Leonardo’s Mountain of Clams and the Diet of Worms

by Stephen Jay Gould

  A second case then seriously compromised the classic textbook example of convergence—the paired eyes of three great phyla: vertebrates, arthropods (with the multiply faceted eye of flies as a primary example), and mollusks (particularly the complex lens eye of squid, so similar in function to our own, but built of different tissues). We had always assumed that eyes in the three phyla evolved three separate times, in complete independence, because they differ substantially in basic anatomy. And we viewed this supposed convergence as a premier example of natural selection’s power to produce organs of similar and optimal function, but built from different materials and evolved from entirely separate starting points. But we now know that eyes in all three phyla share an inherited embryological pathway largely orchestrated by a gene (called Pax-6 in its vertebrate form) retained in all these phyla from a common ancestor, and remaining similar enough to work interchangeably (for the fly version will induce eyes in vertebrates, and vice versa). The end results vary substantially (the multifaceted fly eye is not homologous with our single-lens eye), but the embryological blueprints share a common ancestry, and the eyes of different phyla can no longer be viewed as an example of pure convergence.

  The reversal of opinion during the past decade has been astonishing. Mayr argued that we shouldn’t even bother to look for genetic homology and shared embryological pathways between distinct phyla. We have now moved to the opposite pole of being surprised when we identify a basic gene of developmental architecture in Drosophila and then do not find a homolog in vertebrates. Charles B. Kimmel began a recent paper on this subject by writing: “We have come to find it more remarkable to learn that a homolog of our favorite regulatory gene in a mouse is not, in fact, present in Drosophila than if it is, given the large degree of evolutionary conservation in developmentally acting genes.”

  Still, I guess I haven’t fully accommodated to the change—even though the new perspective suits my hopes and fuels my theoretical prejudices so well—for I never dreamed that my all-time favorite theory of interphyletic union, Geoffroy’s hypothesis of inversion, could possibly be right as well. The basic structuring from front end to back? Fine. Eyes? Why not? But the arthropod belly as the vertebrate back? Kind of silly, however intriguing.

  Except that Geoffroy’s inversion theory, appropriately re-expressed in the language of modern genetics and developmental biology, turns out to be true. In several papers, published during the past two years, and based on work done primarily in the laboratories of Eddy M. De Robertis at UCLA and Ethan Bier at the University of California, San Diego, all essentials of Geoffroy’s theory have been strikingly affirmed in contemporary terms (see especially Holley et al., 1995; De Robertis and Sasai, 1996; François et al., 1994; and François and Bier, 1995; all in the bibliography).

  Geoffroy’s vindication began with the sequencing of a vertebrate gene called chordin. In the toad Xenopus (but working, so far as we know, in a similar manner in all vertebrates), the chordin gene codes for a protein that patterns the dorsal (top) side of the developing embryo, and plays an important role in formation of the dorsal nerve cord. When these scientists searched for a corresponding gene in Drosophila, they discovered, to their surprise, that chordin shares sufficient similarity with sog to make a confident claim for common ancestry and genetic homology. But sog is expressed on the ventral (bottom) side of Drosophila larvae, where it acts to induce the formation of ventral nerve cords. Thus, the same gene by evolutionary ancestry builds both the dorsal nerve tube in vertebrates and the ventral nerve cords of Drosophila—in conformity with Geoffroy’s old claim that vertebrate backs are arthropod bellies, and that the two phyla can be brought into structural correspondence by inversion.

  This intriguing fact cannot affirm Geoffroy’s inversion theory by itself, but De Robertis and colleagues then sealed the case with two additional discoveries. First, they found that a major gene responsible for specifying the dorsal side of flies (and called decapentaplegic, or dpp), has a vertebrate homolog (called Bmp-4) that patterns the ventral side of Xenopus—another reversal consistent with Geoffroy’s hypothesis. Moreover, the entire system seems to work in the same way—but inverted—in the two phyla. That is, dpp, diffusing from the top to the bottom, can antagonize sog and suppress the formation of the ventral nerve cords in Drosophila—while Bmp-4 (the homolog of dpp) diffusing from the bottom to the top, can antagonize chordin (the homolog of sog) and suppress the formation of the dorsal nerve cord in vertebrates. (The preceding figure, taken from the original publications, shows these relationships better than words can convey.)

