Back pain is incredibly common, with 26% of Americans reporting at least one complete day of lower back pain within a 3-month period, consistent with a 2006 study in journal Spine. It is also the leading cause of disability across the world, consistent with a 2014 study in journal Annals of the Rheumatic Bugs.
So why do humans have so-much back pain?
“Because we walk on 2 legs,” said Jeremy DeSilva, a paleoanthropologist at the Dartmouth University. Before humans began walking upright, our mammal ancestors were running around on 4 legs for tens of millions, or even hundreds of millions of years. Mammals with this body-shape have horizontal spine that acts as a suspension bridge, holding-up their torso.
Nearly 7 million years ago, human ancestors had evolved a more upright posture, DeSilva said. Their spine got vertical, allowing them to move around on 2 feet. Experts do not agree on why humans evolved to become bipedal, but one among the major theories is that it helped to transition from the jungles to the savanna. Although, this adaption helped humans flourish, it came with some costs.
“Because evolution can only work with pre-existing anatomies & pre-existing forms, we’ve this spine that evolution has tinkered with,” DeSilva said. “And it has made it good enough. I mean, we are still here. But it does not mean, we do not have problems. Evolution leads to being sufficiently enough to survive. It does not lead to your comfort.”
Bruce Latimer, a physical anthropologist at the Case Western Reserve University in Ohio, described spine as a series of cups (vertebrae) & saucers (disks between vertebrae) balanced on top of each other. Most people have 24 of these cups & 23 disks. Ligaments & muscles help stabilize the stack, but because it is vertical, 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 sequential vertebrae,” DeSilva said.
The natural curve of human spine also causes issues. The spine-curve to balance weight, to allow for flexibility and to avoid blocking the birth canal. But because of this bend, people are susceptible to develop more severe curve, like kyphosis (an outside curvature of upper spine) or scoliosis (a lateral-curvature of spine), DeSilva said. At each curve, spine is prone to fractures.
Modern life in industrialized-countries also plays a role. Core muscles stabilize back, but many people have weak mid-sections. “If you are sitting at a desk all day, slouched-over and you are not working the lower back muscles, then they are easily strained,” DeSilva said.
Although there are many factors, evolution is the main culprit, DeSilva said. After all, our ancient ancestors, including famous Australopithecus Lucy, had back problems too, consistent with a 1983 study in American Journal of Physical Anthropology.
Not all bipeds have as-much back pain as humans, however. Several large terrestrial birds, like ostriches, walk upright on 2 limbs without much of an issue.
“As far as I know, ostriches do not have to go to the chiropractor very often,” DeSilva said. One reason why is that bird’s spine is more diagonal than vertical, so it can act more as a suspension-bridge rather than a tower of cups & saucers. The ostrich also had significantly more time to evolve a high functioning back. “They had a roughly 200-million-year head start on us,” DeSilva said. “When it comes to a bipedal skeleton, we are kind of new kids on the block.”