Game of Thrones star Hafthor “The Mountain” Bjornsson recently broke the deadlift world
record with a 501 kilogram lift… which is absolutely, completely bonkers, but if he
had the lifting power of a leafcutter ant, he’d be able to lift a medium sized sedan
completely over his head, and carry it home.
Not bad.
And if The Mountain had the same relative strength as the taurus scarab dung beetle,
he could pull a fully loaded Boeing 787 Dreamliner.
Compared to us, ants and dung beetles and nature’s other miniature weightlifters are
special because they pull off their amazing feats of strength without bones.
And it turns out, for every species with an internal skeleton like ours, Earth is home
to around 20 species without one – and maybe more.
And that got me thinking, why did we end up with our skeletons on the inside instead of
the outside?
[OPEN]
Hey smart people, Joe here.
It’s time to face the truth.
You’re just meat, in a sack, tied to a bunch of carefully organized rocks.
That is basically what it means to be a vertebrate.
And we owe everything we are and that we do to our bones.
Skeletons are rigid enough for this, and flexible enough for this.
It’s said that we’re built with 206 bones, but 1 in 8 of us have an extra pair of ribs,
some people can even have a pair less.
And that doesn’t count sesamoids in tendons in our hands, feet, and elsewhere.
And speaking of feet, you have 52 bones in your feet alone, twice as many as in your
spine.
Our hands and feet have more than half the bones in our bodies.
Consider that rocks in your fingers were moved by muscles and nerves to help you click this
video.
The oxygen powering the brain that is watching this is being fed by blood made in your bones.
You are only even able to hear me talking thanks to tiny bones in your ears.
All of this from body parts that are about 70% mineral, made of an inorganic material
called hydroxyapatite that is stiff when compressed, combined with flexible protein called collagen
to keep you from shattering.
As a result the 20 or so pounds of bones in the average body can withstand one ton of
compression.
And for most of us, they are the last thing we’ll leave behind.
Bones are pretty awesome.
And also oss-ome.
That’s the Latin root for “bone”…
ANYWAY…
Where did bones come from?
That story goes back at least 1.5 billion years.
Which is a weird place to start because animals… didn’t even exist yet.
But at that moment, violently shifting tectonic plates were washing tons of minerals into
ancient oceans.
The minerals that would one day become skeletons.
Life stayed pretty squishy for a while.
Early multicellular life depended on the water in which it lived to support their skeleton-less
bodies.
But then, around 558 million years ago, through a happy accident of evolution, life split
in two, and two different skeleton stories began.
On one branch, strange creatures began to develop the first hard protective parts, the
precursors of exoskeletons.
And this set off an arms race of armor.
Newly shielded organisms might have gotten eaten less, maybe they were better protected
from the ocean, but partially hard creatures survived more than their squishy friends,
and like I always say, that’s what matters in the game of natural selection.
Early exoskeletons got fancier and we began to see animals with crushing mouthparts, pinchers,
and full suits of armor.
It was during this period that we see the earliest arthropods, the group that includes
modern insects and crustaceans.
But on that other branch, in squishy tadpole-like creatures, something else was happening.
Soft cartilage-like back rods began to form in order to provide scaffolding for muscles
and movement.
And on the outside, some of these fishy creatures evolved a cement-like armor.
This was the precursor of bone, and soon, this hard stuff formed the basis for new structures
like jaws.
Very big jaws.
Later, this mineral armor was slowly internalized, and those early backbones became mineralized,
and together these became the key parts of the vertebrate skeleton as we know it.
Over the eons, nature has stumbled on many different ways of desquishifying organisms.
There’s the lignin and cellulose of plants, calcium-rich shells of mollusks and coral
reefs, and mineral bones like ours.
But insects and crustaceans build exoskeletons made from chains of modified sugars called
chitin.
Chitin is molecularly similar to the cellulose we find in plants, but it’s harder and more
stable.
If you were to zoom into a lobster’s shell on the microscopic scale you’d see chitin
crystals arranged like stacks of plywood.
These special nanostructure arrangements make chitin exoskeletons incredibly tough for their
weight.
And that’s why insects are so strong pound for pound… or gram for gram.
So if exoskeletons are so strong, why don’t we have them?
Like most things in nature, there are tradeoffs.
For starters, opting for an armored exterior skeleton makes growing more difficult.
Every time a lobster or a cockroach is ready to size up, they have to molt, shedding their
old skeleton and leaving them soft and vulnerable for days or weeks while they wait for a new
outer shell to harden.
There’s also a weight problem.
The strength of ants’ legs work at their miniscule mass, but scale that ant up to our
size and it would be crushed under its own weight.
And a few more leg days won’t cut it.
Because as a creature gets bigger, its volume and mass increase faster than the tubular
strength of its hollow exoskeleton legs.
On the other hand, or leg, our internal skeletons provide bigger attachments for muscles than
exoskeletons would, and our bones grow with our muscles as we get bigger and stronger.
But there’s also a tradeoff here.
Bulkier vertebrates have to have more massive bones.
An elephant is about 13% bone by weight, not super agile.
A shrew is 4%, and while it’s quick, it’s easy to squish.
Humans are about 8.5%, a compromise between strength and mobility, and if we had more
bone to be stronger then we wouldn’t move as well.
Another problem is that a human-sized ant would suffocate.
Insects don’t really have blood.
They have very limited circulatory systems filled with a fluid called hemolymph, mostly
full of metabolic stuff and immune cells.
Insects breathe through tiny holes in their exoskeletons that deliver oxygen to their
tissues through a series of internal tubes.
The distance oxygen can travel down the tubes depends on its concentration in the air.
During prehistoric times, two-foot dragonflies did exist, because atmospheric oxygen levels
were higher.
And plus, having all your muscles attached to your outside means that while you’re
strong, you’re not that flexible.
Yeah, let’s see you do this, super strong grasshopper.
So.
Chitin exoskeleton: super strong, but only if you’re small.
A human-sized ant, very lethargic, very crumpled, can’t do yoga.
Remember that split that we talked about earlier?
One branch led to bugs and exoskeletons, and the other branch eventually led to you and
me and every other vertebrate.
And what’s crazy to think about it that we are this way because of luck, or chance.
Because when our ancestors began to build hard bodies, the only ingredients they had
to choose from were what nature provided.
Those shifting tectonic plates a billion and a half years ago?
Well, when slabs of rock that make up earth’s crust rub together, all the minerals they’re
made of wash into the sea.
And one of those minerals, calcium carbonate, happens to be a very useful building material.
Evolution is a lot like a chef stuck at home during COVID quarantine, by which I mean,
you’ve gotta use whatever ingredients you've got around.
Our very distant relatives were bathed in calcium from those grinding tectonic plates,
and so they used this to build the earliest bone-like tissues.
The evolution of bone wasn’t a grand aha! moment in the story of evolution, it is just
one of many such chance events, influenced by the environment, that sent one arm of life
on earth on a completely new course.
And vertebrates aren’t the only ones who ended up building bodies out of calcium.
Invertebrates from mollusks to coral to starfish all use calcium to build their bodies’ support
structures.
Exoskeleton-having animals like insects and other arthropods just went another way, building
chitin exoskeletons, thanks to the chance events of evolution, and if we played the
story of back again from the beginning, perhaps our line would be built differently too.
Which means, as we know now, we’d be very small.
But at least we’d have armor.
And maybe some sweet pincers.
And even some horns.
But then… what would ants look like…
Stay curious