  Second, these scientists also found that the fly gene can work in humans, and vice versa. Vertebrate chordin can induce the formation of ventral nerve tissue in flies, while fly sog can induce dorsal nerve tissue in vertebrates. I regard these three discoveries as forming a tight and well-documented case for Geoffroy’s old theory of inversion.

  Moreover, current results vindicate Geoffroy’s version, not the later scenarios of linear evolution. These data do not support the silly notion that, at a defining moment in the march of evolutionary progress, an arthropod literally flipped over to become the first vertebrate. Rather, as Geoffroy argued so long ago, the two phyla share a common architecture, but in reversed arrangement. In evolving separately from common ancestry, vertebrates oriented the shared design in one manner, annelids and arthropods in the opposite direction. Evolution displays enormous ingenuity and versatility in iterating a set of common genes and developmental pathways along so many various routes of ecology and modes of life. But our brotherhood and sharing, like the still waters of legend, run far deeper than we had dared to imagine. A substantial blast from the past underlies the signs of new designs.

  To end on an admittedly famous note, devotees of B movies will remember one of the all-time classics—the original version of The Fly, with Vincent Price (not that dreadful remake with Jeff Goldblum as our hybrid hero). Focus on the unforgettable last scene: the fly with the man’s head lies ensnared in a spider’s web, as ugly Ms. Eight-Legs moves in for the gruesome kill. In a shrill voice of fear, the fly keeps shouting, “Please help me.” Finally, and mercifully (for the fly’s head on the man’s body has died, so the two creatures cannot be unmixed and properly reconstituted), another character throws a rock at the web, putting fly-man out of his misery. (“They shoot horses, don’t they?”) Perhaps at this crucial moment in the next remake, the rock-wielding mercy killer can offer some zoological advice instead: “Turn over and be a man.”

  VI

  DIFFERENT PERCEPTIONS OF COMMON TRUTHS

  18

  WAR OF THE WORLDVIEWS

  A YEARNING FOR THE “GOOD OLD DAYS” INFECTS US ALL, EVEN THOUGH such times never existed outside our reveries. The nostalgic longing may be universal, but modes of expression vary by culture and social class. We all know the stereotypes. Plebeian Pete wishes that he could still smoke, drink, and eat red meat without raising eyebrows; while Patrician Percival laments that he just can’t find dependable servants these days.

  Stereotypes work by unfair exaggeration, to be sure, but they often build upon a kernel of reality. So consider this statement written in 1906 by a true Patrician Percival: “The latter minister to the former with unconscious service all the time, and with no more arrogant independence than do our domestics generally nowadays.” They don’t make ’em more patrician than Percival Lowell, brother of Harvard President A. Lawrence Lowell, and of poet Amy Lowell—and scion of Boston’s great family in the celebrated ditty:

  And this is good old Boston

  The home of the bean and the cod,

  Where the Lowells talk to the Cabots

  And the Cabots talk only to God.

  In science, as in so many other human endeavors, you don’t have to be rich to succeed; but, oh my, it sure doesn’t hurt either. Charles Darwin inherited a considerable fortune and then increased his stake by shrewd investments. He well understood the intellectual benefits thus acquired, primarily in freed
om and time. Darwin wrote in his autobiography: “I have had ample leisure from not having to earn my own bread.” But Alfred Russel Wallace, the codiscoverer of natural selection, grew up penniless, began his professional life as a schoolteacher, and always lived frugally by his wits as a writer and collector. He probably matched Darwin in intelligence, but never had the time for sustained theorizing and experiment.

