Leonardo’s Mountain of Clams and the Diet of Worms (40 page)

Interestingly, giraffes fight predators (primarily lions) by kicking, but their sexual combats proceed by necking, never by kicking. Thus, this function of the neck may represent a specifically evolved behavior for a particular circumstance.

Giraffes also use their necks in several other ways: as a “lookout tower” to spot predators and other dangers, and as a device to increase surface area and shed heat (giraffes, unlike other large African mammals, do not seek shade and can remain in the sun). Both these functions have been viewed by prominent scientists as a chief reason for the evolution of long necks. In addition, giraffes deftly
shift their center of gravity by appropriate movements of the neck—and these maneuvers are crucial to a wide range of activities, including rising from a lying position, running, and climbing fences and other barriers.

We may now return to the central theme of this essay—the dissociation of current utility from historical origin—and understand why the giraffe’s neck cannot provide a proof for
any adaptive scenario, Darwinian or otherwise. Giraffes do use their long necks to browse leaves at the tops of acacia trees—but such current function, no matter how vital, does not prove that the neck originally evolved for this purpose. The neck may have first lengthened in the context of a different use, and then been coopted for better dining when giraffes moved into the open plains. Or the neck
may have evolved to perform several functions at once. We cannot learn the reasons for historical origin simply by listing current uses.

When we consider the full range of current function, we can be fairly confident that some uses must be secondary, and cannot therefore be the source of historical origin. I can’t imagine, for example, that long necks evolved to help giraffes maneuver in running,
jumping, and getting up—because the problem only arose when giraffes acquired a long neck in the first place, and solutions to problems can’t be causes of the problem.

But other functions may well be original—and the famous reaching for leaves could arise as a largely secondary effect. Since natural selection works fundamentally by differential reproductive success, and since sexual combat so
often acts as a primary determinant of this basic Darwinian benefit, we could state a plausible case for regarding sexual success as the chief adaptive reason for evolving long necks, with the much-vaunted browsing of leaves as a distinctly secondary consequence. In short, we have no basis for any firm assertion about the most famous inquiry among Darwinian just-so stories: How did the giraffe get
its long neck?

This essay therefore features a double whammy in pursuit of a primary theme—the dissociation of current utility from historical origin. In the realm of ideas, current invocation of the giraffe’s neck as the classic case of Darwinian evolution does not grow from firm and continuous historical roots. The standard story, in fact, is both fatuous and unsupported. In the realm of giraffes,
current use of maximal mammalian height for browsing acacia leaves does not prove that the neck evolved for such a function. Several reasonable alternative scenarios exist, and we have no evidence for preferring any plausible version over another.
Caveat lector.

Why, then, have we been bamboozled into accepting the usual tale without questioning? I suspect two primary reasons: we love a sensible
and satisfying story, and we are disinclined to challenge apparent authority (like textbooks!). But do remember that most satisfying tales are false. The seventh-inning stretch predated Mr. Taft, and the story of kingly rising before the Hallelujah Chorus has no established foundation either. Polonius may have been an old bore, but he did give Laertes some good advice in the famous speech that
Laertes surely failed to process because he was trying so hard to leave town. Among other tidbits, Polonius emphasized the importance of overt appearance—and we would do well to remember his counsel. Darwinian evolution may be the most truthful and powerful idea ever generated by Western science, but if we continue to illustrate our conviction with an indefensible, unsupported, entirely speculative,
and basically rather silly story, then we -are clothing a thing of beauty in rags—and we should be ashamed, “for the apparel oft proclaims the man.”

BROTHERHOOD BY INVERSION (OR, AS THE WORM TURNS)

A
S
H
AMLET, IN THE MOST CELEBRATED SOLILOQUY OF
E
NGLISH LITERATURE
, weighs the relative values of life and death, he describes the attraction of suicide (“not to be”) as an escape from
active
insults, including “the oppressor’s wrong, the proud man’s contumely.” But writers and intellectuals worry far more about an opposite fate on life’s potential
“sea of troubles”—erasure and oblivion, the pain of being simply ignored. Samuel Johnson, as recorded by Boswell, expressed this silent arrow of outrageous fortune in a famous aphorism: “I would rather be attacked than unnoticed. For the worst thing you can do to an author is to be silent as to his works.”

I therefore felt special poignancy when I recently read an anecdote about the last years
of a great English physiologist, Walter H. Gaskell (1847-1914). After a distinguished career of solid experimental work on the function of the heart and nervous system, Gaskell switched gears and devoted the entire second half of his professional life (from 1888 on) to promoting and defending an idiosyncratic theory for the origin of vertebrates. The last paragraph of Gerald L. Geison’s long article
on Gaskell in the
Dictionary of Scientific Biography
reads:

We may grieve for Gaskell’s personal fate as an intellectual pariah; but, truth to tell, he had been pushing a pretty nutty theory for the origin of vertebrates. Gaskell believed with all his soul, and with a striking absence of critical questioning, that the evolution of animal life must follow a single pathway of progressive advance mediated by an increasing
elaboration of the brain and nervous system. Gaskell wrote in his major work of 1908,
The Origin of Vertebrates
(the source of all quotes from Gaskell in this essay):

Gaskell identified this controlling principle of linear advance as the “law of the paramount importance of the development of the central nervous system for all upward progress.” In a rhetorical flourish,
he then inverted the Preacher’s famous argument (Ecclesiastes 9:11) for randomness and aimless change without direction: “The law of progress is this—The race is not to the swift, nor to the strong, but to the wise.”

