
Why are bones not made of steel? (2010) - diziet
https://www.materialstoday.com/mechanical-properties/news/why-are-your-bones-not-made-of-steel/
======
tw1010
From "the social function of intellect"[1]: "Henry Ford, it is said,
commissioned a survey of the car scrap yards of America to find out if there
were parts of the Model T Ford which never failed. His inspectors came back
with reports of almost every kind of breakdown: axles, brakes, pistons - all
were liable to go wrong. But they drew attention to one notable exception, the
kingpins of the scrapped cars invariably had years of life left in them. With
ruthless logic Ford concluded that the kingpins on the Model T were too good
for their job and ordered that in future they should be made to an inferior
specification. Nature is surely at least as careful an economist as Henry
Ford. It is not her habit to tolerate needless extravagance in the animals on
her production lines: superfluous capacity is trimmed back, new capacity added
only as and when it is needed"

[1]
[http://biology.unm.edu/pwatson/Humphrey%20The%20Social%20Fun...](http://biology.unm.edu/pwatson/Humphrey%20The%20Social%20Function%20of%20Intellect.pdf)

~~~
masklinn
> It is not her habit to tolerate needless extravagance in the animals on her
> production lines: superfluous capacity is trimmed back, new capacity added
> only as and when it is needed

It's not exactly wrong, but it's way too anthropomorphic and attributes way
too much intent.

Plus there's plenty of natural stuff which at least looks way over-engineered
or redundant/unnecessary in its current form. The Mantis Shrimp's eyes for
instance. The gallbladder also seems mostly redundant, and while one can argue
that the lobed paired design of lungs is necessary considering the common
occurrence of respiratory diseases redundant kidneys seem less than necessary.

~~~
kmano8
I'll keep my paired lungs. Had one spontaneously collapse about 10 years ago,
and had there not been a second in it's own pleural cavity, it would've been a
much worse time for me.

As an aside, one really cool thing that happens in the case of a collapsed
lung, is the trachea will shift toward the healthy side under less pressure.
Called a tracheal deviation
[https://en.wikipedia.org/wiki/Tracheal_deviation](https://en.wikipedia.org/wiki/Tracheal_deviation)

~~~
robbiep
Tracheal deviation in the instance of lung collapse is a sign of a developing
fatal condition, tension pneumothorax. Cool for me as a doctor because I get
to stab a big needle through your chest. Not so cool for the patient

~~~
komali2
Also cool for Mad Max.

------
Gatsky
Bone is an organ with multiple functions beyond structural support. It creates
a reservoir for calcium and phosphate homeostasis and of course contains bone
marrow. It also has attachments for muscles and connective tissue, and adapts
to environmental conditions (eg see a tennis player's forearm). A titanium
bone could do none of these things. The question should really be why are
bones not reinforced with metal or metal composites. In general if greater
strength is needed it can be achieved with thicker bone, which has a far lower
evolutionary barrier than eg metal composite bones. Additionally, the material
properties of the whole biomechanical apparatus need to be considered. A super
strong skinny thigh bone isn't much good if the area available for ligamentous
attachment is so small that the tendons keep tearing. Most injuries involve
connective tissue and muscle rather than bone.

~~~
corey_moncure
Not to mention the role of bone in blood cell synthesis!

The bone is a fascinating organ and sadly misunderstood by most people. They
are, essentially, living rocks. Our bones are innervated, full of living
cells, and constantly engaging in a chemical equilibrium with the body on
CaPO4 as you point out.

Furthermore, the bone needs to grow along with the creature. It has to be soft
enough to accommodate childbirth but allow growth and hardening as the
organism matures. The process by which this occurs in natural bone is complex
and remarkable.

Metal bones can't do any of this stuff.

------
Amorymeltzer
Surprised that this totally blows past the chemical reasons why not. There's
an excellent answer by shigeta on the Biology Stack Exchange[1] to this very
question.

His major answers are that electrical conductivity is likely to be a huge
problem for the nervous system, and that iron and aluminum readily oxidize
(aka rust). Iron oxidization is useful in hemoglobin but would be problematic
in your bones, and while a body could be designed with conductivity in mind,
it would at minimum also take a reworking the nervous system.

1:
[https://biology.stackexchange.com/q/9419/4101](https://biology.stackexchange.com/q/9419/4101)

~~~
x2398dh1
What you pointed out is a very astute answer, but comes from a creationist
perspective rather than an evolutionary perspective. His main answer is:

> Firstly, fully reduced (oxidation state 0) metal has a high energetic cost
> to create in reduced form.

