On Monsters: An Unnatural History of Our Worst Fears (31 page)

A conjoined twin, with two heads sharing one body. Photo by Joanna Ebenstein © 2008. Reprinted by kind permission of the artist and the Vrolik Museum, University of Amsterdam.

Why does this have evolutionary significance? We may not know for sure when or why a mutation arose that fixed the pentadactyl structure for modern tetrapods. The answer to that may lie in the impenetrable swamp of evolution’s historical contingency. But we now know that evolutionary change can happen to entire embryonic patterns, not just individual microvariations. If we extend Alberch’s limb logic to phylogeny we have new tools for understanding evolution. Gould wrote, “Evolution can reduce the number of fingers by stopping the back-to-front generating machine at five. What we now call digit one may only be the stabilized stopping point of a potentially extendable sequence.”
²⁷
Why five digits
survived when eight did not is a question for natural selection. But how five and eight digits were originally built is an issue of development, not selection.

Monsters with supernumerary or missing digits help us to see the causality at work in normal animals. Monsters demonstrate internal constraints, but monsters themselves are extreme cases and therefore deleterious. Smaller scale mutations in these same constraint systems, however, can play a role in evolution.

Do these developmental jumps indicate violations of Darwin’s
natura non facit saltum?
The jury appears to be out on this question. Gould seemed to think that there was, in this new logic of monsters, a kind of vindication of sorts for Goldschmidt’s “hopeful monster” idea.
²⁸
Orthodox neo-Darwinians claim that all macromutations are produced by micromutations (harkening back to Darwin’s claim that species are nothing but stabilized varieties), but Gould wanted to include macromutation as an additional evolutionary mechanism. Despite the jitters produced by these comments among the creationist set, it seems Gould was correct when he argued that such theoretical and empirical discoveries are healthy refinements of Darwinism, not death knells. “Hopeful monster” became a rhetorical banner for biologists who wanted to emphasize development more than natural selection, but nobody except the most marginal characters were willing to throw out the dominance of natural selection, the undisputed adapting force in nature.

Within the past few years this whole business of “hopeful monsters” has become paramount again, in the new science of
evo-devo
, “evolutionary developmental biology.” In recent years biologists have discovered the empirical genetic evidence for the developmental constraint systems that the monsterologists Geoffroy, Owen, Goldschmidt, and others could only theorize about.

Biologists who were working in the 1990s on cracking genetic codes assumed that major phenotypic differences between species would be reflected in major gene pool differences. It seemed reasonable to suppose that we’d find very different genetic ingredients in the molecular makeup of the different animal species; tiny Darwinian mutations would have slowly accumulated diverse pools of DNA information, and hundreds of millions of years of changes would have resulted, neo-Darwinians believed, in very unique genetic codes for humans, mice, and insects. But such was not the case.

In 2001 a provisional map of the human genome was completed, and it surprised everyone. Contrary to expectations, humans turned out to have only around 25,000 genes—about one third the predicted number. And like another in a long history of scientific blows to our collective ego, we also learned that humans and mice had roughly the same number of genes. Even nematode worms were genetically closer to us (quantitatively and qualitatively) than we ever expected.
²⁹
So if the phenotypic diversity of the animal kingdom is not the direct result of genotypic diversity, then it’s not the genetic stuff that causes all the diversity.
³⁰
Biologists began to appreciate the fact that gene
expression
is as important as the genes themselves. An expression system of on/off timing switches has a huge role to play in the morphology, physiology, and even taxonomy of species. Neo-Darwinians had erroneously treated development as if it were just the medium for unrolling the message, but in fact the medium
is
the message. Two similar gene pools could have different switching patterns, and two radically different animals could subsequently result. Expression, we now know, is largely controlled by special timing switches, called homeotic genes. To return to my beleaguered cooking metaphors, DNA is like the ingredients of a cake, and homeotic genes are like the recipe that dictates how much and when the ingredients should be added.

Monsters return in this new investigation because, as always, their various abnormalities help biologists detect the relevant expression mechanisms, both abnormal and normal. The biologist Scott Holley, for example, is currently studying zebra fish monsters in the Department of Molecular, Cellular, and Developmental Biology at Yale University.
³¹
He and his colleagues are exploring the ways that early genetic events can transform limb and body morphologies across vertebrate species. Zebra fish, especially teratological versions, are a relatively new model for studying the formation of the spinal column; humans and fish, for example, have the same genetic process for segmenting vertebrae,
³²
and certain spinal teratologies of the zebra fish appear to unfold similarly in certain cases of human spinal birth defects. So progress in our understanding of the fish may help us prevent or ameliorate spinal deformities in humans.

The focus of Holley’s research is to understand how the genes
express
in the segmentation process (somitogenesis) of the vertebral column. The anterior and posterior axis of the forming fish begins to divide into repeated elements, called somites, which will eventually become vertebrae. Cells transcribe the gene and build the spine downward from top to bottom. The cells build the spine by dividing through mitosis and also by migrating in a distinct downward, then outward pattern. But this building process is regulated further by a “segmentation clock,” a universal biological timing
system that can now be observed across vertebrate species. The growth is pushing downward, but regular oscillations are traveling upward toward the anterior of the developing fish, creating repeated wave-fronts. The waves constitute ebbs and flows of transcription activity. The wave-front, which travels one cell diameter every five minutes, tries to make somites (prevertebrae) everywhere, but the regulating clock gates the activity and thereby creates the necessary gaps between vertebrae. All of this is a complex feedback system of signals which ensures that enough, but not too much, material is growing, but also being channeled correctly. Embryo-genesis is a remarkable dance of dynamic cell division, migration, and tissue complexification, but still a relatively stable process if you back up to observe the level of body plan morphogenesis.

