Details of the evolutionary history from invertebratesto vertebrates, as deduced from the sequences of 18S rDNA.

TLDR

The study sequenced nearly complete 18S rDNA from two chaetognaths, five echinoderms, a hemichordate, and two urochordates. Phylogenetic analysis of these sequences, together with other deuterostomes and protostomes, shows deuterostome monophyly except for chaetognaths, places cephalochordates closest to vertebrates, groups ascidians, larvaceans, and salps as urochordates with early larvacean divergence, and supports chordate evolution from free‑living ancestors.

Abstract

Almost the entire sequences of 18S rDNA were determined for two chaetognaths, five echinoderms, a hemichordate, and two urochordates (a larvacean and a salp). Phylogenetic comparisons of the sequences, together with those of other deuterostomes (an ascidian, a cephalochordate, and vertebrates) and protostomes (an arthropod and a mollusc), suggest the monophyly of the deuterostomes, with the exception of the chaetognaths. Chaetognaths may not be a group of deuterostomes. The deuterostome group closest to vertebrates was the group of cephalochordates. Ascidians, larvaceans, and salps seem to form a discrete group (urochordates), in which the early divergence of larvaceans is evident. These results support the hypothesis that chordates evolved from free-living ancestors.