Basically, it takes a ton of energy to make steel. We are literally burning
gigagallons of over a billions years worth of pressurized organic matter to
have made all of the steel we have made in the past century and a half or so.
We could make a super inaccurate but somewhat plausible estimation that we
have used something like 150 million years worth of the entire earth's
lifeforms at the time's dead bodies and converted that into 150 years worth of
steel. So if my orders of magnitude are correct...it would be something like a
million times more difficult energy-wise for an organisms to create steel from
scratch than calcium-based hardened material.

That's assuming we have burned through 15% of a billion years worth of oil
from bacteria, that 100% of the bacteria converted into oil and then converted
directly into steel, and that there was only a billion years of bacteria to
oil creation. I am sure these numbers are off, but maybe that means it's only
100,000 times or 10 million times more difficult for organisms to make steel
rather than bones.

~~~
fjsolwmv
How is it Creationist? Creationism is just a metaphor for evolution plus a
fictional goal of evolving toward specific human form.

~~~
eesmith
Creationism is not "just a metaphor for evolution." It is a word which covers
a wide range of beliefs, include those who argue that each species was the
result of a specific act of divine creation.

("In Creationism, special creation is a theological doctrine which states that
the universe and all life in it originated in its present form by
unconditional fiat or divine decree." \-
[https://en.wikipedia.org/wiki/Special_creation](https://en.wikipedia.org/wiki/Special_creation)
)

Take this quote from "The Vanishing Case for Evolution" by Henry Morris,
[http://www.icr.org/article/vanishing-case-for-
evolution/](http://www.icr.org/article/vanishing-case-for-evolution/) one of
the founders of the Institute for Creation Research:

> The absence of evidence for evolution does not, by itself, prove creation,
> of course; nevertheless, special creation is clearly the only alternative to
> evolution.

How can special creation - one type of creationism - be both a "metaphor for
evolution" and "the only alternative to evolution"?

Obviously it can't, which is why it's incorrect to summarize creationism as
you did.

Progressive creationism is more accepting of evolution, in that it says
"microevolution" occurs but "macroevolution" does not. Even this cannot be
seen as a metaphor for evolution, because it rejects part of evolution.

Perhaps theistic evolution fits your description
([https://en.wikipedia.org/wiki/Theistic_evolution](https://en.wikipedia.org/wiki/Theistic_evolution)
) but otherwise most types of creationism do not.

------
YeGoblynQueenne
>> But if you had metal bones they wouldn't ever need repairing: titanium
alloy for example has a fatigue strength of about 500MPa which is more than
five times greater than the stresses that it would experience in its life as a
bone.

It's worth remembering that Calcium (the substance bones are made of) is, in
fact, a metal [1]. And our bodies have no problem growing and repairing our
bones. Why wouldn't that happen with steel?

Also- the bit about the stresses that a titanium alloy bone would experience
in its lifetime- yeah, that, probably. We probably have bones as tough as they
need to be, or _needed_ to be in the last million or so years.

________________

[1] A factoid that my 16 year-old self, obsessed with Metal (the music) found
deeply satisfying. "Dude, my bones are made of METAL".

~~~
yorwba
The calcium in bones is pretty much all in the form of calcium phosphate,
which behaves very differently from a pure metal, e.g. with respect to
conductivity. Saying that bones are made of metal is a bit like saying that
stone is made of metal.

~~~
beambot
But... that's exactly what ore is: Stone made partially of metal.

~~~
comicjk
Metal as a material is different from metal elements. Metal element atoms are
common - most kinds of rock contain them. Metal as a material, by contrast,
has metallic bonding, where electrons are delocalized across many atoms. This
is what gives metal materials all their special properties. The metal atoms in
rock do not have metallic bonding; they have covalent and ionic bonding to
nonmetal atoms like oxygen. Therefore rock does not contain metal the
material, even though it contains metal elements.

------
hotsauceror
"...why have our bodies, and those of other animals, evolved so as to make the
particular structural materials that they do make, and not to make others,
especially metals..."

Because that's what's good enough to get through the crucible of successful
genome transmission? Evolution doesn't have an end goal; every "improvement"
came about by random changes, either in the genome or in the environment,
which yielded a reproductive advantage to some new gene or genes. I'm not sure
if the author intended it but this seems to fall into the common fallacy of
believing that evolution is an active process guiding organism toward some
optimal state. Organisms don't evolve "to do" anything. They end up there.
Contemporary genomes are what's left after everything else has died. Not the
conquerors of mountains.

