User:Jtwsaddress42/Projects/Project 4/Sections/Chapter 1/The problems of vertebrate evolution and development

The ancestral origins of the chordates and vertebrates lies within the bilaterian deuterostomes. The Deuterostomes are an eclectic collection of phyla with diverse bodyplans that includes our chordate ancestors. Although the origins of the deuterostomes is murky, it appears that deuterostomes evolved during the late Edicarian/early Cambrian period from a segmented bilaterian ancestor that had pharyngeal gill slits, a hollow nerve cord, along with circular and longitudinal muscles.

The early stem deutetostomes were undergoing a great deal of developmental reorganization as can be seen by the diversity of its major phylogenetic body plans. The branch leading to chordates diverges early and may have been subject to much of the same genetic and developmental instability that accompanied the remodelling of the earlier protosome-like bilaterian bodyplan. By the early Cambrian about 525 million years ago, Myllokunmingia ,which appears to be a very primitive early jawless vertebrate, can be found in the fossil record of the Lower Cambrian Maotianshan shales of China.

Vertebrates undergo several important innovative bursts that organize their anatomy and nervous systems. These seem to be intimately linked to genome duplication events, a somato-visceral fusion of life-stages, and some major adaptive reorganizations. From the beginning, the neural crest has been the great innovation of vertebrates; a fourth germ layer from which, the tissues that weld the somatic and visceral divisions are derived. Following the history of these tissues tells the story of vertebrate evolution and neural organization.