Some scientists have concluded that man is evolutionarily adapted to running long distances in order to hunt down prey. We have springy tendons in our legs and feet that store energy and release it efficiently. Our thin waist and relatively large butt muscles help keep us upright and stable. More important, our hairlessness, sweat glands and ability to breathe through our mouths enable us to run in heat and over longer distances better than animals that get rid of excess heat by panting.

Distance running has also enabled us to maintain contact with other tribes and to spread information. (Indeed, the first Olympic marathon celebrated Pheidippides' purported run in 490 B.C. from Marathon to Athens to bring news of a Greek victory over the Persians.)

But the husky and Arabian horse wouldn't run long distances if not pushed to do so. Animals run because they have to — to eat or avoid being eaten. Man is the only animal that runs simply to do it. Our large brains can convince our frail bodies to keep moving regardless of cost. We may not be the fastest animals, but we can run ourselves into the ground for sport, exhausting our food supply, and making ourselves susceptible to disease, injury and death. That's a feat no pronghorn can touch.

Under the right circumstances, humanoids can outrun any animal on the planet. Distinctive features like Achilles tendons, knee joints or sweat glands make us the finest running organisms on Earth. Our very survival depended on this for millions of years, since we used to hunt and chase our prey in Africa's savannas. Unlike animals, humans, who lack abundant fur, don't gasp in order to dissipate heat, but use their millions of sweat glands. This may be the evolutionary result of having to cope with sun heat when forest habitats were traded for arid savannas. Lieberman also demonstrated that, contrary to the popular belief, running is almost entirely different from walking and the traits he discovered proved that we are more skilled in running than in walking.

Man is the only runner without a balancing tail. The answer lies in compensating this by moving the head, torso, and most importantly, the rear. Together with moving the arms forwards and backwards, this prevents falling in the process. Running animals don't use the same body parts and techniques for loping. As further proof of his theory, Lieberman also states that only runners evolve big tendons. The neck, he says, is very important as well, since, unlike that of, say, chimps, it lets our shoulders twist independently of the head.