~~~
toasterlovin
Evolution is one of the hardest things to actually fully integrate into a
mental model. There are so many subtleties that just do not interact well with
our tendency to ascribe motivation to stuff.

~~~
hotsauceror
It seems fairly intuitive to me. Think of a bunch of marbles falling through a
series of nets. Changes in a particular marble, or in a particular net, might
let the marble fall through. Marbles that fall through one net get to try
again with the next net. Evolution just means comparing marbles that collect
in each net. There's a million answers to the question "how can a marble get
through this net", and the nets don't care if anything gets through at all.

~~~
toasterlovin
FWIW, I don't think the analogy you described is what many people would
describe as "fairly intuitive", though I think it's a reasonable one.

Still, it relies on harnessing our spatial reasoning and I don't think most
people are good at visualizing or reasoning about billions of marbles being
sifted through billions of nets (in the case of simpler organisms) or even
millions of marbles being sifted through ~7,000 nets (in the specific case of
humans).

------
theophrastus
Then there is also the evolutionist's reply to this question: because the
ocean has a lot more calcium than iron, (in ppm Ca: 400, Fe: 0.003[1]). So all
the evolved mechanisms from shells to (cuddle) bone were in place due to
available materials long before any other possibility presented itself.

[1]
[http://www.seafriends.org.nz/oceano/seawater.htm](http://www.seafriends.org.nz/oceano/seawater.htm)

------
ufmace
Also from an engineering standpoint, I would think that just metal-izing the
skeleton wouldn't be very helpful. You'd just switch to breaking all of the
tendons and ligaments and other connective tissue first instead. So you'd need
to strengthen those up too, and in a way that is still as durable and
repairable.

Then all of this stuff is adding more weight, we're going to need bigger or
more powerful muscles to move it around well. Then we'll need bigger/better
heart, lungs, circulatory system to fuel that extra muscle power, and improved
digestive system to get more energy for that stuff into the body. Skin is
seeming kinda fragile with all of this extra bone strength and muscle power,
better toughen that up too.

~~~
logfromblammo
I think I have wondered far more often why bones aren't made from a substance
stiff enough to act as anchor points for tendons, but also flexible enough to
bend over and then snap back to its original shape without breaking. The
bending stiffness and yield strength seem very important for the types of
stresses on a biological organism.

So why aren't our bones made out of polyimide-polyamide-fiberglass composite?

~~~
ufmace
I don't know enough biology to be sure, but I'm pretty sure that some of our
ribs are fairly flexible in this way. I would guess, if we had to come up with
a reason why those bones are flexible and our arms and legs, say, aren't, it's
that the tradeoff in ability to exert large forces in a predictable way and
taking bigger impacts before breaking isn't worth it.

Not that evolution is able to actively design things in that way, of course.
More like it was an evolutionary advantage for those ribs to be flexible, but
any flexibility in arm and leg bones and other major bones was not an
advantage.

------
rlpb
Evolution is also an optimization engine. Living creatures are not over-
engineered because doing so requires additional resources, and resources are
limited. Creatures that are not over-engineered are "fitter".

For example: during a famine, perhaps a growing child with "normal" bones is
much more likely to survive than a steel-boned counterpart who is unable to
find the required extra iron ore and energy needed to grow.

~~~
Pigo
I know this is pretty much a nerdy tangent, but something similar is discussed
often in Star Trek with the Klingon anatomy. Supposedly every major organ has
a redundancy (though I'm not sure they really mean EVERY organ). It's seen as
a strategic advantage by many for beings who live a violent lifestyle, but
some question it as over-engineering. With twice the organs there's the
potential for twice as many things to go wrong during a normal lifespan.

If you consider the fact that human males who live to 80 have a 80% chance of
prostate cancer (IRL not Star Trek), it seems like we live with time-bombs
that do eventually go off.

~~~
SilasX
Did Klingons genetically engineer themselves? If not, this is hard to square
with such a (relatively!) hard-science show. In order for that to happen
through evolutionary processes, you would have to have chance mutations give
them double (or higher) hearts, and that would have to happen for all the
organs, and the selection pressure for warriors that survive (otherwise) fatal
blows would have to be happening all throughout that process.

~~~
Pigo
I've tried to imagine how such a feature could evolve naturally, and I can't
really see how. I believe the impression is that it was an evolutionary
feature that came about because they were constantly losing major organs. BUT
there is quite a bit of genetic engineering and manipulation in Star Trek.

Most of it is to remove genetic birth defects, but there was an entire arc in
Enterprise about how a large population of Klingons were genetically altered
to give them superior strength and skills. So it's possible that it was
related to this, or was engineered at some point in a similar fashion. The
whole arc was only to explain why the Klingons looked different in the early
series and movies. Berman and Braga tried their best to keep the canon intact,
and to have it make sense.

------
CapitalistCartr
Bones can give and break. This can be a benefit. Better to break a bone than
rend the flesh in many cases.