  Percival Lowell (1855-1916) spent his youthful Wanderjahre on several grand tours of Asia, leading to books with such representative titles as The Soul of the Far East and Occult Japan. He then decided to devote his life to astronomy, and began with an ultimate bang (for mucho bucks) by building a private observatory in Flagstaff, Arizona. There he did much useful work, including the prediction of a planet beyond Neptune, eventually found at his observatory by Clyde Tombaugh, and named Pluto in 1930.

  But a lifetime of good work can be swallowed by one unforgettable error. Such a fate seems especially unfair when the understandable lapse of a moment erases the memory of a fine career (Bill Buckner’s gimpy legs, or Pee Wee Herman’s harmless impropriety). But when the error represents an idée fixe, relentlessly pursued over years of research and volumes of writing, then the promoter has built his own coffin. At least Percival Lowell fell before a grand enemy, the god of war himself—the planet Mars.

  In the late 1870s, the Italian astronomer Schiaparelli had described the Martian surface as crisscrossed by long, thin, and straight features that he called canali, meaning “channels” in Italian (with no attribution of causality), not “canals” (with implications of construction by sentient beings). Lowell fell under the spell of these nonexistent phenomena, and spent the rest of his career in ever more elaborate attempts to map and interpret “these lines [that] run for thousands of miles in an unswerving direction, as far relatively as from London to Bombay, and as far actually as from Boston to San Francisco.” (All quotations come from Lowell’s major book on the subject: Mars and Its Canals.)

  Lowell eventually decided that the lines must be true canals, and he developed an ever more elaborate and poignant interpretation. He viewed Mars as a once-verdant world now drying up, with polar ice caps as the only remaining source of substantial water. The canals, he decided, must represent a planetary system of irrigation, built by higher (or at least highly cooperative) beings in a last-ditch effort to funnel spring meltwaters of the ice caps to a parched and more equatorial civilization.

  As I have often emphasized in these essays, the study of error provides a particularly fruitful pathway to understanding human thought. Truth just is, but error must have reasons. If Mars had canals, then Lowell becomes an accurate observer. But a robotic photographer, insentient and without motive, might have done even better. However, since Mars does not have canals, we must ask how Lowell could have deluded himself so mightily—and the answers must embody instructive reasons and motivations. In this essay, I shall not document how Lowell decided that the canals existed, but shall concentrate instead on the logic of his argument for interpreting those supposed structures as products of a higher civilization. I choose this focus because Lowell’s central error persists as a major impediment to understanding both evolution in general, and several key issues in speculations about extraterrestrial life. More immediately, the same error underlies the major public misunderstanding inspired by claims first raised in August 1996 for fossil evidence of life in a Martian meteorite.

  Lowell begins his case with a false argument for extensive vegetation on Mars, an inference from supposedly seasonal changes in coloration over large portions of the Martian surface. Lowell regarded “the existence of vegetation on the planet as the only rational explanation of the dark markings there, considered not simply on the score of their appearance momentarily, but judged by the changes that appearance undergoes at successive seasons of the Martian year.”

  Lowell’s next crucial inference inspired the complaint about servants quoted at the outset of this essay. He argues that the existence of an extensive flora implies a corresponding fauna of complex animals as well:

  Important as a conclusion this is no less pregnant as a premise. For the assurance that plant life exists on Mars leads to a further step . . . It introduces us at once to the probability of life there of a higher and more immediately appealing kind, not with the vagueness of general analogy, but with the definiteness of specific deduction. For the presence of a flora is itself ground for suspecting a fauna.

  Lowell trots out all the familiar examples of interdependency between plants and animals, including pollination by insects and “preparation” of soil by earthworms—though he never seems to realize that particular cases of evolved interaction on an earth already inhabited by animals need not imply a necessary and universal linkage. After all, plants could evolve first, and animals never follow. (Lowell’s forced metaphor about arrogant servants refers to animals who, in their haughty assumptions about superiority, don’t even realize that they minister to plants in return for well-known service in the other direction.)

