The plate on the left contains about equal numbers of colonies of two different bacteria. After the bacteria compete and evolve, the lighter ones have taken the lead in the plate on the right. Courtesy of Michael Wiser hide caption

itoggle caption Courtesy of Michael Wiser

A dog burial in Greene County, Ill. This fossil dates back to about 8,500 years ago. Courtesy of Del Baston, Center for American Archaeology hide caption

itoggle caption Courtesy of Del Baston, Center for American Archaeology

This is an artist's illustration of Kepler-62f, a planet in the "habitable zone" of a star that is slightly smaller and cooler than ours. Kepler-62f is roughly 40 percent larger than Earth. NASA/JPL-Caltech/T. Pyle hide caption

itoggle caption NASA/JPL-Caltech/T. Pyle

Friend Or Foe? Scientists say dogs react differently to the direction of another dog's tail wag. Time & Life Pictures/Getty Images hide caption

itoggle caption Time & Life Pictures/Getty Images

The botulism toxin comes from Clostridium botulinum bacteria, seen here in a colorized micrograph. James Cavallini/Science Source hide caption

itoggle caption James Cavallini/Science Source

The Chalet (right) is the U.S. Antarctic Program's administrations and operations center at McMurdo Station. Reed Scherer/National Science Foundation hide caption

itoggle caption Reed Scherer/National Science Foundation

A helicopter is unloaded from an LC-130 in Antarctica last December. Researchers on this mission were studying the Pine Island Glacier, one of the fastest-receding glaciers on the continent. August Allen/National Science Foundation hide caption

itoggle caption August Allen/National Science Foundation

Would time spent with Anton Chekov, famed for his subtle, flawed characters, make you a better judge of human nature? Hulton Archive/Getty Images hide caption

itoggle caption Hulton Archive/Getty Images

The Clinical Center at the National Institutes of Health in Bethesda, Md. National Institutes of Health hide caption

itoggle caption National Institutes of Health

A worker stands on top of a storage bin on July 27, 2011, at a drilling operation in Claysville, Pa. The dust is from powder mixed with water for hydraulic fracturing. Keith Srakocic/AP hide caption

itoggle caption Keith Srakocic/AP

Space tourist Dennis Tito celebrates after his landing near the Kazakh town of Arkalyk on May 6, 2001. Alexander Nemenov/AFP/Getty Images hide caption

itoggle caption Alexander Nemenov/AFP/Getty Images

Health officials around the world are on constant lookout for the deadly bird flu. Here a worker collects chickens on a farm in Kathamndu, Nepal, where the virus was suspected of infecting poultry last October. Prakas Mathema/AFP/Getty Images hide caption

itoggle caption Prakas Mathema/AFP/Getty Images