Elaine Meinel Supkis

Elaine Meinel Supkis

Walking on two legs costs humans only one-quarter the energy expended by chimpanzees who knuckle-walk on four legs, according to a new research conducted by U.S. anthropologists.

This saving in energy may have been what originally drove our common ancestor to walk upright, anthropologists from University of Arizona and Washington University, St. Louis reported Monday on the latest issue of Proceedings of the National Academy of Sciences.

The research team measured how much oxygen five chimpanzees and four human volunteers burned as they walked on a treadmill: the chimps on four legs or two legs, the humans just on two.

Filler, writing in the journal Neurosurgical Focus on Sunday, said one main clue was a bone feature called the transverse process, which sticks out from the side of the hollow, round vertebrae, Filler said in a telephone interview. This is where muscles attach to the spine.

"The vertebra is transformed in a way that literally reverses the mechanics of the spine," Filler said. "The bone lever of the vertebrae gets switched from bending the spine forward to bending the spine back."

Most vertebrates are oriented forward, to walk on all fours. The transverse process is at the front of each vertebra, facing the animal's belly. This is true of monkeys, too.

But in humans and in the 21 million-year-old fossil of a creature called Morotopithecus bishopi, a tree-dwelling, ape-like creature that lived in what is now Uganda, the transverse process has moved backward, behind the opening for the spinal cord.

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