FOR MORE THAN A CENTURY researchers have been trying to figure out how malaria first arose in humans. The question is urgent, because more than two million people die every year from Plasmodium, the malaria parasite, and understanding its origins might one day lend clues to its complex biology. A piece of the puzzle fell into place in September 2009, when a team of researchers discovered that the main strain that infects human beings—P. falciparum—evolved from another version of the parasite, P. reichenowi, which currently infects chimpanzees. And it happened a mere 10,000 years ago—a moment in evolutionary terms.