~~~
taneq
More importantly, better to break a bone, then let it knit back together more-
or-less as it was, than to bend a bone and be stuck with a permanently bent
limb.

~~~
FreeFull
On the other hand, if a fully broken bone isn't set properly, then you are
stuck with a permanently bent limb anyway.

------
danso
Fun read. I was thinking the answer would simply be kind of the same as the
reasons for not making an entire plane out of whatever its "indestructible"
black box is made of.

------
wellboy
So, to summarize, we don't have a steel skeleton because

1\. We evilved from the sea, where Calcium is far more abundant than metals
2\. Having a skeleton 100x harder than out current one, would require 100x
stronger muscles and tendons, if you don't want them to snap 3\. You couldn't
swim with metal bones, which would again be a problem with 1., and make it
hard to survive as a species if you drown very easily.

Other arguments, which however, were debunked

1\. It takes too much energy to create steel. It doesn't, maybe twice as much.
2\. You also have other bone functions. However, you could also have those
with a semi-steel, semi-organic bone structure. 3\. Bones need to decompose,
so that you don't steal too much resources from the environment. Possible
argument?

------
TheSpiceIsLife
> _So let 's look at the facts. The bones in your body are made from material
> which has a tensile strength of 150MPa, a strain to failure of 2% and a
> fracture toughness of 4MPa(m)½. For a structural material that's not good.
> We can make alloy steels that are ten times better in all three of those
> properties._

Regular structural steel, used for all sorts of typical building applications,
has a yield strength of 250MPa and an ultimate tensile strength of about
400MPa. So bone with it's 150MPa tensile strength, about 60% that of steel's
yield strength, is pretty good!

For comparison, typical structural timbers have a tensile strength of around
18MPa.

------
vlehto
I'm not an expert in the field, but I'm pretty certain the writer of this
piece is neither. He is not using correct units and seems to be misusing the
term "fatigue strength".

The proper way of conducting this kind of material comparison would be finding
the design limit. In this case it would be probably either toughness/weight of
fracture toughness/weight. Then if you are unable to come up with specific
grade of metal that simply beats bone, you have absolutely no case. Typically
alloying for high strength decreases both toughness and fracture toughness.

------
kazinator
Bone in fact sucks at repairing itself in the face of age-related degeneration
such as osteoporosis, arthritis and such.

If you had titanium bones, you wouldn't become hunch-backed and short in old
age, nor would your fingers deform.

~~~
toasterlovin
I would not discount the effect that sedentary lifestyles may have on
degenerative diseases. Modern life is a huge confounding variable when
evaluating the effectiveness of our bodies.

~~~
kazinator
Modern life means having an unprecedented probability of living past forty.

------
lurquer
Incorporating steel into bones (and, presumably shells) would result in
lifeforms that would be very difficult to kill. It may be the case that an
ecosystem cannot establish itself if the members' defense mechanisms are too
strong. It may be physically impossible to evolve muscles strong enough to
crush a steel skull. (Or, at earlier stages, to crush a clam.) Is there not an
evolutionary pressure that tempers the establishment of defense mechanisms
(such as steel-plate covered creatures) that are too effective?

------
nmeofthestate
I'm not so interested in what bones are made of - I've never broken one.
However, I'd love it if my joints could be replaced with something less awful.

~~~
buckthundaz
[https://www.youtube.com/watch?v=LGUhVelktk4](https://www.youtube.com/watch?v=LGUhVelktk4)
[http://www.toddbumgardner.com/hip-and-shoulder-mobility-
the-...](http://www.toddbumgardner.com/hip-and-shoulder-mobility-the-cars-
solution/)

------
Houshalter
Evolution is all about local optimas. If you can't reach your goal with just a
series of small improvements, it can't happen. The article claims an organism
could just eat metal ores. Sure, but to do that you need to start with an
organism that already eats and digests ores. Presumably calcium bones evolved
in an organism that already had a lot of calcium in it's diet for other
reasons.

------
falsedan
For starters: the body would oxidise them & they would rust, and the detached
flakes would travel to other parts of the body, blocking blood vessels or
causing iron poisoning.

A better question would be: why haven't organisms evolved which use iron in
their skeletons?

~~~
comicjk
Bone is a composite material, made of calcium phosphate interwoven with
collagen fibers. It's not clear to me that structural bits of iron would
integrate well into this structure.

