As the worm turns

He then argued, quite cogently within his own framework, that this grand Platonic blueprint paid scant attention to such "insignificant" questions of nitty-gritty daily reality as which side of a universal design happened to point toward the sun. The one grand design has a gut in the middle and the main nerve cords at a periphery. Arthropods orient this periphery down and away from the sun--so we call their nerve cords ventral. But vertebrates orient their spinal cord up and toward the sun--so we call the same structure dorsal in our own kin. One common design in two orientations, insignificantly inverted with respect to an external axis of sunlight and gravity.

But later evolutionary theorists of linear progress had to advance the overtly physical and historical claim that an ancestral lineage of arthropods actually turned over to become the first vertebrates (for the classic statement of the inversion theory in this genealogical form, see William Patten, The Grand Strategy of Evolution, 1920). Gaskell could not abide this indecorous version of his beloved linear progress theory. He could not bear to imagine that the grand march from jellyfish to human, orchestrated by an ever increasing mass of nervous tissue yearning for consciousness, once paused in a stately and orderly progression toward a human pinnacle in order to execute a fancy little flip, a clever jig of inversion, just at the sublime and definitive moment of entrance into the vertebral home stretch.

Gaskell therefore had to keep his stately soldiers upright and uniformly oriented throughout their journey toward the human pinnacle--and he fulfilled this need by crafting the vertebral brain and spinal cord from an arthropod digestive tube, while forming a completely new gut below. By this device, he could keep tops on top and bottoms at the bottom throughout the linear history of animal life, while placing nerves below gut in arthropods, but above guts in vertebrates. Gaskell thought that his move would rescue the theory of linear progress, with its necessary transition of arthropod into vertebrate, from the absurdities of the old inversion theory. "How is it then," he wrote, "that this theory has been discredited and lost ground? Simply, I imagine, because it was thought to necessitate the turning over of the animal." Gaskell therefore invented his peculiar alternative as a refutation of the venerable inversion theory. He wrote of the first vertebrate: "If the animal is regarded as not having been turned over. . . then the ventricles of the vertebrate brain represent the original stomach, and the central canal of the spinal cord the straight intestine of the arthropod ancestor."

How ironic. In order to avoid the "nutty" theory of inversion, Gaskell invented the even nuttier notion of stomachs turning to brains with new guts forming below. No wonder then that subsequent theory cast a plague on both speculative houses and opted instead for the obvious alternative: arthropods and vertebrates do not share the same anatomical plan at all, but rather represent two separate evolutionary developments of similar complexity from a much simpler common ancestor that grew neither a discrete gut nor a central nerve cord. After all, we now know that arthropods and vertebrates have been separated for more than 500 million years, and that "simpler" arthropods did not turn into "complex" vertebrates at some halfway point on a march to a single evolutionary apex.