As the worm turns

Natural History, Feb, 1997 by Stephen Jay Gould

How wonderfully symbolic and real in the double meaning. Geoffroy proposed a theory to unite the architecture of complex animals by comparing vertebrates with segmented worms and arthropods turned over. This theory for the archetype of complex animals became, instead, the archetype of nutty ideas in biology--so nutty that Gaskell felt driven to invent an even crazier theory for the origin of vertebrates, explicitly to avoid the bizarre concept of his own kin as inverted worms. But turning worms also serve as our cultural metaphor for upheaval of accepted ways and thoughts.

I have always loved the boldness of Geoffroy's theory, but I never dreamed that he might be right--even though I have long embraced, as a centerpiece of my own career, his larger view about the importance of inherited architectural pathways as constraints upon the optimizing power of natural selection. Well, the worm has turned twice during the past year--in both actual and symbolic styles. Geoffroy, it seems, was correct after all--not in every detail, of course, but surely in basic vision and theoretical meaning. And the triumph of this surprise, the inversion of nuttiness to apparent truth, stands as a premier example of the most exciting general development in evolutionary theory during our times.

I published my first technical book, Ontogeny and Phylogeny, in 1977. I took pride in this long work on the relationship between embryology and evolution, but I also became quite frustrated because we then knew so preciously little about the potential key to a resolution: the genetic basis of development. How does the genetic code help to orchestrate this greatest miracle of everyday biology--the regular and usually unerring production of adult complexity from the apparent formlessness of the tiny fertilized egg? We knew practically nothing, but we assumed (as documented above) that the major animal phyla, all evolutionarily separate for at least 500 million years, could share no constraining common plan or genetic architecture. Pure Darwinism reigned triumphant and natural selection had built each basic anatomy for its own adaptive utility.

But we can now determine, easily and relatively cheaply, the detailed chemical architecture of genes; and we can trace the products of these genes (enzymes and proteins) as they influence the course of embryology. In so doing, we have made the astounding discovery that all complex animal phyla--arthropods and vertebrates in particular--have retained, despite their half billion years of evolutionary independence, an extensive set of common genetic blueprints for the building of bodies.

Many similarities of basic design among animal phyla, once so confidently attributed to convergence and viewed as a testimony to the power of natural selection to craft exquisite adaptation, demand the opposite interpretation that Mayr labeled as inconceivable: the similar features are homologies, or products of the same genes, inherited from a common ancestor and never altered enough by subsequent evolution to erase their comparable structure and function. The similarities record the constraining power of history, not the building skills of natural selection independently pursuing an optimal design in separate lineages. Vertebrates are, in a sense, true brothers (or homologs)--and not mere analogs--of worms and insects.