Every year, on the third Monday of April, 20,000 people line up to run the world's oldest marathon, in Boston, Mass. — a horrible place to run. The weather can range from rainy to snowy to unbearably warm — sometimes in the same day.

So why do people do it?

Harvard University anthropology professor Daniel Lieberman and his collaborator, University of Utah biology Prof. Dennis Bramble, argue that humans were born to run.

Their groundbreaking study, published in the journal Nature in November 2004, got the scientific community pondering the hypothesis that it's our ability to run, not walk, that sets humans apart as the world's dominant species.

Prof. Lieberman recently spoke with CBC News Online about what makes people really good runners — even though it long ago stopped serving an evolutionary purpose.

What is it that makes us born to run?

We have features in our bodies from our heads all the way to our toes that help us to become good runners — from short toes, a very stable spring-like arch in our foot, and lots of tendons in our legs that act like springs, storing and releasing energy when we run; low, wide shoulders coupled with our heads that allow us to twist our bodies and pump our arms. One of the neat things is that many of these features don't play any role in walking at all, so they're not just a byproduct of being good at walking. There's good evidence of natural selection for running.

You've focused on the role of the gluteus maximus — our big butts.

Everyone always mentions that! The gluteus maximus is actually quite fun because it is the largest muscle in the human body and actually plays very little role in walking at all. You use it when you bend over and have to raise yourself up from being in a flexed posture, from climbing stairs and things like that. But actual walking requires very little effort from that muscle and it turns out it plays two key roles in running. One of the things that makes running different from walking is you're leaning forwards in running, you're always falling. I actually often say that running is a controlled fall. Before your leg hits the ground, your gluteus maximus contracts very strongly and helps stabilize the trunk on your pelvis, on your hips — it keeps you from falling over. The other thing it does is help slow your swinging leg down so it hits the ground properly. What's neat from my perspective, because I not only study running but the evolution of running, is that we can see traces of the gm in the human skeleton. Its origin leaves a big huge scar at the top of the pelvis. It looks like the gluteus maximus got really big around two million years ago.

Others have argued that running played no major role in human evolution. Why do you disagree?

People haven't argued that it played no role. I think they just ignored it. Everybody's focused on walking. There are several reasons. Walking sets us apart from the apes. Walking is truly, deeply fundamental to being human, to being a hominid, and I don't disagree with that in the slightest. The other reason people have not really thought much about it is that we have generally thought of ourselves as really bad runners, and that's mostly because people tend to think about speed. The very fastest human beings in the world — the ones who win gold medals at the Olympics — can run about 10 metres a second, for about 10 seconds, and then they run out of gas. Your typical quadripedal mammal can go about 20 metres a second for about four minutes. I think people are still processing our idea but nobody's come out with one shred of evidence to the contrary and I think most people have had to admit that we are on to something because we are phenomenal runners, it's just we're different kinds of runners than people have been thinking about. We are long-distance runners and endurance runners and we have all kinds of features that enable us to do it and our performance capabilities are outstanding. We can outrun pretty much any mammal in the world, particularly when it's hot and dry.

Why are we still good runners if we haven't needed it for so long?

Actually, 20,000 years ain't so long at all. That's my first response. My second response is that's not how selection operates. Evolution will only get rid of something if it has a selective disadvantage. I would argue that many of us don't need our large brains, either. The fact of the matter is that modern medicine, modern life, technology, culture — all those things enable us to do just fine without natural selection operating. I do believe though that perhaps for millions of years, natural selection did weed out those people who were not good athletes. And so I would imagine the variation in athletic abilities is probably increasing — so that the slow ones, the unfit ones, the ones with poor cardiovascular capabilities — whatever the feature is, if there's a heritable aspect to it, there's much less cost to being unfit and poorly designed now then there used to be.

What were the advantages for early man to being able to run long distances?

Well, I think it has to do with hunting or possibly scavenging, or both. Imagine you're homo erectus two million years ago. You think you are technologically unbelievably sophisticated, because what you have available to you is a sharp stone and you can take that little sharp stone and make yourself a sharp stick. That's it. That's the technological prowess of homo erectus two million years ago. And you've got a hungry family and you want to bring home the bacon, so to speak, and how are you going to do it? You're not just going to walk up to a wildebeest and shove a spear into it because the wildebeest is going to kick you or gore you to death. If you do that a few times, the chances of you getting kicked in the stomach are very high. Big large African mammals are dangerous — even incredibly stupid ones like wildebeest. But what can you do? You can simply chase after it, with a club or a sharpened stick or something like that — and within two to three hours, in the heat, you can — because quadrupeds who can't sweat to cool off overheat quickly when they're galloping. All you have to do as a runner or hunter is to keep that wildebeest or kudu above a trot for long enough and the animal will be unable to kick you or gore you with its horns. It will be easy to dispatch.

What did we as an evolving species give up to gain this ability to run?

Climbing. I think that the big compromise was climbing. It is very hard to have some of the features that are very useful in running and also have other features that are useful in climbing. For example, having low, wide, decoupled shoulders — difficult when you're a runner because you have to twist your body from your head to your pelvis. Climbing animals don't often have tendons. Tendons save a little bit of weight but they mostly act as springs. They're not very useful in walking. Having long toes are a real problem if you're running. I'm not sure if they're a problem walking, but they're really useful if you're climbing trees. I see no reason why you can't be a walker and climb trees. Being a runner and an arboreal creature is really hard.

What role does the skull play in running?

I happen to be a skull guy, so I tend to focus on the skull. It's not so important, but there are lots of interesting little clues in the skull that tell us about running, the most important, I think, being the semicircular canal system, the organs of balance. Humans have much more sensitive semicircular canals than do australopithecines or apes. They act as great gyros; they sense angular acceleration in the head. When you walk, your head doesn't move around very much, but when you run, your head really bounces and jiggles around enormously because you're basically like a pogo stick. There are huge forces of acceleration acting on the head so those more sensitive semicircular canals that we find in the genus homo are surely related to running; they're not related to walking.

Did you get into this line of research because you like to run?

I think that is related to it. I started getting interested in the problem when I was a grad student working towards my PhD and Dennis Bramble, the fellow with whom I am collaborating, was the person who first put the idea in my head. We started fooling around and playing and looking at fossils in the late '80s. He works on general mammalian locomotion and I work on the evolution of humans and so it was a nice mix. As soon as I started thinking about it, it became a no-brainer. I thought about it and Dennis thought about it from every possible angle, and I just don't see how we could possibly be wrong — and that's a dangerous thing to say in biology and evolution — but I think it's one of those ideas that just strangely got missed. There's just so much evidence and it's so obvious to anybody who starts looking at humans and the ability to run long distances — it's just one of those interesting biases that crept into our culture and into our scientific thinking.

That must feel pretty good — that your hypothesis had such an immediate impact.

It's really odd. Really, I'm interested in epistemology — I'm interested in figuring how you figure things out. I'm not really known in human evolution as a storyteller. I'm more interested in understanding underlying biology that enables us to make inferences about evolution. And so, this has cast me in a sort of uncomfortable role, which is performing a hypothesis about why we are the way we are. But I have to say, it's quite fun. I just came back from some physical anthropology meetings and there were lots of talks where people are thinking about running and they're thinking about running in the spinal column and in this and that and the other. Somebody presented really neat data that humans are actually much less costly runners than previous studies have shown. There are many things we don't know — when modern human capabilities evolved — and that will be fun to flesh out. It could turn out, for example, that homo erectus was a good runner but not as accomplished as humans — it was sort of selection had started but hadn't finished the job. There's not enough evidence yet.

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