~~~
falsedan
It's not clear to me what your point is. Are you saying that my suggestion
that the article should have addressed a different (similar) hypothetical is
at odds with your imagination and the physical structure of bone?

~~~
comicjk
It's an answer to your question "why haven't organisms evolved which use iron
in their skeletons". The physical structure of bone would not be improved by
incorporating more iron, so there's no reason for evolution to move in that
direction.

~~~
falsedan
That's not my question… that's the question I would have preferred the article
addressed. I already gave an answer to the question in my original comment.

Also, the 'explanation' begs the question, why wouldn't we see a bone
replacement evolve that's suited for iron reinforcement? Your clarificiation
is a small step down the 'whys', but saying "because bones have evolved to use
calcium" is tautological.

------
yoz-y
How much heavier could one's bones get before they would not be able to swim
anymore?

~~~
mnw21cam
To answer that question, have you ever tried to swim with a 1kg weight belt?
Such devices are commonly used by scuba divers to maintain neutral buoyancy.

Or put it another way, the difference in density between seawater and fresh
water is about 3%, and it's noticeably easier to swim in the sea. So if you
have bone replacements that make you 3% heavier, you should expect to notice
it.

~~~
mannykannot
At least part of the ballast normally carried by divers is to compensate for
the buoyancy of either a wet- or dry-suit, and for the buoyancy of SCUBA tanks
(some are buoyant when empty, at least in seawater [1].) Treading water while
carrying an extra kilogram would be quite a struggle for many people.

On the other hand, the article compares bone to metals on an equal-weight
basis.

[1]
[http://www.huronscuba.com/equipment/scubaCylinderSpecificati...](http://www.huronscuba.com/equipment/scubaCylinderSpecification.html)

------
westmeal
It's a cool idea and forgive me if I'm wrong but don't the bones in the body
create new T & blood cells using the bone marrow? If every bone was replaced
in your body how would you generate new cells?

~~~
BjoernKW
Easy: Exchange the blood for something entirely different, too ...

~~~
westmeal
Trust me I'd replace blood with transmission fluid any day but that's pretty
far from an organic being eh

------
jdonaldson
Bones not only have to be structurally sound, they also have to be
decomposable. Otherwise skeletons would never go away, and it would be too
difficult to reclaim the raw materials from them.

I also have the opinion that bones enable healthier predator/prey
relationships _because_ they're just weak enough to get pulverized by a larger
physical force (i.e. from a predator). I realize teeth could be steel as well,
but in that case muscle would no longer be enough to completely crush a
skeleton.

Nature really seems to value materials that are just strong enough to get the
job done, but just weak enough to get reclaimed easily.

~~~
mannykannot
Even if the current situation is a global optimum in the sense you propose, it
would be purely coincidental, as evolution doesn't do global optimization or
long-term planning, and prey does not adapt for the convenience of its
predators.

~~~
jdonaldson
Everything in evolution is coincidental though, right? It's all about simple
survivability of traits. If one of the traits introduces an imbalance (at a
micro/macro level), it'll eventually sort itself out.

~~~
mannykannot
I think I see what you are getting at, in that the mutations that initiate
each evolutionary step are random, but selection is not random. It is not a
coincidence that Impalas and Cheetahs can run at about the same speed, and it
is not a coincidence that Pronghorns can run as if they were prey for
Cheetahs, as until relatively recently, they were. This does not mean that
every coincidence in biology has an evolutionary origin.

~~~
jdonaldson
That's true... it's impossible to say what's a coincidence and what's not with
respect to a given predator/prey relationship. Usually you see a pretty fair
match up though. I always think it's interesting when you see a species that
is seemingly over-engineered for the purposes of hunting or staying alive:

[http://www.bbc.com/earth/story/20151014-superpowers-of-
the-n...](http://www.bbc.com/earth/story/20151014-superpowers-of-the-near-
invincible-velvet-ant)

[http://www.newsweek.com/orcas-killing-great-white-sharks-
eat...](http://www.newsweek.com/orcas-killing-great-white-sharks-eating-
livers-607002)

------
mattdeboard
Yo this guy just explained the rock eater in Neverending Story

------
grondilu
Aren't all biological tissues made out of proteins? I mean, the very mechanism
of DNA transcription implies that products are necessarily polymer-like, if I
understand correctly. DNA can't make metals. I suppose some purely enzymatic
mechanism could produce 3D-cristals (a metal is a macroscopic aggregate of
microscopic crystals, right?), but apparently no such mechanism exists. It'd
be great though, as we could probably replicate it.

I would also say that nature does make metals, it just needs to go through a
behavioral layer.