Professor Holley’s group has isolated some specific zebra fish mutations in which the on/off switches are not working; the gates are not regulating properly. Malfunctions in the segmentation clock can produce too many or too few vertebrae, or lead the organism to build a spine with too much cell migration and not enough mitosis. The same mutation causes abnormal formation of the vertebral column in fish, mice, and humans.

Professor Holley himself is not particularly concerned with the evolutionary implications of his research, but one can see how the proponents of macromutation could get very excited about it. Small adjustments in the segmentation clock could result in significant changes to the overall length of the body. And these switching systems are not just concerned with anatomical construction; they also regulate the postpartum or post-ovum growth and aging mechanisms of organisms. Neoteny, for example, is the retention of otherwise juvenile traits well into the adult phase of an animal’s life span.
³³
Evo-devo biologists suggest that such adjustments to maturation rates (inter- and intraspecies) could be the result of mutations in the regulating homeotic genes. These timing mutations would be selected for or against, just like smaller variations, depending on whether they conferred advantage to the organism.

In 2006 the Swiss ichthyologist Maurice Kottelat discovered the tiniest fish in the world, the size of a fingernail clipping, living in the acidic peat swamps of Sumatra. This monstrous fish is a mosaic of mature and immature phenotypes: it has a juvenile larval body with mature gonads and pelvic fins. It is as if the sexual equipment of an adult was found on a newborn. It is unclear
why
such a mosaic would be selected for and preserved, but the mechanics of
how
mutations of aging occur are slowly becoming clearer.

Evo-devo biologists love these weird and murky interfaces between homeotic genes, embryogenesis, and species transformations. Theirs is still
very much an emerging field of study, but we have some impressive data so far. Homeotic genes regulate the development of an embryo by regulating smaller scale gene sequences, often acting like repressor molecules that bind onto specific DNA sites and block RNA transcription and subsequent protein production. Some regulating genes, such as Pax genes, are conserved over many species and orders, acting like little tool kits capable of building their same products in whatever context they appear. When the Pax genes that help build eyes are transplanted from an insect to a mouse, or vice versa, they start to build eyes in these radically new environments.

Eight of these homeotic controlling genes are called Hox genes; they can be found in most animals as the controlling system of body morphology. These eight Hox genes are so embedded in the deep grammar of life that biologists believe they’ve been at work in our collective gene pool for over half a billion years. It is possible that these Hox gene systems cause many of the homologous body plans that Richard Owen previously obfuscated by evoking transcendentalism. If these Hox genes turn out to be the real archetypes, then Geoffroy, with his “generative laws,” may have been more correct even than Darwin, with his thesis of contingent shared ancestry.

IN LIGHT OF EVO-DEVO
, what more can we conclude about the nagging question of hopeful monsters? The scientist Sean B. Carrol seems confident that the “specter of a ‘hopeful monster’” has finally been banished by evo-devo.
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In support of the neo-Darwinian modern synthesis, Carrol claims that the gradual (nonsaltational) micromutations, together with the sifting effect of natural selection, are all that is needed to explain evolution—no appeals to jumps, leaps, sports, monsters, or macromutations are necessary. “Evolution of homeotic genes and the traits they control has been very important,” Carrol explains, “but has not occurred by different means than the sorts of mutations and variations that typically arise in populations. The preservation of Hox genes and other tool kit genes for more than five hundred million years illustrates that the pressure to maintain these proteins has generally been as great as that upon any class of molecules. Instead, the evolutionary tinkering of switches, from those of master Hox genes to those of humbler pigmentation enzymes, typically underlies the evolution of form.” In short, Carrol claims that macroevolution is entirely explained by microevolution.

In the United States, where all evolutionary issues are still battling with ridiculous creationist claims, it is easy to understand why biologists want to present a unified Darwinian front against the looming threats
of irrationality. My suspicion is that some of the current reemphasis on gradualism is due to this perceived need to cut off pathways to special creation or various miraculous evolution theories. But the idea, loathed by Darwinians, of nature jumping is not inherently incoherent. The objection that Darwin originally had, and that all biologists still rightly have, is to the idea that a jump could somehow
foretell
the best adaptive direction (the potentially helpful trait) and then modify accordingly to meet the niche demands. Nature cannot see that wings or extra vertebrae would be really helpful to a specific rodent and thereby accelerate an anatomical jump to wings or extra vertebrae. The reason Darwinians are so riled up by the mention of saltation is that it has historically been connected to the idea of a purposeful Nature (e.g., by Owen, Asa Gray, de Vries, neo-Lamarckians). If we strip saltation from this unfortunate teleologi-cal association we find nothing prohibitive about the idea that macromutations can strike out and take their chances, just as micromutations do, in the domain of natural selection. The vast majority of mutations, micro and macro, would be deleterious (or neutral) in the face of environmental pressure, but occasionally they might offer a slight advantage to their possessor. In any case, in a historical science such as evolution theory, which must rationally reconstruct events from deep time, the idea of successful nonteleological macromutations is at least as coherent as gradualism in the reconstruction of phylogeny.

In the end, some of the recent debates about monsters in biology are more semantic than substantive. If you decide to define monsters or teratologies as variations too extreme to reproduce viable offspring, then of course you’re not going to find any monsters acting as launching pads for evolutionary pathways; you’ve begged the question in your definition. But if you define monsters as extreme morphological deviations and leave it at that, the issue of whether or not there are “hopeful monsters” becomes a more empirical point.

Presently, I heard a slight groan, and I knew it was the groan of mortal terror. It was not a groan of pain or of grief—oh, no! It was the low stifled sound that arises from the bottom of the soul when overcharged with awe
.
EDGAR ALLAN POE,
THE TELL-TALE HEART