Hen’s Teeth and Horse’s Toes (6 page)

It is a clear, though lamentably common, error to assume that the current utility of a feature permits an inference about the reasons for its evolutionary origin. Current utility and historical origin are different subjects. Any feature, regardless of how or why it first evolved, becomes available for co-optation to other roles, often strikingly different. Complex features are bursting with potentialities; their conceivable use is not confined to their original function (I confess that I have used a credit card to force a door). And these evolutionary shifts in function can be as quirky and unpredictable as the potentials of complexity are vast. It happens all the time; it virtually defines the wondrous indefiniteness of evolution.

The balancing fins of fish became the propulsive limbs of terrestrial vertebrates, while the propulsive tail became an organ that often aids in balance. The bone that suspended an ancestral fish’s upper jaw to its cranium became the bone that transmits sound to the ears of reptiles. Two bones that articulated the jaws of that reptile then became the other two sound-transmitting bones of the mammalian middle ear. When we see how beautifully our hammer, anvil, and stirrup function in hearing, who would imagine that one bone once suspended jaw to cranium, while two others articulated the jaws. (By the way, before jaws even evolved, all these bones supported the gill arches of an ancestral jawless fish.) And a mode of sex determination that may first have aided a lonely female colonizer apparently became the basis of social systems rivaled only by our own in complexity.

As we probe deeper and further back, the unpredictabilities mount. I discussed the quirkiness of a functional shift toward support of sociality by a sexual system which probably evolved as an aid to colonization. But what about the larger reason for our imperfect and unpredictable world: structural limits imposed by features evolved for other reasons? Social systems, like those of ants and bees, might be of enormous advantage to hosts of other creatures. But they do not evolve largely because it is so difficult to get them started in diploid organisms (only termites have succeeded), while haplodiploid hymenopterans develop them again and again. And going one step further back (I promise to stop here), what about constraints on the evolution of haplodiploidy itself. Haplodiploidy might be a wonderful adaptation to a host of ecologies, but it cannot always be easily evolved.

Assuming that haplodiploids generally arise from diploids, what does it take to turn a haploid creature into a male? Under some systems of diploid sex determination, male haploids cannot easily evolve. A haploid human would not be male, for a single X chromosome induces the development of a sterile female. But other diploids have a so-called XX-XO system of sex determination, where females have two X chromosomes and males have a single X with no accompanying Y (but all other chromosomes in pairs). In such systems, a haploid organism might develop easily and directly into a male. (The XX-XO system is not a prerequisite for haplodiploidy, since more complex modifications can produce male haploids from other modes of diploid sex determination.)

In short, modes of sex determination limit haplodiploidy, haplodiploidy limits sociality, and sociality requires a quirky shift in the adaptive significance of haplodiploidy. What order can we find in evolution amidst such a crazy-quilt of limits to a sensibly perfect and predictable world?

Some might be tempted to read an almost mystical message into this theme—that evolution imposes an ineffable unknowability upon nature. I would strongly reject such an implication: knowledge and prediction are different phenomena. Others might try to read a sad or pessimistic message—that evolution isn’t a very advanced science, or isn’t even a science at all, if it can’t predict the course of an imperfect world. Again, I would reject any such reading of my words about constraint and quirky functional shift.

The problem lies with our simplistic and stereotyped view of science as a monolithic phenomenon based on regularity, repetition, and ability to predict the future. Sciences that deal with objects less complex and less historically bound than life may follow this formula. Hydrogen and oxygen, mixed in a certain way, make water today, made water billions of years ago, and presumably will make water for a long time to come. Same water, same chemical composition. No indication of time, no constraints imposed by a history of previous change.

Organisms, on the other hand, are directed and limited by their past. They must remain imperfect in their form and function, and to that extent unpredictable since they are not optimal machines. We cannot know their future with certainty, if only because a myriad of quirky functional shifts lie within the capacity of any feature, however well adapted to a present role.

The science of complex historical objects is a different, not a lesser, enterprise. It seeks to explain the past, not predict the future. It searches for principles and regularities underlying the uniqueness of each species and interaction, while treasuring that irreducible uniqueness and describing all its glory. Notions of science must bend (and expand) to accommodate life. The art of the soluble, Peter Medawar’s definition of science, must not become shortsighted, for life is long.

MODERN GEOLOGY BEGAN
, or so the usual story goes, with the publication of a book so oddly named that it almost surpasses the peculiarity of the title later assumed by its author, Nicolaus Steno, a Dane by birth and a Catholic convert who became Titular Bishop of Titiopolis
(in partibus infidelium)
in 1677. (Titular bishops “preside” over areas in pagan hands and therefore unavailable for actual residence—in the realm of infidels, as the Latin subtitle proclaims. The old bishopric of Titiopolis is now part of Turkey.) As his real job, dangerous enough in Protestant lands, Steno ministered to the scattered Catholic remnants of northern Germany, Norway, and Denmark.

