Bilaterian animals—that is, bilaterally symmetric animals with distinct anterior and posterior ends—are often thought to have evolved from a common ancestor with a medial, ventral nerve cord. Common molecular patterns along the body axes of animals as diverse as fruit flies, annelid worms and humans support this scenario. Andreas Hejnol and colleagues look at the mediolateral neuroectodermal patterning system in a wide range of animals, including Xenoturbella (a basal bilaterian) and various lophotrochozoans (such as annelids, brachiopods and rotifers). They observe that the final anatomical configurations of the central nervous system are unrelated to the patterning system. They conclude that similar central nervous system architectures are likely to have arisen many independent times across the bilaterian group—an example of convergent evolution.
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