Scenarios for the making of vertebrates.

Over the past 200 years, almost every invertebrate phylum has been proposed as a starting point for evolving vertebrates. Most of these scenarios are outdated, but several are still seriously considered. The short-range transition from ancestral invertebrate chordates (similar to amphioxus and tunicates) to vertebrates is well accepted. However, longer-range transitions leading up to the invertebrate chordates themselves are more controversial. Opinion is divided between the annelid and the enteropneust scenarios, predicting, respectively, a complex or a simple ancestor for bilaterian animals. Deciding between these ideas will be facilitated by further comparative studies of multicellular animals, including enigmatic taxa such as xenacoelomorphs.