Why do so many people have back pain?

Back pain is incredibly common, with 26% of Americans reporting at least one full day of lower-back pain within a three-month period, according to a 2006 study in the journal Spine. It's also the leading cause of disability across the globe, according to a 2014 study in the journal Annals of the Rheumatic Diseases.

So why do humans have so much back pain?

"Because we walk on two legs," said Jeremy DeSilva, a paleoanthropologist at Dartmouth University. Before humans began walking upright, our mammal ancestors had been running around on four legs for tens of millions, or even hundreds of millions, of years, he told Live Science. Mammals with this body shape have a horizontal spine that acts as a suspension bridge, holding up their torso.

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About 7 million years ago, human ancestors evolved a more upright posture, DeSilva said. Their spine became vertical, allowing them to move around on two feet. Experts don't agree on why humans evolved to become bipedal, but one of the major theories is that it helped to transition from the jungles to the savanna. Although this adaptation helped humans flourish, it came with some costs.

"Because evolution can only work with pre-existing anatomies and pre-existing forms, we have this spine that evolution has tinkered with," DeSilva said. "And it's made it good enough. I mean, we're still here. But it doesn't mean we don't have problems. Evolution leads to being just good enough to survive. It doesn't lead to your comfort."

Bruce Latimer, a physical anthropologist at Case Western Reserve University in Ohio, described the spine as a series of cups (vertebrae) and saucers (disks between the vertebrae) balanced on top of each other. Most people have 24 of these cups and 23 disks. Ligaments and muscles help stabilize the stack, but because it's vertical, the disks are prone to slippage. 

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"Humans are the only mammal that we know of that as we age, we can get spontaneous fractures of our vertebrae just from having that weight on top of each successive vertebra," DeSilva said.

The natural curve of the human spine also causes issues. The spine curves to balance weight, to allow for flexibility and to avoid blocking the birth canal. But because of this bend, people are susceptible to developing more severe curves, such as kyphosis (an outward curvature of the upper spine) or scoliosis (a lateral curvature of the spine), DeSilva said. At each curve, the spine is also prone to fractures.

Modern life in industrialized countries also plays a role. Core muscles stabilize the back, but many people have weak midsections. "If you're sitting at a desk all day, slouched over, and you're not working the lower back muscles, then they're easily strained," DeSilva said. 

Although there are multiple factors, evolution is the major culprit, DeSilva said. After all, our ancient ancestors, including the famous Australopithecus Lucy, had back problems, too, according to a 1983 study in the American Journal of Physical Anthropology. 

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Not all bipeds have as much back pain as humans, however. Some large terrestrial birds, such as ostriches, walk upright on two limbs without much of an issue. 

"As far as I know, ostriches don't have to go to the chiropractor very often," DeSilva said. One reason why is that the bird's spine is more diagonal than vertical, so it can act more as a suspension bridge rather than a tower of cups and saucers. The ostrich also had significantly more time to evolve a high-functioning back. "They've had a roughly 200 million-year head start on us," DeSilva said. "When it comes to a bipedal skeleton, we're kind of the new kids on the block."

Originally published on Live Science.