In this June 2013 photo provided by National Geographic, diver Susan Bird, working at the bottom of Hoyo Negro, a large dome-shaped underwater cave in Mexico's Yucatan Peninsula, brushes the Naia skull found at the site. Paul Nicklen/AP hide caption

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A Filipino chicken vendor in Quezon City, east of Manila, Philippines. Researchers say Pacific island chicken are genetically similar to the variety found in the Philippines, but different from South American chicken. Rolex Dela Pena/EPA/Landov hide caption

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Area A of the Happisburgh archaeology site in Norfolk, Britain, where coastal erosion has revealed mudflats containing 800-thousand-year-old footprints. Martin Bates/British Museum/EPA/Landov hide caption

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Human evolution is an unfolding process with chapters yet to be written; no one really knows where we're going. But we can look back to earlier chapters, with ancestors like Australpithecus afarensis, including the individual we call "Lucy" (seen above), for an understanding of how evolution works and what has happened to us over time. Tim Boyle/Getty Images hide caption

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Language may have evolved in concert with tool making. Sergey Lavrentev/iStockphoto.com hide caption

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When anthropologists tallied the use of emotional words through a century of literature, they included many books without clear emotional content — technical manuals, for example, and automotive repair guides. Steve Debenport/iStockphotography hide caption

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A member of the Ya̧nomamö people at Irotatheri community in Venezuela's Amazonas state, near the Brazilian border, in September 2012. Leo Ramirez/AFP/Getty Images hide caption

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