Endurance athletes sometimes say they're "addicted" to exercise. In fact, scientists have shown that rhythmic, continuous exercise — aerobic exercise — can in fact produce narcoticlike chemicals in the body.
Now researchers suggest that those chemicals may have helped turn humans, as well as other animals, into long-distance runners.
The man behind the research is University of Arizona anthropologist David Raichlen, a runner himself. He does about 25 miles a week.