In the chordate and vertebrate central nervous system, sensory and motor nerve tracts cross from one side to the other as they connect the brain with sensory receptors and motor neurons. These ¿decussations,¿ crossings in the form of an X, relate each side of the brain to the opposite side of the body. The protochordates derive from an invertebrate ancestor, but no such contralateral arrangement occurs in any invertebrate phylum. No adaptive benefit of decussation has been established. What might explain the evolution of decussation? Method: A brief review of relevant features of comparative morphology of invertebrates, chordates and vertebrates leads to an explanatory model of decussation. Result: A ¿somatic twist model¿ of invertebrate-vertebrate transition accounts for decussations as byproducts of a more momentous change; the relocation of the neuraxis from the ventral to the dorsal aspect of the body. Evidence is presented that this inversion proceeded by means of a twisting of the body 180 degrees on its axis just behind its anterior pole. This rotation aligned the neuraxis with the dorsal head ganglia and brain and by twisting the nerve tracts it brought decussation in its wake. Conclusion: Decussation evolved as a byproduct of a genetically determined partial inversion of the body plan, which resulted in a 180 degree rotation posterior to the brain and oropharynx.
