Several hundred million years ago, fishlike creatures developed rudimentary fingers and toes, which helped them drag their way onto dry terrain. At Harvard Medical School, developmental biologist Susan Dymecki just found a crucial genetic step that gave rise to those indispensable digits. Dymecki normally studies how genes shape the brains of mice. In a largely trial-and-error process, she and her lab workers manipulate the mouse genome to see how different parts of the brain develop. These experiments produced an unexpected mutation: an animal that lacked fingers and toes. Dymecki found she had knocked out the DNA sequence activating a gene called BMPRIB, which evidently contains the genetic information that sculpts cartilage into digits.
When switched on by a different stretch of DNA, BMPRIB becomes active within the brain. "We think that the sequence we knocked out was acquired sometime later during evolution, and it led to the gene being expressed not just in the nervous system but also in the limb," Dymecki says
