Several human congenital deformities include defects in the heart and forelimbs. Stainer and Yelon's group now shows that Hand2 directs TBX5 . A common developmental link between our hearts, hands and jaws has evolutionary resonances with what we know about the early evolution of the vertebrates. The very earliest and most primitive vertebrates had neither jaws nor paired fins. Later on, vertebrates evolved jaws.
Jaws are an obvious distinguishing feature, but there is a host of others, including paired limbs.
Studies on fossil jawless fishes called cephalaspids offer an intriguing insight into the evolution of the gnathostome state. Cephalaspids were primitive, armoured creatures. Although they lacked jaws, they had paired forelimbs and are believed to have been more closely related than lampreys to the gnathostome stock. Yelon, D. et al . The bHLH transcription factor Hand2 plays parallel roles in zebrafish heart and pectoral fin development.