Did humanity's ability to run long distances turn us into the world's dominant species? That's what a couple of scientists -- Dennis Bramble of the University of Utah and Daniel Lieberman of Harvard -- argued last week in Nature (PDF link). Many animals are much faster than humans at sprinting short distances, but they have no endurance. Humans are one of the few animals (other than dogs, hyenas, and horses) that can run for minutes and even hours at a time. Running imposed a big cost on homo erectus: A physiology engineered for the marathon is ill-suited for climbing trees, which means we couldn't as easily forage for fruit or escape predators. But, as the scientists argue, long-distance running allowed us to roam more widely in search of the high-protein food necessary to evolve our huge brains, small intestines, and small teeth, all of which eventually allowed to us to create the Xbox and The Simple Life. Ooo yeah.

The scientists have compiled a list of 26 physiological features that make humans human, all of which evolved partly to allow us to run -- such as the ligament at the back of the neck that allows us to hold our heads steady even while bounding (as per the diagram above, taken from the Nature paper.) Most other animals can't do this, which is what got Bramble and Lieberman originally interested in this problem. Thirteen years ago, as the New York Times reports, they were watching a pig run on a treadmill ...

"Dennis and I noticed how the pigs can't hold their heads still while running," Dr. Lieberman recalled. "Any good human runner keeps his head still because of the nuchal ligament, a tendon in the back of the neck."

Interestingly, another crucial part of our ability to run is our development of a big, meaty butt. Even our closest relatives -- apes, chimps and monkeys -- don't have them:

Have you ever looked at an ape?" Dr. Bramble said. "They have no buns."

Dr. Lieberman, a paleontologist, explained: "Your gluteus maximus stabilizes your trunk as you lean forward in a run. A run is like a controlled fall, and the buttocks help to control it."

So the next time you're in the gym checking out your ass in the mirror, take comfort: All civilization rests upon what you see.