Before the Backbone: Views on the Origin of the Vertebrates

Description

The general reader with some background in comparative anatomy, will find this reference an informative discussion on historical views (eg, Garstang, Gislén, Romer, etc), not so historical views (eg, Jefferies), and modern anatomical and molecular results concerning how vertebrates arose. A wide varieties of views are supported, but a number of conclusions are nonetheless formulated. Gislén's view of the carpoid as an echinoderm with chordate affinities is considered more correct than Jefferies' view of the carpoid as a chordate with echinoderm affinities. Recent molecular evidence supports chordates diverging from (echinoderms and hemichordates), and within the chordates, urochordates diverging from (cephalochordates and craniates). Larval paedomorphosis as the mechanism originating the vertebrates is unlikely, and the sessility of tunicates is probably a derived trait. While strong homologies between homeobox genes and organ systems in both arthropods and vertebrates are acknowledged, it is noted that molecular methods set deuterostome phyla clearly apart from protostome phyla, suggesting the direct ancestry of the vertebrates is not from the arthropods. Molecular methods also indicate that the amphioxus is not a degenerate vertebrate, but essentially a primitive one, and elaboration of its features leads to the emergence of the vertebrate head.