The success and ecological dominance of the vertebrates can be attributed to several unique traits that arose during the evolution of vertebrates from a deuterostome ancestor. A limited list of novelties that evolved in the vertebrate lineage includes the following:
• The notochord
• Populations of cranial placodes and neural crest cells
• Paired pectoral and pelvic limbs
By comparing the morphology, ontogeny, developmental genetics, and fossil history of vertebrates and other chordates (such as the cephalochordate amphioxus and urochordate ascidians), it is possible to trace the evolutionary origins of these structures. Conserved regulatory genes control the pattern of these new developmental fields and tissues, revealing once again that new developmental programs have evolved through the co-option of existing regulatory genes and circuits and the expansion of ancestral patterning roles.
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