The book, published in 1669, bears a title considered “almost unintelligible” by its chief translator from the original Latin. It is called
De solido intra solidum naturaliter contento dissertationis prodromus
, or
Prodromus to a dissertation on a solid body naturally contained within a solid
. A prodromus is an introductory discourse, but Steno never wrote the promised dissertation because his religious interests, following his conversion in 1667 and his ordination in 1675, led him to abandon his distinguished career as a medical anatomist and, by fortuitous introduction at the very end of his scientific work, a geologist.

Why a solid within a solid? And what can such a cryptic phrase have to do with the origin of modern geology? Posing a problem in a startling and novel way is the virtual prerequisite of great science. Steno’s genius lay in recognizing that a solution to the general problem of how solid bodies get inside other solids might provide a criterion for unraveling the earth’s structure and history. But Steno did not formulate his problem by rational deduction from his armchair. As so often happens in a human world, he drifted toward it after an accidental beginning.

Like many anatomists, Steno became interested in the resemblances of humans with other animals. He decided to dissect sharks and made some important discoveries. He demonstrated, for example, that the tight coils of the spiral intestine yielded the same total length (within a more confined space) as the meandering intestine of mammals. In October 1666, during Newton’s great year, or
annus mirabilis
, and a month after London burned, Steno received for study the head of a giant shark caught at the city whose English name, Leghorn, is as peculiar as Steno’s two titles. (The name refers neither to limbs nor musical instruments, but represents a poor English rendering of the old spelling, Ligorno, for the town now called Livorno in Italian.) Steno, like so many intellectuals, was working at the nearby city of Florence under the patronage of Ferdinand II, the Medici grand duke. In examining the teeth of his quarry, Steno recognized that he had accidentally bought into one of the major scientific debates of his age, the origin of
glossipetrae
, or tongue stones.

These fossil sharks’ teeth could be collected by the barrel, especially in Malta. In twentieth-century terms, their origin cannot be doubted. They are identical with the teeth of modern sharks in outward form, detailed structure, and chemical composition—therefore they cannot be anything but sharks’ teeth.

Yet the identity in form that makes us so certain led to another potential interpretation in Steno’s time—for God, the author of all things, often created with striking similarity in different realms to display the order of his thoughts and the glorious harmony of his world. If he had made a world with seven planets (sun, moon, and the five visible planets of an older cosmology) and seven notes in a musical scale, why not imbue rocks with the plastic power to form objects precisely like the parts of animals? After all, the
glossipetrae
came from rocks and rocks were created as we find them. If the tongue stones are sharks’ teeth, how did they get
inside
rocks? Moreover, the earth is only a few thousand years old, and tongue stones inundated European collections. How many sharks could have infested Mediterranean waters in so short a time?

A mid-eighteenth century illustration of why
glossipetrae
(A, B, and C) must come from the mouths of sharks.
FROM DE CORPORIBUS MARINIS LAPIDESCENTIBUS (ON PETRIFIED MARINE BODIES) BY THE SICILIAN ARTIST-SCIENTIST AUGUSTINO SCILLA
.

Steno observed that his shark had hundreds of teeth and that new ones formed continually as old teeth wore down and fell out. The numbers of
glossipetrae
from Malta no longer foreclosed an origin in sharks’ mouths, even under the Mosaic chronology (which Steno did not question). According to the common legend that great scientists are unprejudiced observers who can shuck constraints of culture and see nature directly, Steno came to his correct conclusion—that
glossipetrae
are fossil sharks’ teeth—because he made better observations. Steno was a fine observer, but he was also an adherent to the new mechanical philosophy that insisted on physical causes for phenomena and viewed detailed internal similarity as a sure sign of common manufacture in the mechanical sense. Steno did not see better; rather, he possessed the conceptual tools to interpret his excellent observations in a necessary way that we continue to regard as true.

But Steno then abstracted the problem of
glossipetrae
in a remarkably original manner—and achieved with this great insight his role as the founder of modern geology. The tongue stones found within rocks, Steno reasoned, were problematic because they were solids enclosed within a solid body. How did they get in there? Steno then recognized that all the troubling objects of geology were solids within solids—fossils in strata, crystals in rocks, even strata themselves in basins of deposition. A general theory for the origin of solids within solids could provide a guide for understanding the earth’s history.

Taxonomy is often regarded as the dullest of subjects, fit only for mindless ordering and sometimes denigrated within science as mere “stamp collecting” (a designation that this former philatelist deeply resents). If systems of classification were neutral hat racks for hanging the facts of the world, this disdain might be justified. But classifications both reflect and direct our thinking. The way we order represents the way we think. Historical changes in classification are the fossilized indicators of conceptual revolutions.

The French scholar Michel Foucault uses this principle as his key for understanding the history of thought. In
Madness and Civilization
, for example, he notes that a new method of dealing with the insane arose in the mid-seventeenth century and spread rapidly throughout Europe. Previously, madmen had been exiled or tolerated and allowed to wander about. In the mid-seventeenth century, they were confined in institutions along with the indigent and unemployed, a motley assemblage by modern standards. We might regard this classification as senseless or cruel, but as Foucault argues, such a judgment will not help us to understand the seventeenth century.

