Spine specialist trying to figure out why people so often have bad backs says he has come up with a new theory about when and how early humans evolved the ability to walk upright.

The uncannily human-looking backbone of a 21 million-year-old precursor of humans and apes gives the first clue, said Dr Aaron Filler of the Cedars Sinai Medical Center in Los Angeles.

A major change in the vertebrae that allowed this pre-human to stand upright and carry things also made it easier to crush and strain the spongy discs between each vertebra, Filler, a medical doctor with a doctorate in anthropology proposes.

That, in turn, explains why back pain is a leading cause of disability, he said. Writing in the journal Neurosurgical Focus on Sunday, Filler 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. Great apes, such as chimpanzees, share this feature.

The fossil was discovered in the 1960s but no one noticed the important change until 1997, when paleontologist Laura MacLatchy of the State University of New York at Stony Brook reported on the remarkable features of Morotopithecus.

"That means that upright posture bipedalism goes back 20 million years, not just 5 or 6 million years," said Filler.

In his study and in a book published last week called The Upright Ape - a new origin of the Species, Filler argues that this common ancestor, and ancestors going back many millions of years before, walked upright. Homo sapiens, the human species, continued upright, while apes evolved back toward all fours, he argues.

"When you look at most ape species, their spines and most of their bodies still look pretty monkey-like," Filler said.