I've just being doing some googling to try to understand this research better.

It seems that it's been known for a while that more primitive animals have/had functionally distinct front (sensory) and rear (motor + autonomic) neural systems, with different genes (Otx, Gbx) associated with each. In 2003 it was shown that an extant species, acorn worms, have this front/sensory, rear/motor neural division, assumed to date back to the pre-chordates.

In vertebrates we also see this association of Otx gene expression in the fore/midbrain and Gbx in the hindbrain, but it had been assumed that these "two halves" of the brain had fused long ago... What this new research shows is that this fusion only happens during development of the embryo, which seems to be a fairly common phenomena "Ontogeny recapitulates phylogeny" whereby embryonic development follows the same steps as evolution (e.g. human embryo also has a tail, chicken embryo has teeth).

Of course the front/sensory and rear/motor neural systems aren't totally isolated, since the sensory signals serve to steer the motor programs running in the back, and our brain still follows this primitive division of functionality.