BEIJING, July 16 (Xinhuanet) -- A new theory about how humans evolved into upright walking creatures with a predilection for back pain and disabilities dates back 21 million years to the precursor of humans and apes, according to a spine specialist at Cedars Sinai Medical Center in Los Angeles.

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

    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.

    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.

    He also said humans evolved a new structure of muscles that pull the body from side to side while standing.

    "This is very important for carrying an infant or child," Filler said. "From the point of view of back pain, now we have big muscles doing this heavy work that never did before. They can get torn and strained."

    The backward orientation also allows the cushiony discs to get crushed, Filler said. "In most animals the vertebrae get spread apart when they carry infants on their backs when on all fours," he said.

    (Agencies)