  Having established (to his satisfaction) that animals must inhabit Mars, Lowell asks what level of complexity these animals must have reached, never doubting that life, once begun, must evolve to higher and higher states: “Once started,” Lowell writes, “life, as paleontology shows, develops along both the floral and the faunal lines side by side, taking on complexity with time.”

  Lowell locates the mechanism of evolutionary advance in adaptive necessities imposed by a cooling planet. In a planet’s hot youth, the simplest forms will flourish in a tropical bounty. But steady cooling requires greater organic complexity to weather increasingly harder times. When the going gets tough, to cite the current cliché, the tough get going:

  It [life] begins as soon as secular cooling has condensed water vapor to its liquid state; chromacea and confervae [unicellular plants and animals in the terminology of Lowell’s time] coming into being high up toward the boiling-point. Then, with lowering temperature come the seaweeds and the rhizopods, then the land plants and the lunged vertebrates. Hand in hand the fauna and flora climb to more intricate perfecting, life rising as temperature lowers.

  Mars, at a greater distance from the sun and a smaller size than Earth, must have cooled further than our planet. Martian animal life must therefore be more advanced than Homo sapiens. For all our vaunted wealth and technology, what have we built that a telescopic observer on Mars might recognize as a product of advanced life? The Great Wall of China? Our largest cities? Nothing, in any case, to match the scale of the Martian canals:

  Nor is this outcome [evolution to higher complexity] in any sense a circumstance accidental to the earth; it is an inevitable phase in the evolution of organisms. As the organism develops brain it is able to circumvent the adversities of condition; and by overcoming more pronounced inhospitality of environment not only to survive but spread. Evidence of this thought will be stamped more and more visibly upon the face of its habitat. On earth, for all our pride of intellect, we have not yet progressed very far from the lowly state that leaves no records of itself.

  In particular, the increasing harshness of Martian environments implies a mental response from evolving life:

  In an aging world where the conditions of life have grown more difficult, mentality must characterize more and more of its beings in order for them to survive, and would in consequence tend to be evolved. To find, therefore, upon Mars highly intelligent life is what the planet’s state would lead one to expect.

  Put together all these conjectures and inferences—that Mars has vegetation; that plants imply animals; that both plant and animal life must progress to greater complexity; that planetary cooling and increasingly more challenging environments inspire evolutionary progress; that Mars has cooled further than Earth; that beings more advanced than Homo sapiens must therefore inhabit Mars—and the supposed canals cry out for interpretation as technological devices for husbanding the depleting resources of a cooling and drying planet.

 
; Schiaparelli’s “channels” must really represent what his Italian word conveys in English—true canals of planetary scale, built by higher creatures to tap the only available supply of water:

  Dearth of water is the key to their character . . . So far as we can see the only available water is what comes from the semi-annual melting at one or the other [polar] cap of the snow accumulated there during the previous winter. Beyond this there is none except for what may be present in the air. Now, water is absolutely essential to all forms of life; no organism can exist without it. But as a planet ages, it loses its oceans . . . and gradually its whole water supply. Life upon its surface is confronted by a growing scarcity . . . [The canals are], then, a system whose end and aim is the tapping of the snow-cap for the water there semiannually let loose; then to distribute it over the planet’s face . . . From the fact, therefore, that the reticulated canal system is an elaborate entity embracing the whole planet from one pole to the other, we have not only proof of the worldwide sagacity of its builders, but a very suggestive side-light, to the fact that only a universal necessity such as water could well be its underlying cause.

  We should admire, Lowell suggests, not only the high intelligence that could build such a system, but also the superior moral qualities of beings that can cooperate (as we seem so singularly unable to do) at a planetary scale! “The first thing that is forced on us in conclusion is the necessarily intelligent and non-bellicose character of the community which could thus act as a unit throughout its globe.”

  Martian civilization may be doomed, despite this grand and noble planetary attempt to ward off disaster. But at least we may take courage and comfort in higher evolutionary stages thus implied for future earthly life (while we may hope for a different outcome):