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Let's pick up the pace and talk about running now. Endurance athletes sometimes say they're addicted to exercise. In fact, scientists have shown that rhythmic, continuous exercise - aerobic exercise - can produce narcotic-like chemicals in the body. Well, now researchers suggest that those chemicals might have actually helped turn humans - and other animals - into long-distance runners. NPR's Christopher Joyce explains.

CHRISTOPHER JOYCE, BYLINE: David Raichlen is an anthropologist at the University of Arizona and a runner - about 25 miles a week. Being human, Raichlen has some tools that help -short toes that don't get in the way, for example, and big joints in the legs to absorb shock. But he thinks humans are also wired to run.

DAVID RAICHLEN: Wired to run, meaning that our brains are probably - have been sort of rewired from an evolutionary sense, to encourage these running and high aerobic activity behaviors.

JOYCE: Many anthropologists believe that 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. But first, let's take a look at this thing they call runner's high.

DR. CHRISTINA MORGANTI: My name is Christina Morganti. I'm an orthopedic surgeon in Annapolis, and I've been a runner for a long time.

JOYCE: Since you were just a tot.

MORGANTI: A young, wee child. Yeah.

JOYCE: Morganti runs races, but not for medals.

MORGANTI: To be honest, I don't really care about the competition as much as that feeling you get when you're in really good shape. That's what I'm looking for.

JOYCE: I caught up with Morganti, literally, on a trail in Maryland. It was 7:30 in the morning on a sunny, spring day.

MORGANTI: You know, when you first start, you feel a little stiff and kind of logy. And then once you get started, everything loosens up and...

JOYCE: As a doctor, Morganti knows what regular running does for her body.

MORGANTI: Your heart gets stronger. It gets bigger. Your - the amount of blood your heart can pump is more.

JOYCE: That's stroke volume.

MORGANTI: Stroke volume, yup. Oxygen metabolism gets more efficient.

JOYCE: That means your blood vessels and muscles absorb more oxygen. Running also builds new bone. But when I ask her about runner's high, she lights up.

MORGANTI: Oh, it's really like an empowerment. And it's Zen at the same time. You feel strong and light, and you feel relaxed.

JOYCE: Morganti injured herself running two years ago; she had to stop.

MORGANTI: And everything else fell apart. My ability to cope with the stresses of life; my organizational skills juggling, you know, job and motherhood; everything like that wasn't as acute as it was when I was able to run and be fit.

JOYCE: It's now about three miles into the run. Morganti is getting her rhythm, but also some pain.

MORGANTI: I'm actually a little bit tired. I have a hamstring injury; I'm starting to feel that a little bit now. But I'm feeling like, what a beautiful day, and how great it is to be out here. And I don't care about that.

JOYCE: That's actually a problem - her not caring. Morganti treats runners for injuries, and she says they're the worst patients.

MORGANTI: The treatment is to stop running. They won't; they don't want to. And it's - a lot of the behavior is not unlike the patients we have that are seeking drugs. It's really similar. It's an addiction.

JOYCE: A drug addiction. And that's where we return to David Raichlen's experiment.

When people exercise aerobically, their bodies can actually make drugs - cannabinoids, the same 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. Maybe it's an evolutionary payoff for doing something hard and painful, that also helps them survive better, be healthier or hunt better, have more offspring.

So he put dogs - also distance runners - on a treadmill, as well as ferrets. Ferrets are not long-distance runners. And the dogs produced the drugs, and the ferrets did not.

RAICHLEN: It suggests that some level of aerobic exercise was encouraged by natural selection, and it may be fairly deep in our evolutionary roots.

JOYCE: Raichlen published his research in the Journal of Experimental Biology. He says it's not conclusive; he needs to test a lot more animals. Some anthropologists say it's actually walking we evolved to do, and running is just incidental.

But don't tell that to Christina Morganti. She's just finished her run, and she's happy.

MORGANTI: It's almost like a little tingle you get for several hours after, and then a calming you have the rest of the day. And then you sleep well that night and then the next day, you're ready to go again.

JOYCE: Even without an antelope to chase down. Christopher Joyce, NPR News.

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