On Wednesday, Dr. Hintermann and her colleagues showed just how old some of those pieces were: The recipe for building hands was borrowed in part from the one for our nether regions. Dr. Hintermann and her colleagues carried out their study by tracing the activity of genes in developing embryos. Those signaling molecules switch genes on by grabbing DNA at a precise location, like a key turning in a lock. Scientists have identified some of the locks that enable embryos of humans and other species to grow limbs. When 5DOM was snipped out of a mouse embryo’s DNA, the embryo grew legs but failed to grow feet.