Pritchard's proficiency in both mathematics and biology can be traced in part to a bum knee. He and his father, William G. Pritchard, an applied mathematician who moved the family from England to the United States to take a faculty position at Pennsylvania State University, shared a love of running. Pritchard's first scientific paper, which he wrote as an undergraduate with his father as coauthor, analyzed the effects of wind on sprinters and concluded that the women's world record in the 100 meters, set by Florence Griffith-Joyner at the Olympic Trials in 1988, should have been disallowed because of a strong tailwind. However, the anemometer, which recorded a wind speed of zero, appears to have malfunctioned. (“Running officials have occasionally asked us about our results,” says Pritchard, “but they don't want to change the record.”)

Pritchard planned to run the 1500- and 5000-meter track events for Penn State when he entered as a freshman. But a knee injury forced him to redshirt for a year, which meant he was in college for five years. That gave him plenty of time to double-major in biology and mathematics while still competing on the track and cross-country teams. A freshman class taught by Andrew Clark on population genetics—the study of changes in DNA sequences in populations of organisms over time—intrigued him. “He was tremendously gifted mathematically,” Clark says. “When a student like that comes along, you pay attention.”

After graduating from Penn State, Pritchard moved to Stanford University to work with Marc Feldman, a prominent geneticist who had also been Clark's graduate adviser. In Palo Alto, Pritchard quickly fell in with the vibrant community of students and faculty members surrounding Feldman and Luca Cavalli-Sforza, who pioneered the use of population genetics to study the movements of human populations over broad historical periods. “I was fascinated by the idea that you could use the mathematics of population genetics to learn about human history,” Pritchard says.

In 1998, he returned to England for a postdoctoral fellowship at Oxford University, and there he wrote a computer program that dramatically changed how geneticists think about the genetic relationships among people. Known as structure, the program analyzes the differences in DNA sequences in a sample of individuals. It then sorts the individuals into groups based on their genetic similarities.

Pritchard and Noah Rosenberg, a friend from Stanford who's now a geneticist at the University of Michigan, led a team of researchers who used structure to analyze genetic data from more than a thousand people drawn from 52 worldwide populations. At the time, geneticists thought the extensive genetic overlaps among all humans would make it difficult to divide people into categories. But structure clearly sorted the people into groups centered on continents or parts of continents, including sub-Saharan Africa, western Eurasia, eastern Asia, and the Americas. The resulting paper, published in Science in 2002, was named “Paper of the Year” by The Lancet.

Newspaper stories heralded the results as providing a biological basis for traditional racial classifications, but Pritchard interprets the results somewhat differently. For him, the patterns in our genomes reflect the earth's geography and the history of populations as much as they do the classifications societies use to divide individuals into groups. Anatomically modern humans evolved in Africa sometime before 150,000 years ago. They spread into the rest of the world and gradually replaced the more archaic forms of humans living in other parts of Africa and in Eurasia, including the Neanderthals in Europe, Homo erectus in Asia, and the most recently discovered Homo floresiensis in Indonesia.

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