Look at the apes.

These tree-dwelling creatures have extremely long arms (with a span up to 2m or 6 ft!) while their legs are short. That's why a "skinny" roughly 200 kg (500 pounds) male gorilla can be no taller than 1.7-1.8 m (5.5-6 ft).

So, how to explain the inverse human anatomy, which may seem silly for apes, with the short limbs and long legs?

The energy cost of walking and running is linked primarily to the effort made by muscles to lift and move the limbs.

That's why Dr. Herman Pontzer, assistant professor of physical anthropology in Arts & Sciences, University of Washington in St Louis focused on how much energy does it actually take to get around and how longer legs influence this process. He designed a mathematical model for assessing energy costs for two and four-legged animals. "All things being equal, leg length is one of the major determinants of cost. If two animals are identical except for leg length, the animal with longer legs is more efficient", said Pontzer.

Fossil evidence reveals that 2.5 million years ago, when humans passed from Australopithecus ape-human type to Homo, human like type, a big increase in leg length occurred.

Scientists believe this is an adaptation for running and walking over larger distances, the real break away from tree life and adaptation to terrestrial life. "If you greatly increase the distance that you travel each day, then you'd expect evolution to act on walking efficiency. That way, the energy you save on travel can be spent instead on survival and reproduction", said Pontzer.

His LiMB model is an equation that predicts the rate of force generation (energy use) while walking and running linked to limb length. Longer legs were found to require less force production and lower energy cost.

Pontzer experimented his mathematic models by putting people, goats and dogs on a treadmill and measured the oxygen consume while walking and running at various speeds, a measure of energy production and waste. The LiMB model revealed itself to be more effective than others in assessing locomotor cost, including contact time and body mass, being valid for animals with either two or four legs.