unpronounceable sequence
chth
, only records an orthographic convention in converting Greek letters to Roman. This four-consonant sequence represents two Greek letters,
chi
and
theta
, one transliterated
ch
, the other as
th
. Both belong to a five-letter Greek word for fish, and ichthyosaur means “fish lizard.” We meet the same orthographic problem in such words as ophthalmology. But never despair and remember that things could always be worse. What would you do if that four-letter sequence came right at the beginning of a word—as it does in a common barnacle with the most forbidding name of
Chthamalus
.)

In considering the convergence of ichthyosaur upon fish, we marvel most at the form and location of fins and paddles—the machinery of swimming and balancing. The fore and hind paddles are, perhaps, least remarkable, for ancestral structures are clearly present as front and back limbs of terrestrial forebears—and these can be modified, as whales and dolphins have done, to forms better suited for sculling than for walking. But the dorsal (back) and caudal (tail) fins are boggling in their precision of convergence with analogous structures in fishes. For the terrestrial ancestors of ichthyosaurs obviously possessed neither back nor tail fin, and ichthyosaurs therefore evolved these structures from scratch—yet they occupy the position, and maintain the form, that hydrodynamic engineers deem optimal for propulsion and balance.

The classic painting of an ichthyosaur by Charles R. Knight. Note the fish-like position of the fins.
Courtesy of Department of Library Services, American Museum of Natural History
.

Yet just as ichthyosaurs themselves developed these fishlike features in a graduated transition from terrestrial ancestors, so too did our understanding of their extensive convergence grow piece by piece. To be sure, the basic similarity with fishes had never been doubted. In fact, the first two published references, both in 1708, mistook ichthyosaur vertebrae for the backbone of a fish. Both the celebrated Swiss naturalist J. J. Scheuchzer, in his
Querelae Piscium
(Complaints of the Fishes) and the German J. J. Baier, in his work on fossils from the area of Nuremberg, presented figures of ichthyosaur vertebrae for a most interesting purpose: to maintain that fossils are true remains of creatures that once lived, and not some manifestation of a plastic force inherent in rocks and ordained to establish global order by eliciting parallel forms in the organic and inorganic realms (an idea that strikes us as absurd today, but that made lingering sense within a neo-Platonic ideology not yet fully dispersed by the causal theories of Newton and Descartes).

Both Scheuchzer and Baier argued that these “fish” fossils recorded the devastation of Noah’s flood. Scheuchzer’s work is written as a humorous conversation among fossil fishes annoyed at humans who do not recognize their organic nature and affinity with living relatives. As for Baier, I recently had the pleasure of purchasing a copy of his rare work, without the slightest expectation that I would soon, or ever, have any practical or immediate use for this beautiful book. What a pleasure, then, to read his two page discussion of “ichthyospondyli” (fish vertebrae), with its conclusion that we must view them “
pro piscibus vere petrificatis…pro universalis Diluvii reliquiis
”—as truly petrified fish, remains of the universal flood.

Better evidence, primarily from bones of the skull and paddles, revealed the reptilian nature of ichthyosaurs by the early nineteenth century, but strong convergence upon fishes remained the prevailing theme of most writing. Nonetheless, though skeletons revealed the streamlined body and fishlike paddles, two missing pieces conspired to prevent any full appreciation for the true (and awesome) extent of convergence—for the back and tail fins, as soft structures, had not been discovered. All the early reconstructions—by Buckland, Conybeare, de la Beche, Hawkins, and other worthies of early English geology—showed a slithering serpent without back or tail fins, not the reptilian embodiment of a swordfish. How, then, did the two key pieces fall into the piscine puzzle?

Richard Owen’s note of 1838 stands as the chief document in this resolution. Thanks largely to keen insight and uncanny field work from Mary Anning, and to support from the demented and eccentric Thomas Hawkins (whose monographs of 1834 and 1840 must rank as the craziest documents ever written in paleontology), many good skeletons of ichthyosaurs were collected in England during the early nineteenth century. Owen had noticed an apparent peculiarity in one fine specimen—a sharp downward bend in the sequence of rear vertebrae at about two-thirds the distance from the back flippers to the end of the tail. Owen gave little thought to this tailbend, reasoning that it only represented an anomaly (probably a postmortem artifact) of a single specimen. But when skeleton after skeleton showed a tailbend in the same position, Owen realized that he had stumbled upon a phenomenon worthy of explanation. Owen wrote:

Caricature of ichthyosaurs by Henry de la Beche, made in the early nineteenth century before the back and tail fins had been discovered.
Courtesy of Department of Library Services, American Museum of Natural History
.

Illustration of ichthyosaur tail bends taken from Richard Owen’s 1838 article.
Courtesy of Department of Library Services, American Museum of Natural History
.

Owen then drew the right conclusion for the wrong reason and correctly inferred the existence of a tail fin. He argued that the constant position of the tailbend must record an attachment of some structure at this point. He rightly conjectured that this organ must be a tail fin, and he even predicted its vertical position (as in fishes) rather than a horizontal orientation (as in whales). But he wrongly assumed that the bend must represent a dislocation (probably after death) of an originally straight vertebral column—perhaps because the tail fin bloats with gas as the animal begins to decay, thereby fracturing the vertebral column at the front border of the fin. Owen then added other conjectures, and wrote: