The first steps that our earliest human ancestors took on two legs may arguably be the biggest ever, for both a man and mankind. Why the switch from all fours to just two limbs? The answer, according to a new study published today in: to save a few calories.

Anthropologist Herman Pontzer and colleagues at Washington University in St. Louis compared the energy expended when chimpanzees walk on either four or two legs with that used by humans walking upright. Their findings: people use a whopping 75 percent less oxygen, a direct measure of energy use, than chimps do perambulating on either two or four legs.

The study is the best evidence yet to support energy saving as the driver for bipedalism in our ape ancestors. It shows savings occur in hominins' upright posture, a result of lengthening leg bones and outward tilting pelvises.

"Since we can understand how anatomy drives the energy cost, we can ask what evolution would tinker with to make bipedalism cheaper," says Pontzer whose used high speed video and biomechanical measurements to show that chimps sauntering on two legs used the large muscles of their hips causing them to expend more energy than humans who confine most of their stepping movement to the small muscles in their ankles. Humans also take long strides compared with chimps' quick, short energy-guzzling steps.

The last common ancestor shared by humans and chimps lived about five million years ago, during a time when Earth was becoming dryer, the rainforest was shrinking, and sources of food were becoming fewer and more widely scattered. Scientists have long hypothesized that hominins evolved to bipedalism to save energy as they faced longer and longer treks to find food.

Daniel Schmitt, a biological anthropologist at Duke University, says that Pontzer's study is important because it demonstrates how incremental changes in both femur (thighbone) length and pelvis tilt save energy. "He's created a way to continue being a biped once you start," Schmitt says.

In contrast to earlier findings that bipedalism may have evolved in treetops, this study says that upright locomotion may have evolved in a four-legged, knuckle-walking ape ancestor.

Bipedalism is one of the first specifically hominin qualities that developed after our ancestors and ape ancestors split from one another. "What's cool about bipedalism is that it's first," Pontzer says. Now, he adds, we have a possible reason and mechanism for its evolution.