Malaria may not be an ancient plague but rather a fairly young one¿about 10,000 years young. According to a study published Friday in Science, the initial spread of the disease parallels that of agriculture and early human civilization. So by looking at mutations in our own genes, scientists can track malaria's origins.

"This is a striking example of how infectious disease can shape the path of human evolution," says lead author Sarah Tishkoff of the University of Maryland. She looked at the human gene G6PD (glucose-6-phosphate dehydrogenase) and found that some people have a mutation that makes them immune to malaria. When tracing the mutation back, she found that it emerged simultaneously with early agriculture.

The link between agriculture and malaria lies with the animal that transmits the disease: the mosquito. Scientists estimate that between 7,000 and 12,000 years ago, temperature and humidity in Africa increased dramatically and created lakes and pools of water, breeding grounds for bugs. At the same time, larger human populations that stayed in one place made it easier for the disease to spread. "One mutation found throughout Africa arose within the past 3,830 to 11,760 years," Tishkoff says. "This estimate is consistent with archaeological and historical documents that show malaria has had a significant impact on humans only within the past 10,000 years, since the origination of agriculture."

Tishkoff found another mutation of G6PD in the Mediterranean region, the Middle East and India, which developed more recently¿1,600 to 6,640 years ago. She thinks that the army of Alexander the Great may have brought a different strain of malaria to the regions when it conquered them in the fourth century B.C. "This study," Tishkoff says, "demonstrates how the environment, culture, genes and history interact to shape patterns of variation in the modern human genome."