Weighing the Evidence for a Deuterostome Branch of Animals and Implications for Understanding Chordate Origins

For over 100 years, the deuterostome clade has been one of the few unchallenged branches in the phylogeny of the animal phyla. Deuterostomia includes the echinoderms and hemichordate worms and also our own phylum of chordates. Molecular phylogenies have shown that some other phyla previously linked to deuterostomes by shared morphology and embryology (most notably the chaetognaths or arrow worms) are in fact members of the second major branch of bilaterian animals: the protostomes. Several supposedly deuterostomian characters found in chaetognaths are therefore common to both branches of bilaterian animals, weakening the support for Deuterostomia. Recent studies of molecular data show equivocal support for the deuterostome clade. The deuterostome clade even seems to get some artefactual support from well-known sources of systematic error. The weak and possibly nonexistent support for Deuterostomia has important consequences for our understanding of bilaterian evolution and the origins of the chordates.