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Imagine, for a moment, how our earliest ancestors felt when they came down from the trees and stood on two legs. There are benefits, of course: free hands to carry food. There were also downsides. On the ground, it was a tiger-eat-monkey world, and two legs were slower than four.

But one prominent biologist says, not so fast. Humans invented something better than speed - endurance running - which helped change the course of evolution.

For our series "The Human Edge," NPR's Christopher Joyce went to Cambridge, Massachusetts, to investigate and work up a sweat.

Professor DAN LIEBERMAN (Human Evolutionary Biology, Harvard University): So how far shall we go?

CHRISTOPHER JOYCE: Dan Lieberman starts running toward a bridge.

Prof. LIEBERMAN: We can either go that way - that's about a mile and a half - or we can go that way; that's about a mile.

JOYCE: A rather distant bridge.

(Soundbite of laughter)

Prof. LIEBERMAN: If you want to abandon ship, that's fine. I don't blame you. It's not...

JOYCE: He looks like a human greyhound. Lieberman's already run three miles this morning. He's an evolutionary biologist at Harvard University, and he studies running and how it makes humans unique. Even when he's running a marathon, he's thinking about how the body does it.

Prof. LIEBERMAN: Sometimes I do kind of bizarre things when I'm running and, you know, move my arms in funny ways just to think about it. And I get these strange looks, and realize I should probably not be doing this in public.

JOYCE: Today, it's his footwear that draws attention. They're like gloves -skintight, no heels, no support. Lieberman studies barefoot runners and finds they naturally land on the balls of their feet - it's too painful to land on the heel. Planting your foot on the ball transfers and stores elastic energy in the foot ligaments and the Achilles tendon.

Prof. LIEBERMAN: And then they act like rubber bands. They're springs. And they recoil, and they push you back up into the air as you start to take your jump.

JOYCE: Lieberman says that makes barefoot running - the way our ancestors ran -more efficient than running with shoes.

But even if early humans ran more efficiently, they still couldn't run as fast as a four-legged meal. So, Lieberman says, they evolved into marathon machines.

Prof. LIEBERMAN: Most animals are designed for speed, for power, not for endurance. And we're a very special species in having been selected for endurance, not speed.

JOYCE: We grew longer legs and lighter feet. The joints in the legs and the pelvis got bigger to absorb a lot of impact. We grew a bigger butt muscle.

Lieberman says these and other changes allowed us to run down and exhaust prey, like antelope and kudu. He notes that so-called persistence hunters in Africa have been known to do that. And the payoff would have been big for early humans: lots of high-calorie meat to feed a bigger brain.

At this point, my brain is oxygen-starved, so we stop running. Lieberman says I lack meat motivation.

Prof. LIEBERMAN: In fact, I tell people, you know, when you're in the marathon and you're wondering at mile 16 or 17 what on earth you're doing, remember, you're chasing a kudu. That's what you're doing. You're re-enacting that chase from a million years ago.

JOYCE: Does it work?

Prof. LIEBERMAN: Oh, absolutely.

(Soundbite of laughter)

Unidentified Man: All right, going in three, two, one.

(Soundbite of treadmill)

JOYCE: At Harvard's 19th century Peabody Museum, one of Lieberman's graduate students runs on a treadmill. Electrodes stuck on his body record muscle contractions. He wears a dog collar around his forehead, with accelerometers to measure head movements.

Lieberman is creating a computer model of how we run. He thinks it will tell him how the earliest humans evolved to run. He believes running made us what we are. To prove it, he sometimes asks his volunteer runners to do some strange things.

Prof. LIEBERMAN: There are no humans out there with faces as large as Neanderthals. So people wear weights in their mouths, which then changes the center of gravity of the head.

JOYCE: They put weights in their mouths?

Prof. LIEBERMAN: Oh, yeah. So we have special weights.

JOYCE: It sounds horrible.

Prof. LIEBERMAN: Yes, it is.

Unidentified Man: Three, two, one.

(Soundbite of laughter)

JOYCE: But understanding head control is important. If you don't keep your head still, you can't focus your eyes. Lieberman says modern humans, unlike apes, have a special muscle that connects each arm to the neck and the head. When you swing your arms, they become counterweights to stabilize the head.

Clearly, we're runners. And millions of years ago, we started putting more meat on our menus. That's clear from cut marks on animal bones, and our unique digestive system and our bigger brain. Did endurance running make that happen? Lieberman thinks yes, though the proof is tough to tease out.

Prof. LIEBERMAN: When you find bones, they - you know, none of them come with labels on them telling you how they ran or how they walked, or why they're the way they are in the first place - or why we are the way we are, right? But they pose questions about, you know, what were the transitions in human evolution? Why did those anatomical features change?

JOYCE: Some scientists say just because modern African hunters can run down prey doesn't prove our ancestors did. There's no hard evidence of it. Maybe it was better hunting tools that got us more meat. Maybe what Lieberman sees as the body's running adaptations just made for better walking.

And some suspect that Lieberman's passion for running colors his conclusions. He says no.

Prof. LIEBERMAN: Oh, I'm an evolutionary biologist first and a runner second, absolutely. I mean, I do love running, but it's not the only thing I love.

JOYCE: Lieberman is passionate about exercise. He says it's fundamental to being human. It's essential to the way our bodies function and to keep them healthy. He says running is the purest form of exercise.

Prof. LIEBERMAN: Adding information about how exercise - how we evolved to exercise, and how exercise is actually woven into our bodies by natural selection in a special way, I think, adds extra relevance.

JOYCE: Especially, he says, as our modern lifestyle threatens to make human exercise obsolete.

Christopher Joyce, NPR News.

NORRIS: To see how human bodies are built for running, we have a video for you at npr.org.

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