Stone artifacts dating back 15,500 years suggest humans may have arrived 2,000 years earlier than previously thought. Courtesy of Michael R. Waters hide caption

itoggle caption Courtesy of Michael R. Waters

An evacuee is screened for radiation exposure at a testing center on Tuesday in Koriyama city, Fukushima prefecture. Officials are concerned about radiation leaking from the nuclear plant nearby. Wally Santana/AP hide caption

itoggle caption Wally Santana/AP

The FBI used a new DNA fingerprinting technology to analyze the anthrax used in the 2001 mailings. These anthrax spores can live for many years. Janice Haney Carr/CDC Public Health Image Library hide caption

itoggle caption Janice Haney Carr/CDC Public Health Image Library

Researchers used hatchling loggerhead turtles, like this one wearing a special harness, to see how the turtles responded to changing magnetic fields as guides for migration. Courtesy of Nathan Putman hide caption

itoggle caption Courtesy of Nathan Putman

Scientists hope to use a fungus to keep malaria-carrying mosquitoes like the Anopheles gambiae species from growing resistant to insecticides. Courtesy of James Gathany/CDC hide caption

itoggle caption Courtesy of James Gathany/CDC

Robert O'Rourke was diagnosed with idiopathic pulmonary fibrosis in late 2006. The lung disease causes scarring in the lungs, which makes breathing difficult. Courtesy of Robert O'Rourke hide caption

itoggle caption Courtesy of Robert O'Rourke

An American black bear from the Kenai Peninsula in Alaska. A new study found that a bear's metabolism in hibernation drops by nearly 75 percent while its body temperature falls just slightly. Oivind Toien/Institute of Arctic Biology, University of Alaska, Fairbanks hide caption

itoggle caption Oivind Toien/Institute of Arctic Biology, University of Alaska, Fairbanks

Hazardous materials experts enter the Hart Building of the U.S. Senate on Nov. 7, 2001, in Washington. The building was closed after an anthrax-laced letter was found in then-Sen. Tom Daschle's office. Stephen Jaffe/AFP/Getty Images hide caption

itoggle caption Stephen Jaffe/AFP/Getty Images

Fleas push off the ground from the ends of their legs, not from the part of the leg that resembles a knee. Gregory Sutton hide caption

itoggle caption Gregory Sutton

Researchers have sequenced the genome of the common water flea, Daphnia, shown in this false-color micrograph. About 35 percent of the genes are brand new to science. Jan Michels/Christian-Albrechts-Universitat zu Kiel hide caption

itoggle caption Jan Michels/Christian-Albrechts-Universitat zu Kiel

With just 600 residents and no cities, Sark, located about 80 miles off the coast of England in the English Channel, is the first dark-sky island. Phllip Capper/Flickr hide caption

itoggle caption Phllip Capper/Flickr

The space shuttle Challenger explodes shortly after lifting off from the Kennedy Space Center in Cape Canaveral, Fla., on Jan. 28, 1986. Bruce Weaver/AP hide caption

itoggle caption Bruce Weaver/AP

To eat, chitons use their teeth, which look like black bulbs with bluish highlights, to grind up rock. Lyle Gordon/Northwestern University hide caption

itoggle caption Lyle Gordon/Northwestern University

The planet Kepler-10b, seen in this artist's drawing, is about 1.4 times the size of Earth and is the smallest planet ever discovered outside of our solar system. NASA hide caption

itoggle caption NASA