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. 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.  "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. Read more...

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