A pro-Russian rebel walks in a passage at a local market damaged by shelling in Petrovskiy district in the eastern Ukrainian town of Donetsk. Mstislav Chernov/AP hide caption

itoggle caption Mstislav Chernov/AP

Britain's Prime Minister David Cameron speaks at a news conference in Downing Street, London on Friday after the U.K. raised its terror alert level. Paul Hackett/PA Photos/Landov hide caption

itoggle caption Paul Hackett/PA Photos/Landov

NFL Commissioner Roger Goodell, shown here in January, has sent an open letter to NFL team owners explaining the league's new policies for preventing and punishing domestic violence and sexual assault. Andrew Burton/Getty Images hide caption

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Researchers raised two groups of walking, air-breathing Polypterus senegalus — one on land and one on the water. They discovered that each group was able to adapt to be best suited to its environment. A. Morin, E.M. Standen, T.Y. Du, H. Larsson/McGill University hide caption

itoggle caption A. Morin, E.M. Standen, T.Y. Du, H. Larsson/McGill University