XMRV, a mouse virus, may be an artifact of laboratory experiments rather than the cause of chronic fatigue syndrome. Whittemore Peterson Institute hide caption

itoggle caption Whittemore Peterson Institute

A study published in the journal Science last December suggested that a bacterium found in California's Mono Lake was able to substitute arsenic for phosphorous. But that conclusion has many critics. Miss Bliss 55 via Flickr hide caption

itoggle caption Miss Bliss 55 via Flickr

Shoppers crowd a narrow street outside Tsukiji market in Tokyo on Dec. 31, 2010. Japan has relatively tight social rules. And that makes sense, according to researchers. When people are squeezed together, they have an incentive to cooperate. Toru Yamanaka/AFP/Getty Images hide caption

itoggle caption Toru Yamanaka/AFP/Getty Images

Mixing antidepressant Paxil and the cholesterol-fighter Pravachol may lead to an unexpected increase in blood sugar, researchers say. Joe Raedle/Getty Images hide caption

itoggle caption Joe Raedle/Getty Images

CT scans of fossil Hadrocodium skulls allowed scientists to reconstruct its brain. The olfactory bulbs, located at the front of the brain, grew steadily larger as millions of years passed. Matt Colbert/University of Texas at Austin hide caption

itoggle caption Matt Colbert/University of Texas at Austin