Two-Legged Walking an 'Energy Cheap' Option

(HealthDay News) -- Humans' move to two-footed, upright walking may have evolved because it uses less energy than walking on four legs, new research suggests.

Upright walking, or bipedalism, is a critical difference between humans and apes and is considered a defining characteristic of human ancestors.

Some scientists have speculated that bipedalism evolved in humans because it was a more energy-efficient way of foraging.

"For decades now, researchers have debated the role of energetics and the evolution of bipedalism," David Raichlen, an assistant professor of anthropology at the University of Arizona, said in a prepared statement. "The big problem in the study of bipedalism was that there was little data out there," he said.

In a study published July 16 in the online early edition of the Proceedings of the National Academy of Sciences, Raichlen and colleagues collected data on the energy individual bodies expended when five chimpanzees and four adult humans walked on a treadmill. The chimpanzees had been trained to walk on four legs (quadrupedally) and bipedally.

The humans walking on two legs only used one-quarter of the energy that chimpanzees that "knuckled-walked" on four legs did, the researchers found.

The chimpanzees, on average, used the same amount of energy using two legs as they did when they used four legs. The researchers attributed this to their different gaits and anatomy.

Shorter steps and more active muscle mass was associated with more energy used in walking upright, since the chimpanzee with the longest stride was most efficient walking upright.

"We and many others have found these adaptations [such as slight increases in hindlimb extension or length] in early hominins, which tells us that energetics played a pretty large role in the evolution of bipedalism," said Raichlen.

More information
The Public Broadcasting Service has more about walking upright.