SWALLA, BJ: Origin of the Vertebrates from…….. Worms?

The origin of the vertebrates has been studied intensively for over a hundred years, but recent advances in understanding developmental gene expression have allowed new ways of testing hypotheses. Urochordate and hemichordate phylogenies have shown that the deuterostomes are divided into two great clades. The chordates, including urochordates, cephalochordates and vertebrates make up one clade. Echinoderms and hemichordates are sister groups in the other clade. Hemichordate worms have a collagenous skeleton seen clearly in collagen preparations. These results suggest that collagenous skeletons and, perhaps, filter-feeding pharyngeal systems are basal deuterostome characterisitics. Former chordate hypotheses suggest that the deuterostome ancestor was a colonial, sessile hemichordate, or was a feeding larva, similiar to present day echinoderm or enteropneust larvae. However, both of these organisms lack a well developed collagenous skeleton. We suggest, instead, that the deuterostome ancestor was a small, solitary, soft-bodied worm with pharyngeal gill slits that were used for filter feeding and oxygen capture. Subsequently, urochordates and echinoderms are presumed to have lost their collagenous skeletons. In both cases, this may have been a result following the evolution of a protective outer coating, the cellulose-like tunic in urochordates and the calcareous endoskeleton in echinoderms. All of these events are likely to have preceded the Cambian Explosion.