To elucidate the evolutionary origin of nervous system centralization, we investigated the molecular architecture of the trunk nervous system in the annelid Platynereis dumerilii. Annelids belong to Bilateria, an evolutionary lineage of bilateral animals that also includes vertebrates and insects. Comparing nervous system development in annelids to that of other bilaterians could provide valuable information about the common ancestor of all Bilateria. We find that the Platynereis neuroectoderm is subdivided into longitudinal progenitor domains by partially overlapping expression regions of nk and pax genes. These domains match corresponding domains in the vertebrate neural tube and give rise to conserved neural cell types. As in vertebrates, neural patterning genes are sensitive to Bmp signaling. Our data indicate that this mediolateral architecture was present in the last common bilaterian ancestor and thus support a common origin of nervous system centralization in Bilateria.

Understanding the early evolution of animal body plans requires knowledge both of metazoan phylogeny and of the genetic and developmental changes involved in the emergence of particular forms. Recent 18S ribosomal RNA phylogenies suggest a three-branched tree for the Bilateria comprising the deuterostomes and two great protostome clades, the lophotrochozoans and ecdysozoans. Here, we show that the complement of Hox genes in critical protostome phyla reflects these phylogenetic relationships and reveals the early evolution of developmental regulatory potential in bilaterians. We have identified Hox genes that are shared by subsets of protostome phyla. These include a diverged pair of posterior (Abdominal-B-like) genes in both a brachiopod and a polychaete annelid, which supports the lophotrochozoan assemblage, and a distinct posterior Hox gene shared by a priapulid, a nematode and the arthropods, which supports the ecdysozoan clade. The ancestors of each of these two major protostome lineages had a minimum of eight to ten Hox genes. The major period of Hox gene expansion and diversification thus occurred before the radiation of each of the three great bilaterian clades.

DNA sequence analysis dictates new interpretation of phylogenic trees. Taxa that were once thought to represent successive grades of complexity at the base of the metazoan tree are being displaced to much higher positions inside the tree. This leaves no evolutionary ''intermediates'' and forces us to rethink the genesis of bilaterian complexity.A deep reorganization of the metazoan phylogenetic tree is presently taking place as a result of the input of molecular data. Far from being an exercise confined to a small circle of aficionados, the changing views on the pattern of animal interrelationships has profound consequences for understanding the underlying processes of animal diversification. As has repeatedly been stressed, we shall never be able to reason on the evolution of development and the way it has shaped animal diversity unless we have a reliable history of the path taken by this diversification. Here, we highlight the salient recent results based on genetic data, especially the displacement of taxa long thought to represent successive grades of complexity at the base of the metazoan tree, to much higher positions inside the tree. This leaves us with no evolutionary ''intermediates'' and forces us to rethink the genesis of bilaterian complexity. The reappraisal of animal evolution rests on several congruent approaches ranging from primary gene sequence analysis to qualitative molecular signatures within appropriate genes. Each of them, however, has its methodological difficulties; we shall, therefore, also try to briefly pinpoint the issues of contention and discuss the strength of the present view.Preliminary Comments on the Reliability of Phylogenetic Trees. To an outsider, the field of phylogenetic reconstruction may appear to be full of controversies and uncertainties. There have been acrimonious debates over the best methodology to use for reconstruction (phenetics vs. cladistics) and over the relative merits of morphological vs. molecular data. Worst of all, contradictory trees have kept pouring in, often with insufficient critical assessment. Obviously, we cannot review the whole field here, but we wish to emphasize three points to justify our reasoned optimism and confidence in the recent molecular phylogenies.