Carol Downing is still haunted by memories of last year's marathon. But she's excited about reuniting with other survivors. Michael Dwyer/AP hide caption

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Despite Worries, Boston Survivor Heads Back To The Start Line

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Runners train in Ngong, Kenya, in 2012. The country has produced the world's best distance runners for decades, and most belong to the Kalenjin people. Michael Steele/Getty Images hide caption

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How One Kenyan Tribe Produces The World's Best Runners

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Researchers say our brains are probably wired from an evolutionary sense to encourage running and high aerobic activities. Above, a man runs past the Sydney Harbour Bridge on April 22. Ryan Pierse/Getty Images hide caption

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'Wired To Run': Runner's High May Have Been Evolutionary Advantage

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