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.

The finding rests on a molecular comparison of the genomes of the two parasites. Stephen Rich, an evolutionary geneticist at the University of Massachusetts Amherst, and his colleagues measured the diversity of the genomes, a rough proxy for age (genomes tend to acquire genetic components over time). Reichenowi's genome can be 20 times more diverse than falciparum's, which means reichenowi is much older. “It seems that malaria has been in chimps as long as they've been chimps,” Rich says.

Following the trail back to the origin of reichenowi is a more complicated problem, not least because malaria is so widespread. “In terrestrial vertebrates, we find it virtually everywhere we look,” Rich observes. “We're only getting started.”