Acorn worms have a dispersed but patterned nervous system.

Lowe/Elsevier

Complex nervous system patterning—usually assumed to have coevolved with advanced, centralized nervous systems—may have arisen before nerves consolidated into a central nerve chord, according to Christopher Lowe, John Gerhart (University of California, Berkeley, CA), Marc Kirschner (Harvard Medical School, Boston, MA), and colleagues.

Their idea runs counter to the prevailing theory of dorsoventral axis inversion. The ventral nerve chords in arthropods (such as Drosophila) and dorsal nerve chords in chordates (such as humans) have been thought to be related via an inversion event some time during evolution. In the new theory, however, the original ancestor is proposed to have had a dispersed nervous system that converged centrally in independent dorsal and ventral events.

Reconstructing chordate evolution is tricky for several reasons. The rapidity of the Cambrian explosion and the soft bodies of the ancestors...

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