Why classify together the poor, the unemployed, and the insane; what common theme could inspire such an ordering? Foucault argues that the birth of modern commercial society led to a new designation of the cardinal sin, the one that had to be made invisible by confining all those who, for whatever reason, wallowed in it. That sin was idleness, and Foucault shows that sloth replaced the old medieval curse of pride as the most fundamental of the seven deadly sins in seventeenth-century texts. It mattered little that the insane did not work for biological or psychological reasons, and the unemployed for want of opportunity.

Steno also reordered the world in a way that must have seemed as curious to his contemporaries as the amalgamation of madness and poverty seems to us. As his contemporaries gathered the idle, Steno identified solids within solids as a fundamental class of objects, divided them from everything else, and developed a set of criteria to sort his solids into subdivisions representing the different causes that fashioned them. The great
Prodromus
is, fundamentally, a treatise on a new system of classification for solids within solids—a classification by common genesis, rather than superficial similarity of outward appearance. Steno’s revolution in thought arises from his altered classification—and his curious title, so understood, could not be more devastatingly appropriate. I have read the
Prodromus
many times, but when I finally understood its message, just last month, that bizarre title sent a shiver up my spine.

The
Prodromus
has usually been misinterpreted by geologists who attribute Steno’s success to his use of modern observational methods. (In fact, although the
Prodromus
is sprinkled with astute observations, its longest section is a speculative discussion on the origin of solid bodies, based on the incorrect premise that all solids must be generated from liquids, and that the form of a solid indicates the motions of the liquids that produced it.) His translator writes, for example: “At a time when fantastic metaphysics were rife, Steno trusted only to induction based upon experiment and observation.” But the
Prodromus
reports no real experiments and only a modest number of observations. It succeeded primarily because Steno followed a metaphysic congenial with our own, but relatively new in his time.

Geologists have also judged Steno inappropriately by searching the text for gems of “modern” insight, rather than by understanding its argument as a totality. Thus, the commonest statement about the
Prodromus
, often the only statement made by geologists, holds that Steno presented the crystallographic law of the constancy of interfacial angles—that however much the size and shape of crystal faces vary, the angles between them remain the same. Well, perhaps he did, but the “law” appears as two throwaway lines in a figure caption, and has little relation to Steno’s major theme or argument. (It arises simply as a corollary to his speculations about inferring the motion of fluids from the form of solids precipitated from them.)

No, the
Prodromus
is, as its title states, about solids in solids and their proper classification by mode of origin. It is founded upon two great taxonomic insights: first, the basic recognition of solids within solids as a coherent category for study and, second, the establishment of subdivisions to arrange solids within solids according to the causes that fashioned them.

Steno uses two criteria for his subdivisions. (They are blessedly obvious once you state the problem, but Steno’s revolution is the statement itself.) First, in what might be called the principle of molding, Steno argues that when one solid lies within another, we can tell which hardened first by noting the impress of one object upon the other. Thus, fossil shells were solid before the strata that entomb them because shells press their form into surrounding sediments just as we make footprints in wet sand. But surrounding rocks were solid before the calcite veins that run through them because the calcite fills preexisting channelways just as Jello matches the flutes of a mold. The principle of molding allows us to establish the temporal order of formation for two objects in contact. In a world still regarded by many of Steno’s contemporaries as formed all at once by divine fiat, this criterion of history struck a jarring chord and eventually forced a transposition in thought.

Early in the
Prodromus
, Steno stated the problem that he wished to solve with his second criterion: “Given a substance possessed of a certain figure, and produced according to the laws of nature, to find in the substance itself evidences disclosing the place and manner of its production.” His solution, the basic principle for any historical reconstruction, holds:

Past processes cannot be observed in principle; only their results remain. If we wish to infer the processes that formed any geological object, we must find clues in the object itself. The surest clue is detailed similarity—part by internal part—with modern objects formed by processes we can observe directly. Similarity can be misleading—and great mistakes have been made in applying Steno’s principle—but our confidence in common origin mounts as we catalog more and more detailed similarities involving internal structure and chemical composition as well as external form.

Thus, Steno argues, sedimentary rocks must be the deposits of rivers, lakes, and oceans because they “agree with those strata which turbid water deposits.” Fossil shells once belonged to animals, and crystals precipitated from fluids as we make salt or rock candy today.

With these two principles—molding and sufficient similarity—Steno established both prerequisites for geological, or any historical, reconstruction: he could determine how and where objects formed, and he could order events in time. Steno’s genius, to say it one more time, lay in establishing this new conceptual framework for observation, not in the acuity of the subsequent observations themselves. Steno’s break with older traditions stands out most clearly in his complete failure, save in one sheepish passage, to consider the primary subject that obsessed his colleagues: the identification of goals and purposes for all things, including what we now regard as purely physical processes of uplift, erosion, and crystallization. In one fleeting passage, Steno cites dissimilarity of function as a reason subservient to his usual argument about internal resemblance for stating that rocks and bones form differently. But he quickly adds the disclaimer, “if one may be permitted to affirm aught about a subject otherwise so little known as are the functions of things.” As Foucault also argues, the subjects you leave out of your taxonomies are as significant as the ones you put in.