Development of the annelid axochord: Insights into notochord evolution

Abstract

Origin of the spine lies in a worm The notochord, the developmental backbone precursor, defines chordates—the group of animals to which humans belong. The origin of the notochord remains mysterious. Lauri et al. report the identification of a longitudinal muscle in an annelid worm that displays striking similarities to the notochord regarding position, developmental origin, and expression profile. Similar muscles, termed axochords, are found in various invertebrate phyla. These data suggest that the last common ancestor of bilaterians already possessed contractile midline tissue that, via stiffening, developed into a cartilaginous rod in the chordate line. Science , this issue p. 1365