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 of chordates make it impossible to construct precise evolutionary trees. And chordates' closest major relatives, the echinoderms, have added so many bizarre anatomical features that they are next to useless for comparisons. Thus, the new study subject is the acorn worm. These hemichordates are a lesser-known lineage but, like the chordates, they are bilateral deuterostomes, in contrast to arthropods, which are protostomes and thus more distantly related.
The basic result of the new paper is simple. Although acorn worms have distributed nerve nets rather than nerve chords, they express 22 genes in sets of anterior, midlevel, and posterior domains, whose relative positions are conserved with the patterns found in chordates. The authors argue that the ancestor of deuterostomes (and probably protostomes, as well) had a similar combination of a patterned but noncentralized nervous system.
If nerve chords formed twice during evolution, they might be expected to have formed in two different ways. The evidence here is split: the chords from arthropods and chordates are started off by two similar inducing molecules but finished off by very different molecules.
Lowe acknowledges that during evolution acorn worms may have lost a central nervous system rather than having failed to gain it. He expects that more examples like the acorn worm in other branches of the evolutionary tree will help support the new model. But, he warns, “this is going to continue to be a really difficult problem to resolve.” ▪