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2016-08-17. From Fins Into Hands: Scientists Discover a Deep Evolutionary Link.

posted Aug 19, 2016, 12:51 PM by Alan Gould
By Carl Zimmer, The New York Times. For GSS Losing Biodiversity chapter 3. Excerpt: To help his readers fathom evolution, Charles Darwin asked them to consider their own hands. “What can be more curious,” he asked, “than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions?” Darwin had a straightforward explanation: People, moles, horses, porpoises and bats all shared a common ancestor that grew limbs with digits. ...On Wednesday, a team of researchers at the University of Chicago reported that our hands share a deep evolutionary connection not only to bat wings or horse hooves, but also to fish fins. ...The fossil record shows that we share a common aquatic ancestor with ray-finned fish that lived some 430 million years ago. Four-limbed creatures with spines — known as tetrapods — had evolved by 360 million years ago and went on to colonize dry land. ...For over two decades, Neil H. Shubin, an evolutionary biologist, has investigated this transition in two radically different ways. ...In 1996, a team of French researchers studying mice discovered genes that are essential for the development of their legs. When the scientists shut down two genes, called Hoxa-13 and Hoxd-13, the mice developed normal long bones in their legs. But their wrist and ankle bones failed to appear, and they did not grow any digits. This discovery suggested that Hoxa-13 and Hoxd-13 genes tell certain cells in the tetrapod limb bud that they will develop into hands and feet. Dr. Shubin knew that fish have genes related to Hoxa-13 and Hoxd-13. He wondered what those genes were doing, if anything, in developing fins. An experiment on fish might give him and his colleagues a clue. “But we didn’t have the means to do it until technology caught up with our aspirations,” Dr. Shubin said. ...thanks to a new gene-editing technology called Crispr. Scientists can use it to readily alter genes in virtually any species. ...Andrew R. Gehrke, a graduate student... engineered zebrafish so that he could follow individual cells during the development of embryos. ...cells that switched on the Hox genes started to glow. They kept glowing throughout development, until they reached their final location in the fish’s body. ...When the fins were fully developed, ...the fin rays were glowing. In a similar experiment on mice, the digits and wrist bones lit up. “Here we’re finding that the digits and the fin rays have some sort of equivalence at the level of the cells that make them,” Dr. Shubin said....  http://www.nytimes.com/2016/08/18/science/from-fins-into-hands-scientists-discover-a-deep-evolutionary-link.html