How did the bat get its wings?

The question of how bats evolved wings highly adapted for powered flight from limbs equally well adapted for walking has been a topic of fascination for centuries. In On the Origin of Species, Charles Darwin proposed how divergent limb morphologies evolved through "successive slight modifications" of a conserved pattern, stating "there will be little or no tendency to alter the original pattern" and that "The bones of a webbed foot might have all its bones, or certain bones, lengthened and the membrane connecting them increased so as to serve as a wing". One hypothesis fitting well with Darwin's ideas and with comparative molecular data is that divergence in form arises primarily from changes in the dynamics of gene expression during development. We are testing this hypothesis using a functional genetic approach comparing limb morphogenesis between mouse and bat. We initially focused on Prx1 , a homeobox gene required for developing limb skeleton elongation. A limb‐specific Prx1 enhancer was identified using a LacZ reporter assay in transgenic mice. The endogenous limb enhancer of the mouse Prx1 gene was then replaced with that of the bat by gene targeting in mouse ES cells. Targeted mice express the mouse Prx1 gene under the control of the bat limb enhancer, develop forelimbs that are significantly, albeit slightly, lengthened and display elevated levels of Prx1 mRNA in limb bones during development.