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Comment by YeGoblynQueenne | Hacker News Reader

>> 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".

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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.

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

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.

>> We probably have bones as tough as they need to be, or _needed_ to be in the last million or so years.

Well, yeah. I think evolution makes you as good as you need to be to reproduce before you die, rather than optimise every characteristic.

I would hazard a guess that necessary reactions involving calcium require (a lot) less temperature than those that would be necessary to build a bone-like structure out of iron or aluminium.

afaik Aluminium also requires an electric current is run through it while being refined. It's more than complicated metals like iron.

Well you can't eat as much titanium as you can eat calcium so there is that.

That's because neither you nor anything in your diet (nor anything in your diet's diet) has titanium teeth and jaws.

The Earth's crust is 3.6% calcium, and we get more than enough of that in our diet...it's 5% iron. There's even a lot of titanium - fully 0.6%, much higher than 'rare' elements like carbon! [0]

I don't think we'd have any trouble getting it if the metabolic and biological systems that are employed to get us calcium were instead employed to produce titanium.

[0]: https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth...

We take in calcium as +2 ions, and use it in the same state when making calcium phosphate for bones. That helps because calcium, like most metals, takes a lot of energy to reduce to its metallic form. Titanium and iron are also naturally found as ions. Getting bulk quantities of the metallic form would be a waste from an evolutionary standpoint, when it could just make a strong mineral-based composite material like bone.

> There's even a lot of titanium - fully 0.6%, much higher than 'rare' elements like carbon! [0]

Almost all our carbon goes back to the air.

This leads to the interesting concept that when you are losing weight, you are actually exhaling most of your weight loss.

Edited: Downcommenter is correct. Sorry, glitched. I was thinking about various things and pulled plants into the mix when the article is only about humans.

What do you mean by "We don't eat carbon"? All food that humans consume is mostly made out of carbon, hydrogen, oxygen and nitrogen. The carbon is indeed breathed out in the form of carbon dioxide, but no animal is capable of taking carbon from the air and integrating it into their body.

unless you consider plant life some form of animal ;)

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