There’s a good chance you’ve had back pain.
And I'm saying that not because I know anything about you, but because you are human.
The WHO estimates that 60-70% of people in industrialized countries will experience low
back pain sometime in their adult life . They also label it as the single biggest cause
of disability in the world.
And it can happen in old people, young people, athletes, sedentary folks — everyone is
subject at some point.
Back pain is a kinda nebulous cluster of symptoms with lots of technical diagnoses.
Things like age, sitting all day and a lack of flexibility can predispose us to back pain,
and when I worked in a physical therapy clinic we would assess all those things as part of
a patient’s treatment plan.
But no matter what condition is causing back pain,
it's still a fact that you're more likely than not to feel it at some point in your life.
And to understand why, we need to go back in time to when our ancestors roamed the earth.
No, no, past that.
Nope, more still.
That’s better.
Humans belong to a group of animals called vertebrates, meaning we have a backbone, really
a bunch of tiny backbones called vertebrae.
When vertebrates fist showed up, all of animal life happened in the oceans.
The support and buoyancy of water made it totally fine to squish and squirm through
the ocean like jellyfish or microbes did.
But eventually, fish with bony spines showed up.
Then some of those fish evolved lobe-fins, which gave rise to tetrapods, vertebrates
with four limbs.
And that was a good body plan that would eventually let tetrapods walk on land.
But without support of water they needed to adapt to fight gravity.
They had the advantage of having four limbs to distribute their weight across, but they’d
also need to evolve things like strong shoulders and pelvic girdles, as well as tweak their
spine.
And one of the biggest influences on the spine’s anatomy was the animal’s locomotion, or
how it moves around.
Imagine a lizard.
Its spine bends side to side, lifting its limbs off the ground, moving forward using
this kind of pivoting motion.
But if you wanted to sprint or gallop, your spine would need to flex and extend.
And that’s exactly what happened.
Animals evolved longer limbs that went under their body instead of out to the side which
started a new style of locomotion that allowed some higher speeds.
Think of how a cat sprints compared to a crocodile.
There’s some debate about which came first, the limbs or the spine, but that’s a story
for another time.
This spine was such a good design, that mammals kept it when some of them went back to aquatic
life.
Like dolphins can flex and extend their spines while sharks bend side to side.
There's a reason that sharks can’t do backflips.
That anatomical plan of spinal cord protected by bony vertebrae has stayed relatively consistent
in mammals.
And then our ancestors started walking on two legs.
This shift to bipedalism is probably where back pain comes from.
If we look at how chimps walk
They use a flexed hip and a flexed knee, which they need because their center of gravity falls here.
By walking upright, humans could take longer strides which saved energy during long bouts
of walking.
And to do that, we evolved a lordotic lumbar spine, it curves like this which puts our
center of gravity above our hips.
We have different curves throughout our spine.
Lordotic in our 7 cervical vertebrae, kyphotic in our 12 thoracic vertebrae, again, lordotic
in the 5 lumbar, and our sacrum is one piece, but it looks like a kyphotic curve.
That lordotic curve made it so our trunk’s weight came down here, which made a compressive
force here which is cushioned by the intervertebral discs between our vertebrae.
But that also results in a shear force here.
That’s our first big pain point.
Our bodies are constantly trying to resist this shear force through our ligaments and
muscles.
Okay, if the spine is going to evolve, its connection with the legs, the pelvis, had
to evolve too.
Chimp pelvises are longer while our ilia are shorter and broader, which makes sense.
A wide, short ilium and broad sacrum gave the lowest lumbar vertebrae plenty of clearance,
making sure it didn’t hit the pelvis while we extended our legs during long strides.
Plus, human babies have freakishly large skulls, so the human pelvis needed to get freakishly
wide anyway.
Our femurs had to get longer and straighter too, which gave us even longer strides along
with femoral heads and acetabula, or hip sockets, that lent themselves to hip extension.
But in this transition, we lost muscle mass in our erector spinae, a muscle group that
straightens the spine.
Old World Monkeys have massive erector spinae while humans have, as a 2005 review put it,
a “semi-competent” erector spinae that they think might be why we’re the only animal
that gets scoliosis.
So by about 3 million years ago our ancestors had wide pelvises, long legs, and curvy spines
better suited for walking than climbing around trees.
And really, our spinal and pelvic anatomy hasn’t changed too much since then.
So then what about this structure predisposes us to injury?
Well, different research groups have different hypotheses, pointing at everything from different
muscle type composition to chronic strain on our spinal ligaments required by the shear
forces.
Right, none of these studies have to do with modern risks like how much you sit or how
bad your posture is — these studies are more interested in the structure of the skeleton
than lifestyle.
A few years ago, researchers had an idea called the “ancestral shape hypothesis which goes like this:
There’s enough variation within modern humans that some of us have vertebrae shaped
more like our ancestors’, who had vertebrae better suited for knuckle walking
like gorillas or tree climbing like orangutans.
Not talking on two legs.
And we can measure that similarity with a technique called geometric morphometrics — basically
applying geometry to 3D scans of a body part.
According to the ancestral shape hypothesis, people with vertebrae that are more similar
to our ancestors will get injuries more than others.
So in 2015, researchers tested this hypothesis.
They collected vertebrae from human, chimp, and orangutan samples and scanned them with
special machines, which let them analyze the shape.
Then they looked for signs of disc herniation, which is one of the most common causes of
low back pain.
This kind of injury happens when the gelatinous contents of the intervertebral disc push out
of the disc, usually posteriorly into nerves, or vertically into another vertebral body.
That vertical squish causes a little divot called a Schmorl node, which was evidence
that the vertebrae had a disc herniation.
Sure enough, the human vertebrates that were shaped more like the other primates had higher
rates of Schmorl nodes and thus higher rates of disc herniation.
Evolving a lordotic spine also put as at risk of a kind of stress fracture to the posterior
part of the vertebrae called spondylolysis, or what I call /spondies/...isn’t that fun?
Now, disc herniations are usually associated with spine flexion while spondylolysis is
associated with lordosis — apes don’t usually get spondies.
If people with disc herniations have really similar morphologies to ape vertebrae, then
by this logic, people with spondies should have really dissimilar vertebrae.
So the same researchers from the ancestral shape hypothesis study came up with the “overshoot”
hypothesis.
Like evolution started curving the spine towards an upright posture and never stopped.
So both the big curve and individual vertebrae predisposes backs to spondylosis.
With that in mind, the research team did another round of scans and got a nice little proof
of concept as a result.
Humans with spondylolysis had differently shaped vertebrae than their healthy human
counterparts (other than the obvious fracture), and those humans with spondies had very different
shaped vertebrae than apes.
So that’s the tradeoff.
Our spines have this shape because it lets us walk on two legs, but as a consequence,
we have vertebrae that set us up for back pain.
Now, when the day comes and you go seek medical help for back pain, your doctor isn’t going
to pull up this video and gripe about evolution.
That would be a very unhelpful doctor.
They’ll hopefully do some imaging and some functional tests and give you a proper diagnosis.
But there’s something beautiful about this phenomenon.
We often point to DNA as the unifying part about biology.
We share so much genetic material in common and that's cool, but that’s even more true
if we’re talking about back pain.
Modern humans are products of evolution, we all experience something our ancestors felt,
that people all across the world that we’ve never met will feel — the terrible design
of the human spine.
I hope you like this video.
I’ve got more musculoskeletal videos for you to watch in this playlist right here if
you’re interested.
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Have fun, be good, see you next time.