Did the ctenophore nervous system evolve independently?

Recent evidence supports the placement of ctenophores as the most distant relative to all other animals. This revised animal tree means that either the ancestor of all animals possessed neurons (and that sponges and placozoans apparently lost them) or that ctenophores developed them independently. Differentiating between these possibilities is important not only from a historical perspective, but also for the interpretation of a wide range of neurobiological results. In this short perspective paper, I review the evidence in support of each scenario and show that the relationship between the nervous system of ctenophores and other animals is an unsolved, yet tractable problem.