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ORDOVICIAN PERIOD

488-444 million years ago

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The evolutionary modifications of the early gnathostomes included differentiation of the duplicated hemoglobin genes, coronary blood vessels, mechanisms of cardiac control similar to those of terrestrial vertebrates, venous valves, and new blood vessels. In addition to lymphatic organs, they evolved antibodies. A genetic accident (the insertion of a bacterial transposon into the genome) which occurred in basal gnathostomes would allow their descendants to shuffle antibody components to create greater antibody diversity.

THE CIRCULATORY SYSTEM
The genome duplication events created myoglobin and hemoglobin genes from an ancestral globin gene followed by a and ß genes from the ancestral hemoglobin. The enzyme carbonic anhydrase became increasingly efficient (Tufts, 2003). Gnathostomes evolved coronary blood vessels to service cardiac muscle (Kardong 460; Hardisty 242). The vagus nerve and the neurotransmitter ACh inhibit the heart (in lampreys, both stimulate the heart) (Hardisty 251). Catecholamines increase both heart rate and contractibility (Hoar, 1970). The intrinsic conduction system of the heart was modified to allow faster transmission of impulses between atria and ventricles (Webster, 1974, p. 54)

The number of aortic arches was reduced to 6 (Romer). Venous valves evolved to better return blood to the heart. (Webster, 1974, p. 67; Hoar, 1983).
The posterior portion of posterior cardinal veins was interrupted (Romer p. 474), lateral abdominal veins evolved (which mammals retain only as umbilical veins) (Romer p. 477), and the hepatic portal vein was no longer contractile (Guenter 154). More elastic fibers formed in the ventral aorta (Prosser, 1973). The number of blood vessels traveling to skeletal muscle increased and subclavian and iliac vessels serviced the pectoral and pelvic fins (Hardisty). Gnathostome circulatory systems no longer possessed accessory hearts (contractile portions of blood vessels other than the branchial heart in Amphioxus and hagfish) and the reduction of sinuses between vessels led to a reduction in blood volume (Webster, 1974, p. 57; Forster, 200; Kardong).

LYMPHATIC SYSTEM
Gnathostomes possessed both a thymus (Hoar, 1983) and spleen (Romer 452). Gnathostomes not only possessed true antibodies, but also different antibody classes, including IgM. Gnathostome antibodies possessed V, D, J, and C regions (Hohman, 1993, Roux, 1998). Of the terminal lytic proteins C5-C9 which function in the complement cascade of higher gnathostomes, C5 and C8 are known in sharks (Zarkadis, 2001). MHC genes themselves seem to have arisen with the in the evolution of the gnathostomes, coinciding with the proposed rounds of genome duplication which seem to have occurred at the base of this group (Ohta, 2000; Bartyl, 1994).
The adaptive immunity of higher vertebrates depends on the ability of recombination activating genes (RAG) to randomly join segments of antibody genes to create an enormous variety of antibodies and T cell receptors. The RAG proteins in vertebrate genomes seem to have resulted from the insertion of a bacterial transposon of the Hin recombinase family and the RAG sequence functions in a way similar to transposons. When the RAG transposon was inserted into the ancestral immunoglobulin gene, it seems to have separated V and J segments which were once part of the same exon (Kasahara, 2004). The RAG rearrangements only affect one type of leukocyte-it did not interrupt the preexisting innate responses (Du Pasquier, 2004). All gnathostomes can rearrange antibody and T-cell receptor segments using RAG genes. RAG I is homologous to integrase and RAG II is homologous to integration host factor; these two genes cause site specific recombination in bacteria (Bernstein, 1996).