"In high school, I wondered whether the Jamaican Americans who made our track team so successful might carry some special speed gene from their tiny island. In college, I ran against Kenyans, and wondered whether endurance genes might have traveled with them from East Africa. At the same time, I began to notice that a training group on my team could consist of five men who run next to one another, stride for stride, day after day, and nonetheless turn out five entirely different runners. How could this be?"

We all knew a star athlete in high school. The one who made it look so easy. He was the starting quarterback and shortstop; she was the all-state point guard and high-jumper. Naturals. Or were they? The debate is as old as physical competition. Are stars like Usain Bolt, Michael Phelps, and Serena Williams genetic freaks put on Earth to dominate their respective sports? Or are they simply normal people who overcame their biological limits through sheer force of will and obsessive training?

The truth is far messier than a simple dichotomy between nature and nurture. In the decade since the sequencing of the human genome, researchers have slowly begun to uncover how the relationship between biological endowments and a competitor's training environment affects athleticism. Sports scientists have gradually entered the era of modern genetic research. In this controversial and engaging exploration of athletic success, Sports Illustrated senior writer David Epstein tackles the great nature vs. nurture debate and traces how far science has come in solving this great riddle.