From: Live Science
By
Tara Santora
October
5, 2021 – 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.
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.
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."
No comments:
Post a Comment