488-444 million years ago



A second genome duplication event occurred in the ancestors of the gnathostomes. The additional set of genes allowed a more complex differentiation of many regions of the body, including the mouth region. Jaws evolved from the first pair of pharyngeal arches and jawed vertebrates, the gnathostomes, diversified in the ancient seas.

A second genome duplication event occurred at the base of the gnathostome lineage after its divergence from that of lampreys (Stadler, 2004; Hahn, 1998; Escriva, 2002; Hoyle, 1998). There are a number of cases in which a single invertebrate gene is homolgous to four vertebrate genes. This is observed in Hox clusters, syndecan, myc, BMP (5-8), EGFR/ERBB2-4, ENGR, GPC, ID, JAK (non-receptor kinases), MEF (MADS box enhancing factors), NOTCH, Src kinases, and Src-related kinases. There are a number of gene families in which three vertebrate genes are homologous to a single invertebrate gene, perhaps after one member was lost; these include aldoase, Alzheimer -amyloid protease inhibitors, ankyrin, Bruton's tyrosine kinase, cadherin, calmodulin, caudal homeobox genes, collagen type IV, cathepsin (cystein protease), Dlx homeobox proteins, E2A transcription factors, exrin, glioblastoma family zinc fingers, hedgehog, insulin receptors, integrin chains, laminin chains, laminin chains, MyoD transcription facots, myosin heavy chains, nitric oxide synthases, Pbx homeobox, Raf kinases, Ras, retinoblastoma, retinoic acid receptors, Stat, tenascin, and Wnt (wingless signalling factors (Spring, 1997). The observation that shared regions between human chromosomes indicate large-scale duplication, has allowed the positions of additional genes (such as PBX and NOTCH family members) to be predicted and located (Katsanis, 1996). Regions of human chromosomes 1, 6, 9, and 19 are homologous and contain trilogues and tetraloges (such as notch, PBX, heat shock proteins, retinoid receptors, tenascin, calcium channels, collagen alpha chians, ABC transporters, complement proteins (Ohno from Muller, 1998 ; Lundin, 1993).

Following this genome duplication event came one of the most important changes in vertebrate history--the evolution of jaws. A shark skull is depicted below, representing the most primitive group of jawed vertebrates alive today. In jawless fish, the pharyngeal arches contain cartilaginous bars for support. Anatomical and genetic evidence suggests that the first pair of pharyngeal bars were modified to become jaws. The following view of a shark skull pictures both the jaws and the pharyngeal bars.

shark jaws

shark jaws