Evolution and the receptive eye(2012.6.1[Fri.])

 

  If you said that comparative morphology and embryology are fields of study intended to reduce the morphological lexicon to a manageable size, it would be a biting joke, but one not without a kernel of truth. Evolutionary diversity comes dressed in many forms, and a look at the embryonic primordia and ancestral characters that stand behind many organs and their functions frequently reveals that these have arisen from common source. The cartilage that suspends the jaw from the neurocranium, the columella in the reptilian middle ear, and the mammalian stapes can all justifiably be referred to as “hyomandibular derivatives” It’s a question of discovering homology.

  But is that really all there is to it? By equating the morphology of this mammalian bone with some or other cartilage in the shark head, we deprive it of its specialized function, leaving only a venerable name that fits it into its body plan. For talking about the origin of the bone, that’s plenty – it’s the A-student answer. But what of the enigmatic morphological changes that led to the structure of the mammalian ear? Embryology needs to start answering these questions (and dropping that confident honor student’s façade) soon if it hopes to preserve its status among the sciences. To do so, however, will require us to gird ourselves for a kind of madness.

  Emanuele Tesauro, a noted critic of mannerist literature, wrote “Madness is nothing more than a fable for taking one thing for another” (as quoted in Die Welt als Labyrinth. Manier und Manie in der europäischen Kunst. Beiträge zur Ikonographie und Formgeschichte der europäischen Kunst von 1520 bis 1650 und der Gegenwart). The evolutionary changes in living things, and the diversity they engender can fairly be likened to a labyrinth, in which disciples of rationality and logic are doomed to fail, and whose sole prescription is to struggle against one’s own powerlessness. There once was a morphologist who took on the problem of the segmented formation of the animal, particularly vertebrate, head, which led him to the Kopfproblem of the evolutionary origins of this phenomenon, of which he said, “The study of segmentation is comparable to the study of Apocalypse – that way leads to madness.” Perhaps so, but only if that madness is true can we say that the seeker may at least be confronting the problem. Is there any deeper mystery, or one that has more defied understanding these past two centuries, than animal evolution? If we accept that, then we must agree that there is no scientific challenge or undertaking more compelling than comparative morphology. It reduces the absolute primacy of knowledge to an irrelevance, turns our common sense to nonsense, all the while beckoning to us from a place in which easy logic and reason are nowhere to be found...

  In important ways, a good morphology paper is like a good mystery novel. Meraviglia and illusion reveal the truth where knowledge and understanding are helpless. To our feeble imaginations, such mysteries can only appear as monsters, unleashed before our understanding as free-ranging metamorphoses. It is no place for common sense. Indeed, to study evolutionary biology requires one to unlearn those sensibilities.

  What then should we look for in tracing the evolution of morphological traits; what are the methods to this madness? Evolution has inevitably discarded old and created new tools, but have we truly comprehended how that process has played out? The only honest answer is No. Our common sense abandons us here. This is no simple purification or amelioration of form, but rather a nightmarish, aggregate of phantasmagorically real shapes recalls those seen in the mannerist movements of the late Renaissance and Baroque periods or the surrealism of the 20th century. And here too we revisit the phenomena advocated by those schools – archetype, divergence, fusion, analogy.... Fins become hands, or wings. What once was a gill next is used for biting, and then for perceiving vibrations in the air as sound. If we peer even more closely into tissues, we find never before encountered families of molecules and cells, embracing new relationships and inventing new functions. As if witnessing the “...intercourse between an umbrella and a Singer on an anatomical table.”

  There is no such thing as trivial conservation in the endlessly various evolution of form. Segments (such as cervical vertebrae in mammals) that are tightly conserved in one taxon, shift into protean diversity in another. The same is true of gills – lampreys reliably have nine pairs, while the counts differ in their hagfish relations, which frequently boast more than any other extant vertebrates. In arthropods as well, the number of limbs is fixed in only a fraction of taxa. Centipedes commonly have only odd numbers of segments, but the occasional mutation leading to an even number is not unknown. It is not the case that only anatomical features are conserved while more minute characters are left free to change. Our senses and sensibilities are powerless. How then are we to define a body plan armed only with such blunt tools? Is this the only thing that has not changed?

  Just as in human art and invention, biological evolution proceeds by freely seeking out the greatest possible distances between form and form, function and function. It is an Ars Combinatoria using embryonic elements and cells. It does not develop systematically, but as the consolidation and stabilization of chance encounters; the invention, and abandonment, of conserved patterns. These transformations resemble the delirious and hallucinatory visions of the 1930s surrealists, and that perhaps is the true face of evolution.

  The rich fecundity of evolution and life itself appears to us as a mannerist Pantheon of transfigurations, in which anything may be permitted, and where finding the impossible is the more difficult task. Its transformations are not just the normative, conservative, expected, typical and (in the Haeckelian sense) recapitulatory, but frequently (and even more often!) show endless deviations and compensations, exceeding the norm in ways that could rightly be called pathological or perverse.

  If only biological evolution followed the orderly course that Haeckel imagined, understanding would surely be simpler and free of worry. But the reality of it could never be captured by predictable, quantitative forms, for it is no less than an accretion of capricious and qualitative transformations. This is how we humans see evolution, giving rise in its almost violent instability to the dynamism that comprises its essence.