Advocates for a single line of progress encounter their greatest stumbling block when they try to find a smooth link between the apparently disparate designs of invertebrates and
vertebrates. In addressing this old problem, Gaskell adopted the standard strategy of linear progress theorists from time immemorial: identify the most complex invertebrate and attempt to forge a link with the simplest vertebrate. Gaskell, again following tradition, selected arthropods as the invertebrate pillar for his bridge, and then tried to build the span by his law of neurological complexification.
He wrote:

So far, so conventional. Gaskell’s theory becomes idiosyncratic, and a bit bizarre,
in his chosen mode for forging the improbable link of arthropod to vertebrate. Among the plethora of prominent differences between these phyla, one central contrast has always served as a focus for discussion, and a chief impediment to any linear scheme. Arthropods and vertebrates share some broad features of general organization—elongated, bilaterally symmetrical bodies, with sensory organs
up front, excretory structures in the back, and some form of segmentation along the major axis. But the geometry of major internal organs could hardly be more different, thus posing the classical problem that has motivated several hundred years of dispute and despair among zoologists.

Arthropods concentrate their nervous system on their ventral (belly) side as two major cords running along the
bottom surface of the animal. The mouth also opens on the ventral side, with the esophagus passing between the two nerve cords, and the stomach and remainder of the digestive tube running along the body
above
the nerve cords. In vertebrates, and with maximal contrast, the central nervous system runs along the dorsal (top) surface as a single tube culminating in a bulbous brain at the front end.
The entire digestive system then runs along the body axis
below
the nervecord. (The accompanying figure from Gaskell’s book illustrates this cardinal difference in an unconsciously amusing way.) But could evolution (or a sensible divine creator, for that matter) turn an arthropod with belly above nerve cords into a superior vertebrate with brains on top and gut below?

Gaskell proposed a pretty
wild scheme for such a transformation, and his loss of respect (and students) followed his inability to construct a cogent defense. Gaskell argued that the dorsal gut of arthropods evolved into the vertebrate brain and spinal cord as a proliferation of nervous tissue fueled the upward march of progress. This new nervous tissue began to surround the old gut, eventually choking off all digestive function
like a strangler fig around a host tree, or an anaconda squeezing the lifeblood from a pig. The modern vertebrate brain surrounds the old arthropod stomach, thus explaining the ventricles—the interior spaces between the folds of the brain—as remnants of an ancestral digestive space. Similarly, the central canal of the spinal cord represents the old arthropod intestine, now surrounded by nervous
tissue.

But this putative solution only engendered an even more troubling problem: If the arthropod gut became the vertebrate nervous system, then what organ can serve as a precursor for the vertebrate gut? This problem stymied Gaskell, and he opted for a deus ex machina that eventually satisfied no one but himself (and perhaps his one last auditor): the vertebrate digestive tube simply arose
de novo
, to suit an obvious need. Gaskell concluded:

Can we extract any message from Gaskell’s failed theory beyond a stodgy, if appropriate, warning about the virtues of caution and sobriety? I certainly think so, for I have
long held, and expressed as a mainstay of these essays, that when fine scientists devote their careers to theories later judged nutty or crazy, interesting and instructing reasons always underlie the paradoxical advocacy. This principle certainly applies in Gaskell’s case because we can identify both a generally constraining bias and a personally compelling reason that drove Gaskell to the odd
idea of stomachs turning into brains and new guts arising from nothing but inchoate potentiality.

Gaskell’s dubious but unquestioned conviction about linear progress served as the general bias that led him to propose an almost alchemical scheme of transmutation from arthropod to vertebrate. But an understanding of the history of this subject also reveals a particular reason that interacted with
his general conviction about progress to lead him down a path of increasing irrelevance and loneliness. In short, Gaskell proposed his own nutty theory because he couldn’t abide the older and standard account, also judged by history as a prime case of nuttiness, for linking arthropods and vertebrates.

Think about the basic contrast, and the most obvious way to produce alignment. Arthropods grow
ventral nerve cords with the gut above; vertebrates develop with a dorsal nerve tube and a gut below.
Presto turno
—and one becomes the other. Why not just invert a segmented worm or an insect to produce the vertebrate pattern? Turn a bug on its back (as Kafka did, come to think of it, when he changed his protagonist into a roach in “The Metamorphosis”), and the internal geometry of a vertebrate
emerges—nerves above guts.

I don’t mean to be frivolous or cavalier about complex and serious matters. All participants in the history of this debate know perfectly well that an inverted worm or insect doesn’t become a vertebrate,
tout simple
and all nice and clean. More than a few knotty problems and inconsistencies remain. To cite the dilemma most widely discussed in the literature, the esophagus
of an inverted bug runs upward through the nervous system (right in the area that will become the vertebrate brain), to emerge at a mouth on top of the head. Clearly this will not do (and has not done in any real vertebrate)! So the inversion theory for deriving vertebrates from arthropods must argue that the old brain-piercing mouth atrophied and closed up, while a new ventral mouth developed
at the front end of the vertebrate gut. Forming a new mouth at the end of an old tube may not be quite so bold or improbable as building an entirely new gut from nothing (as Gaskell’s theory required), but no evidence for such a scenario exists either, and the whole tale smacks of fatuous special pleading to save an otherwise intriguing idea.