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 thinks humans are "Wired to run, meaning that our brains ... have been sort of rewired ... to encourage these running and high aerobic-activity behaviors." Many anthropologists think early humans learned to run long distances to chase down and exhaust prey, like antelopes. Meat is one payoff for runners. But Raichlen thinks there may have been another reward: a runner's high. He designed an experiment to test this idea. When people exercise aerobically, their bodies can actually make drugs - cannabinoids, the same kind of chemicals in marijuana. Raichlen wondered if other distance-running animals also produced those drugs. If so, maybe runner's high is not some peculiar thing with humans. So he put dogs - also distance runners - on a treadmill. Also ferrets, but ferrets are not long-distance runners. The dogs produced the drug, but the ferrets did not. Says Raichlen: "It suggests some level of aerobic exercise was encouraged by natural selection, and it may be fairly deep in our evolutionary roots."

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