It's in the evolution of the human spine

Jul 16, 2007 10:33 GMT  ·  By

80 % people experience back pains along their life. And in 90 % of the cases, this takes long to treat, over 6 weeks. The most frequent causes are diseases like arthrosis, muscular contracture, trauma, osteoporosis and inflammatory processes, the majority linked to back muscles and bones. Now a spine specialist has come with a new theory about why people experience back pains so often, in connection to our evolution of 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. 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," believes Dr. Aaron Filler of the Cedars Sinai Medical Center in Los Angeles, a medical doctor and anthropologist.

"That, in turn, explains why back pain is a leading cause of disability. One main clue was a bone feature called the transverse process, which sticks out from the side of the hollow, round vertebrae. This is where muscles attach to the spine. The vertebra is transformed in a way that literally reverses the mechanics of the spine. The bone lever of the vertebrae gets switched from bending the spine forward to bending the spine back." said Filler.

The vertebrae of most mammals have a transverse process oriented forward, in the front of each vertebra, for walking on all fours, facing the animal's inner cavity. This is also the situation in monkeys.

But in a 21 million-year-old creature called Morotopithecus bishopi, an arboreal, ape-like species discovered in present-day Uganda, the transverse process has moved backward, behind the opening for the spinal cord, just like in modern apes (chimps, gorilla, orangutan, gibbons) and humans.

Even if Moropithecus was first found in the 1960s, only in 1997, paleontologist Laura MacLatchy of the State University of New York at Stony Brook spotted this remarkable trait in Morotopithecus.

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

"Homo sapiens, the human species, continued upright, while apes evolved back toward all fours. When you look at most ape species, their spines and most of their bodies still look pretty monkey-like. 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. 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." said Filler.

"The backward orientation also allows the cushiony discs to get crushed. In most animals the vertebrae get spread apart when they carry infants on their backs when on all fours. What further differentiates humans from apes is the positioning of the place where the spine attaches to the hips," continued Filler, who made ample comparisons between the backbones of gibbons, chimpanzees, macaque monkeys, living and extinct species of other mammals groups but also pre-human species.