the simplicity of a limb

In their paper on Evolvability , Marc Kirschner and John Gerhart discuss the separation of concerns within our genetic code. They pay special attention to limbs: The limb is a complex structure with precisely placed bone, cartilage, muscle, nerves, and vascular elements, and one might think it is difficult for such a structure to change in evolution. 1 Every vertebrate uses the same genes to…